Part Forty Four – Over-thinking things – Part One

From the very start of this project I have now realised that I have wasted hundreds of hours thinking far too much about things that really don’t need worrying about.  In this episode, I’m going to try and focus on a few of them, I’ve no doubt more will come up in later posts.


I mentioned in a reply to Mike in the previous part that I had wasted a lot of time doing things that I thought would be needed for the building control completion inspection, but that it turns out weren’t.  The same is true about so much of this build, that I think it’s well worth trying to summarise some of them in a single post.

Firstly, I should point out that as a retired scientist I find it far, far too easy to get sucked in to investigating details and finding out why things behave as they do.  Sometimes this is a good thing, but 90% of the time it leads to a great deal of wasted effort.  The classic is the initial house design.  I was obsessed with how to heat it and making sure it had enough passive solar gain in winter.  It didn’t even cross my mind that it might  need cooling, neither did it cross my mind during the design stage that having a house deep into a cut out into a south-facing hillside, near the bottom of a fairly sheltered valley, would mean that the micro-climate around the house was significantly warmer than the average for our area, which is on the edge of Salisbury Plain – a notoriously cool place!

One consequence of this is that the local climate data from the Met Office isn’t very representative of the temperatures we get in this valley, and even less representative of the local temperature around our house, set, as it is, 2.5m down into the hillside and sheltered on the North and part of the East by the ground a couple of metres away.  This meant that the climate data I used both in my own spreadsheet and in PHPP was in error.  My estimate is that in summer our average local temperature is around 2 to 3 deg C warmer than the Met Office data set for the area, and in winter at least 1 deg C warmer, maybe a bit more.

The effect of this is to skew the heating requirements downwards, and create an overheating risk that didn’t show in my spreadsheet, PHPP or SAP.

Curiously, most of the overheating comes from the East, not the South.  The sun rises well North of East in summer, so the East elevation has a long period of direct exposure.  This shows clearly on the cladding; the larch on the East side has faded a great deal more than that on the South or West faces.  The bedroom with a window facing East overheated badly, so this has been mitigated by applying heat reflecting film to the outside of that window and by replacing the very thermally lossy thermal store with the Sunamp PV (the services area is adjacent to the East bedroom).

The kitchen East-facing window also gets a lot of solar gain, as does the South facing one.  The dark grey of the stone window internal window cills, and the kitchen worktops, exacerbate this, by heating up and convecting and conducting heat into the room.  By contrast, the solar gain from the South facing gable is not that massive.  The large roof overhang limits it, but adding solar reflective film has also made a big difference, such that cooling is not often needed now.

I wasted more time thinking about complex heating and cooling control systems than you would believe.  I built several versions of rather sophisticated controllers, using internal and external sensors measuring air temperature (inside and outside), floor slab temperature, relative humidity, flow and return temperatures from the floor heating, you name it.  I had weather compensation, predictive heat demand from rate of change of outside temperature and very accurate control of the floor slab temperature, which, in a low energy house, is directly related to the heat output at any time and the house internal temperature.

All this was a complete and total waste of time and money.  At a guess I spent around 500 hours playing around with different systems, plus a few hundred pounds building ever-more sophisticated controllers.  I can’t believe, now, that controlling the temperature of a passive house is really very, very simple.  There is no need for any complex controls or expensive valves, sensors, etc.  The house will pretty much look after itself when it comes to heating in winter, with just a very simple room thermostat controlling the whole ground floor under floor heating.  It’s a complete and total waste of effort, time and money to invest in anything clever or sophisticated to regulate the heating, when a simple and cheap room thermostat works so exceptionally well.

We’re in the heating season now, and have had temperatures down as low as -6 deg C on a couple of nights.  The only heating has been from the under floor system, as we don’t ever use the Genvex active MVHR for heating at all – it just isn’t needed and, anyway, it tends to dry the air out a bit too much.  The coldest temperature I’ve seen in the house has been about 20 deg C first thing in the morning, the warmest (this December) about 21 deg C.  The floor slab sensor now only displays the temperature, it doesn’t control anything, and that shows a steady temperature of around 21.2 to 21.8 deg C.  The heating system comes on about once every two to four days, for around an hour, and usually pumps water at about 23 to 24 deg C around the floor slab.  Any hotter than this and the house will heat up too quickly, and there will be a noticeable overshoot in the floor, and room, temperature.  On very cold nights, the heating will come on in the morning. A couple of weeks ago we had three nights in succession where the heating came on for an hour or so every morning.  The room thermostat is on the wall, in the shade, on the ground floor, pretty much in the very centre of the house.  It’s currently set to 20.5 deg C, and has a 0.1 deg C switching hysteresis.  This means it turns on at 20.5 deg C, and off at 20.6 deg C.

Because there is a lag between the floor slab warming up and the room air warming up, the room temperature will continue to slowly increase through the day as the slab gives up heat, even though the heating is off.  This seems ideal, as it means that the temperature tends to peak in the evenings, at around 21 deg C if there’s been no one in the house and no solar gain, then drop very slowly overnight on a very cold day.  The rate of change of temperature is so slow that the subjective effect is that the house is at a very constant temperature all day.  This isn’t surprising, as the daily room temperature variation rarely exceeds 1 deg C in winter and is often only around ¼ of that, unless the weather is very cold.  Our old house, even with double glazing, good loft insulation and cavity wall insulation starts to cool around half an hour after the heating goes off, and loses around 1 deg C every couple of hours if the heating is left off during the day in winter.  We often see a 4 to 5 deg room temperature variation in winter in the old house, and it’s very noticeable that the new house just feels far more comfortable even if the heating hasn’t been on for two or three mornings.

So, the moral of this tale is to forget about complex heating controls in the low heat loss house.  Forget about zoning, as all the rooms will end up being close to the same temperature, forget about programmers and clever thermostats, as all that is needed is a low temperature heat source into the floor plus a simple high resolution room thermostat situated near the centre of the house.

Had I know this at the start I’d have been amazed.  In fact, quite frankly I’d never have believed it.  I’ve wasted hundreds of hours trying to solve a problem that turned out to be imaginary!

I’ve just realised that this is all getting a bit too long, and that there are other things I’ve wasted time over-thinking that I’ll need to put in other posts.  To finish, these are the diagrams of the floor  heating and cooling system.  This excludes the Genvex active MVHR, which is separate and only ever used occasionally for summer cooling now; it’s never used for heating in practice, and has it’s thermostat set permanently to 19.5 deg C, and if the floor heating is working then the house never ever gets this cool, so it’s really just a backstop in the event of failure of the main system:

The above diagram shows the ground floor part of the system.  The thermostatic valve on the UFH manifold is normally set to around 24 deg C, which is right near the very bottom of its control range.  If the weather turns extremely cold (below about -5 deg C or so) then there is a very slight benefit in turning this temperature up by a degree or two, but also a risk that by doing so there will be a temperature overshoot.  The system is very sensitive to small changes in flow temperature, and if there was one thing I might, possibly, change it would be to add a very limited weather compensation capability to the flow thermostat.  I’m not sufficiently convinced yet that it’s worth the effort, for the handful of days in a year when it might, possibly, give a small benefit.  It’s worth noting that a lot of thermostatic mixer vales won’t control down to 23 to 24 deg C or so.  Ours is a Wunda manifold, and the mixer valve uses a sensor set into a pocket in the flow manifold, and as far as I know is one of the few that will control well down at such low flow temperatures.

For those that have doubts about the effectiveness of such a simple control system, here is some temperature data from the house logger.  The logger collects data every 6 minutes and measures and records lots of different parameters, including room Co2 and room RH, but this plot just shows the room temperature (red) at around 1.2m above floor level in the living room and outside air temperature (blue) measured at the sheltered North face of the house.  For the first few days of this plot I had the room thermostat set to 21 deg C, which turned out to be a bit warm, so I turned it down to 20.5 deg C, a change that can be seen around 11th November:

From the above, it’s clear that the outside temperature varies very wildly, whilst the room temperature is relatively stable, not changing by more than about 1 deg C.  Most home thermostats can’t regulate to 1 deg C, the best they normally manage is around 2 deg C or so, and, as mentioned before, our old house changes temperature inside by several degrees per day, with the thermostat set to 21 deg C and the gas boiler running for a fair part of each day to try and keep the house around 21 deg C.

The most important single aspect of this whole under floor system, and the one that has the most dramatic impact on the evenness of the house temperature, is the fact that the circulating pump is running all day long, every day (but turned off at night) whether heating is needed or not.  This circulates water around the whole floor slab and evens out the temperature, taking heat from the warmer areas and using it to heat to cooler areas.  I can’t underestimate how important this is – it really does make a massive difference to the comfort level, all at the cost of less than 20 watts to run the low energy Grundfos pump on the under floor manifold all day.


The diagram above is mainly concerned with the DHW system, which uses pre-heated water from the ground floor buffer tank.  I’ve included it here for completeness, and as it shows the expansion vessels and filling loops for the under floor piping and ASHP (filled with antifreeze mixture) and the buffer tank filling loop (filled with normal inhibitor).  The buffer tank is run at a very low pressure, 0.3 bar cold and 0.5 bar warm, with a 1.5 bar pressure release valve.  This is because the buffer tank isn’t a pressure vessel, and is designed for a working pressure of not more than about 1 bar.  This arrangement was used to avoid having a header tank and overflow – it means the system can be sealed, a significant advantage.

35 thoughts on “Part Forty Four – Over-thinking things – Part One”

  1. That’s interesting because I have approached the house build from the opposite direction. I just hope I haven’t over simplified the heating and DHW aspects of the build. I have accepted the output from PHPP which is 11kWh/m²a for specific space heating demand and a heating load of 9W/m². Interestingly SAP shows an energy use of 8kWh/m²a but I have not used that figure. The PHPP indicates this demand can be met by a Genvex compact unit for space heating and DHW, so for space saving reasons and simplicity this is what we will use. We have 250W of heating in the form of towel rails in the three bathrooms and electric UFH in the kitchen. Any extra heating required on especially cold days will be met by a fan heater. It will be interesting to see whether I have over simplified things, only time will tell. We did also suffer from overheating in the sitting room which has a large ¾ glazed gable end. The overheating was solved using an external film on the glazing.

  2. Peter, I don’t think you’ve over-simplified things at all, what you describe will, I’m sure work well.

    I’m finding that the house is very tolerant to small changes in heat input, around 100 to 200W, with no discernible effect on room temperature. I work over there on my own every day during the week, but that doesn’t show on the room temperature measurements, so I’m assuming that the ventilation rate, plus the ability of the house to absorb a small amount of extra heat, is enough to damp out any temperature change to a level below that which I can reasonably measure (or it just gets lost in the noise of the daily small temperature variations plus the 0.1 deg C resolution of the data logger sensors).

    However, there’s a threshold of around 300 to 500W additional heat input at which the room temperature starts to increase, sometimes quite rapidly. It’s noticeable on the days when we have a few people around, especially when the weather is mild.

    I’ve found the external film very effective at reducing solar gain. It wasn’t installed until relatively late in the year, but there was an immediate decrease in the frequency and time that the cooling systems ran. The only downside has been that it’s made the hall slightly darker, but not to a significant extent. The privacy the “stainless steel” reflective finish gives is worth the small light loss, in our view. For the East facing bedroom we used the very pale tinted film, that has a pretty low light loss, as there were no privacy issues there, and you can’t really tell the film is there, apart from the large reduction in solar gain.

    1. Clearly, having a pump running 365 x (24-8) = 5840 hours a year is a significant consumer of energy. Grundfos offer a large number of pumps for the domestic market (as do Wilo and others). Can you say which model you chose? (and whether there were any key factors? do you circulate to the 1st floor?)

      Also, as you point out, the hysteresis of your thermostat is 0.1 deg C . Would you mind saying where you sourced that?

      Finally, you deserve a two week winter holiday 🙂 Will you turn the system off when you leave? If you do, two or three days before you return, will you have the ability to turn it on (remotely)? If you cant, and its still winter, what temperature do you expect to find the house at?

      1. Hi Ian,

        The pump only runs during the day; it’s on a time switch, so comes on at around 6 am and goes off at around 9pm, so runs for about 15 hours a day. It’s rated power on the setting I’m using is 20W, but when I measured it I found that it was really only drawing 14W, probably because the head is almost non-existent with the three pipe loops paralleled. So, over the course of a year, the pump uses 365 x 15 x 14 = 76.65 kWh, but, even in winter, at least half of that energy would come from the PV system during the day, so the reality is that it probably costs around £5 a year to run. Even if it cost double that a year I’d reckon it to be good value, because the impact of evening out the heat in the slab is tremendous, as effective much of the time as running the heating. The UFH pipes are only on the ground floor, and we’ve found that there’s no need for any heating upstairs at all, the bedrooms are usually around 1/2 to 1 degree cooler than downstairs, which seems fine. The towel rails (on timers) in the bathrooms tend to make them a bit too warm, so the timer settings need to be adjusted so they don’t come on for too long.

        The pump I have is the Grundfos UPS2, set to run at it’s lowest speed (it has three speed settings). Wilo do a very similar pump, in fact the DHW preheat pump I have upstairs is the equivalent Wilo model, but that runs on speed 2, as I want to make sure the plate heat exchanger has plenty of flow to circulate the water from the buffer tank through it as fast as possible.

        The thermostats are Computherm Q3RF units, there’s some more details in this blog entry: I can’t recall exactly where I bought them, but have a feeling it was from Poland. I see there’s a seller on Ebay selling them for around £37 at the moment. I think we paid around £35, but that was over a year ago. You may find a better price if you hunt around. As supplied they have 0.3 deg C hysteresis, but there’s a link on the back to switch them to 0.1 deg hysteresis.

        I leave all the systems on all the time, heating and hot water, as the cost is so tiny, that it isn’t worth the bother of turning them off and then having to wait a day or two for the house temperature to stabilise. The house makes us a lot more money than it costs to run; so far we’ve generated around 17,000 kWh and used about 6,000 kWh in total, and the PV wasn’t fitted until around 6 months after the electricity supply went live. Overall I think we pay around £300 to £400 a year in electricity charges and receive around £900 to £1000 a year back for the electricity we generate! The only things I turn off when not there is the boiling water tap (which is on a time switch as well) and the wifi. Everything else just stays on.

        1. “The pump only runs during the day; it’s on a time switch”

          “The heating system comes on about once every two to four days, for around an hour, and usually pumps water at about 23 to 24 deg C around the floor slab”

          With apologies to President Clinton, “It depends upon what the meaning of the word ‘on’ is LOL.

          I think from the diagram when your heating thermostat calls for heating, the ASHP is enabled (so as to provide heat) and the electrically operated thermal shutoff valve is opened to permit heated water from the ASHP to pass into the UFH circuit.

          Is that correct? (I’ve read through your different posts and can’t quite see the detail here?).

          Conversely when the heating thermostat cries “enough” the ASHP shuts down and the electrically operated thermal shutoff valve is closed.

          Meanwhile, the UFH UPS2 just keeps on circulating…

          (Btw I am trying to extrapolate here to spec a control system where all heat energy – couple of hours a day I hope – is supplied by my LPG combi boiler. I think my use case is quite simple: thermostat calls for heat – boiler Heating ON & boiler Circulator ON and electrically operated thermal shutoff valve OPEN; thermostat says “enough” and boiler Heating OFF & boiler Circulator OFF and electrically operated thermal shutoff valve CLOSES. My UFH UPS2 keeps on circulating also. And of course the DHW component of the boiler is operational during the day to fire up when a tap opens. Unlike your profit centre, my setup is clearly going to be a cost centre but am hoping them costs aint gonna be too high.)

          1. Hi Ian,

            Yes, you’re right, when the time switch is “on” technically the whole heating/cooling system is on, but 90% of the time there is no heating or cooling going on, because none of the thermostats is calling for heat. During this time, the manifold is isolated from the ASHP by the thermo-actuated valve on the return, and the buffer tank is isolated by its electrically operated ball valve, so the manifold pump is just recirculating water around the floor slab.

            It’s because water has such a high heat capacity (about 5 times greater than concrete) and because it’s a slightly better thermal conductor than concrete that it’s so effective at moving heat around the slab.

            You’re spot on about the heating thermostat, it does exactly as you describe. There is also a buffer tank thermostat, that can turn the ASHP on and direct flow to that tank only, for DHW pre-heat, plus, to reduce the chance of short cycling the ASHP when there is only a very small demand for heat, the buffer tank valve is also opened when the floor heating comes on, so the ASHP has somewhere to dump heat, really. When the ASHP is in heating mode the buffer tank can get slightly above the tank thermostat set point, as it will ultimately heat to the ASHP flow temperature, which is a few degrees warmer than the buffer tank thermostat setting. In heating mode the buffer tank thermostat is ignored, which has no ill effects, it just means there might be a bit more preheat available for the hot water system.

  3. Great Post Jeremy reassuring in many ways but pointing up the old ‘if you cannot measure it you cannot control it’ adage very well on the sensitivity of the stat and the UFH mixer valve. Can I ask how you determined that the air heating dried the air more than desired, was it by measuring the RH or just your lungs as instruments?

    I also have to say that although you now regret the work you did in ‘thinking things through’ you would be kicking yourself now if the simple system did not work as you would want and then have to do it anyway AND those of us who watch your work with interest would not have learned all we have about how to do about where the dragons live.

    1. Hi Mike,

      Pretty much everything is a consequence of measurements, rather than just feel. I put temperature sensors in all over the place and hooked them up initially to the heating/cooling controller, but now they just go to a data logger. The same data logger records CO2 concentration and relative humidity and records everything to an SD card, as a .csv text file that any spreadsheet programme can import. So, the outcome is really the result of having measured lots of things, trying lots of control strategies, then realising that nothing complex was needed at all, and a simple room thermostat did the job better than my “clever” weather-compensating, floor, outdoor, room and flow temperature monitoring, system could!

      With hind sight I’ve learned a lot, and pretty much all of it is directly applicable to a house with a similar long duration thermal time constant, no matter what it’s made from. The key is that houses that have a long thermal time constant pretty much look after themselves in terms of keeping the temperature of the surfaces stable, as long as things like excessive solar gain are addressed and they are insulated with a suitably high decrement delay material (or have the equivalent by combining materials to get an overall long decrement delay time).

      Dry air is a problem in winter, when in cold weather the RH drops a lot outside and the MVHR doesn’t help. Add in air heating via an active MVHR and it tends to deliver air with an RH down around 20%, which quickly drops the house down to the uncomfortable zone, below about 30 to 35%. We had exactly this problem with the last place I worked. To save running costs (because of the maintenance and disinfection cost) the humidifier modules were left out of the air handling system. The result was that the RH dropped below 30% in the building in winter and people started getting asthma attacks, skin rashes etc. Retrofitting the humidifier modules cost more than it would if they’d been fitted with the build (they’d been specified by the designers, it was our idiot “works and bricks” manager who objected, because his budget would have to cover the maintenance!).

      The active MVHR lowers the humidity even more in cooling mode, because it extracts water from the delivered air, but as the outside RH is usually moderately high when cooling is needed this isn’t a problem, or at least, not a problem I’ve seen.

      It makes me chuckle reading back through early posts on Ebuild, as I was really focussed on the heating system. As it turns out, heating is a doddle to manage, as there is so little of it needed. Getting a good hot water system takes a great deal more thought.

  4. “three pipe loops paralleled”
    I suspect I dont know what a domestic circulator is capable of …

    With specific reference to the first diagram.

    There is one circulating pump shown (Grundfos UPS2). With electrically controlled thermal shutoff valve closed, UPS2 circulates water around UFH Loop 1 only.

    With electrically controlled thermal shutoff valve open, UPS2 circulates water around UFH Loop 1 and around ASHP Loop 2 – water leaves UPS2, passes around the UFH loop, then splits, some travelling to the ASHP loop and some back around the UFH loop. Some heated water from the ASHP Loop 2 will then pass through the Thermostatic mixer valve.

    Is that correct?

    But where’s the 3rd loop? If its (I guess) the Buffer tank, how is water actually circulated to that if the electrically controlled thermal shutoff valve is closed (no heating required?)

    1. Hi Ian,

      There are three loops of UFH pipe set in the floor, really because the stuff comes in 100m rolls and there’s about 300m of pipe in the floor, so it needed three loops. They are all connected in parallel at the manifold. Often separate loops are used to zone heating, to allow different amounts of heat to be emitted into each zone, by use of the combination of flow control valves (which adjust the flow rate in each loop, on the manifold) and electrically operated thermal actuators fitted to the return manifold to turn zones on and off individually. With a low temperature system like this, with a near-constant temperature house, there’s no benefit in trying to zone heating, as the MVHR will tend to try and even the temperature out over the course of a few tens of hours anyway.

      In our case, I’ve got manual zone valves that are all wide open and the flow control valves are adjusted to give an equal flow rate in each of the three loops (there are slight differences in flow resistance between each, depending on the number of bends). I’ve drawn the floor loop as just one loop of pipe for simplicity, but in reality imagine that loop is really three, 100m loops of pipe, all in parallel. The fact that they are all fully open and connected in parallel is my reason for thinking that the head that the circulating pump is working against is pretty low. This is borne out by the fact that the flow rate doesn’t increase (on the manifold flow meters) if I turn the pump speed up. At it’s lowest speed it’s already flowing as much water around the pipe loops as it can.

      So, addressing your points in order:

      – With the electrically controlled thermal shutoff valve closed, UPS2 circulates water around UFH Loops 1 , 2 and 3, as they are all in parallel.

      – With the electrically controlled thermal shutoff valve open, UPS2 circulates water around UFH Loops 1, 2 and 3, with warm water from the ASHP being allowed into the manifold via the thermostatic mixer valve. The opening of the electrically controlled thermal valve allows the UFH manifold to be connected to the ASHP circuit. The flow is primarily just around the pipes, with a small amount of warm water coming in via the mixer, just enough to maintain the set UFH flow temperature, and slightly cooler water exiting via the thermal valve to the ASHP return. In this case the buffer tank valve will also be open and the heat exchange coil in the buffer tank will be in parallel with the manifold. There is a pump inside the ASHP (only on when the ASHP is turned on for heat or cooling) that circulates warm water around the ASHP circuit.

      The third case is when there is no call for floor heating or cooling, and the buffer tank thermostat calls for heat. Under that condition the manifold thermal valve stays closed, keeping the floor circuit isolated by closing the return to the ASHP circuit, and the buffer tank valve opens, allowing the ASHP to only heat the buffer tank for DHW preheat.

      Hope this makes things a bit clearer.

      1. “The third case is when there is no call for floor heating or cooling, and the buffer tank thermostat calls for heat. Under that condition the manifold thermal valve stays closed, keeping the floor circuit isolated by closing the return to the ASHP circuit, and the buffer tank valve opens, allowing the ASHP to only heat the buffer tank for DHW preheat.”

        so under those circumstances, its the small circulator in the ASHP which circulates the hot water?

        (And in the second case, you have a situation where the ASHP circulator is operating in series with the UPS2 – which I assume causes no issues and probably requires no comment LOL)

        1. Hi Ian,

          Yes, it’s the pump inside the ASHP that circulates water around and provides a head of pressure at the flow side, whenever the ASHP has been turned on by a call for heat (or cooling) by a thermostat.

          This pump provides a pressure against the inlet of the thermostatic mixer valve, with warmer water available that the valve can draw off into the manifold, as required. Typically the ASHP flow will be around 40 deg C (any warmer than this and it tends to run very much less efficiently) and the flow manifold will be around 23 to 24 deg C when the heating is on, so only a small amount of “fresh” warm water is drawn in through the mixer to maintain the flow manifold at the set temperature. “Excess” water is drawn back to the ASHP return via the open thermal valve. Because this “short circuits” the ASHP (the thermostatic mixer valve is almost closed, because of the low floor pipe flow temperature setting), the buffer tank valve is also opened, to give the ASHP a path in parallel with the floor system and reduce the pressure in that circuit. As an additional safety measure, and also to deal with the floor cooling case, where the buffer tank valve is closed and the thermally actuated valve is slow to open (several minutes), there is a standard adjustable pressure bypass valve fitted across the ASHP flow and return, just as you would have on a normal heating system circuit where all the radiators had thermostatic valves. This bypass valve opens if the pressure in the ASHP flow pipe gets too high (from the ASHP pump) and diverts the flow directly to the return. The ASHP can then sense the lower differential temperature between flow and return and modulate down, or even turn off, as required.

          The best way to think of it is that when the ASHP is commanded on by a thermostat, it provides a positive pressure at the flow pipe and a negative pressure at the return pipe. That pressure is used to push a small amount of warmer water through the thermostatic mixer, as required. When in cooling mode the thermostatic mixer opens fully, as it’s trying to get the flow manifold up to around 23 to 24 deg C, but is being fed with flow water at 12 deg C, so all it can do is open right up and allow pretty much all the ASHP flow to pass through. The floor is still wholly driven by the UPS2 pump though, as with the thermostatic mixer valve open there is a path for the ASHP flow straight across the return pipe manifold to the return thermal valve and hence back to the ASHP return. The UPS2 “sucks” some water from immediately after the thermostatic valve and pushes it around the floor pipe loop.

  5. “Dry air is a problem in winter, when in cold weather the RH drops a lot outside”

    Not true if you live near the coast! The temperature can be low but the RH high, although it doesn’t feel humid. I live just under 5 miles from the sea and the RH here is usually between 70% and 100% during the winter. Today RH is 86% here and 88% at Manston (Met station) which is just under 3 miles from the sea.

    1. Hi Peter,

      During the cold spell a week or so ago, the RH dropped to around 40% inside the house, which is about as low as I’ve seen it. The temperature was around -5 deg C outside, though, so I think most of the available water vapour in the air had condensed out.

      One potential problem with MVHR is that it ventilates a great deal more effectively than natural ventilation, and the extracts are all from rooms that would, if left to themselves, increase the humidity in the house and all the fresh air coming in will be quite dry in cold weather if you live somewhere like we do, on the edge of Salisbury Plain. As a consequence the indoor and outdoor humidity tend to track more closely than they do in our current house (with natural ventilation) where activities like showering, cooking etc all tend to increase the RH inside the house to some extent, as there’s nothing driving fresh air in and the extractors are far from being 100% effective.

    2. I’m finding our house get very dry when the weather out side is dry and cold, we are over in the east and it may be 80% RH out side and down as low as 30% in the house. I find we have to run a humidifier to get moisture back in the air or we start giving each other electric shocks and get dry skin.

      But this happened in our old solid walled leaky house as well, we would light the wood burner and have to turn the humidifier on as well, but definitely more noticeable in the new house

      1. Welcome!

        Since writing the comment above, and after having left our new house empty for a week during a cold spell, I set a new record for RH of 19%. This is very uncomfortably dry, but I’ve noticed that it increased a fair bit with just the two of us in the house, moving new furniture in. An hour later and it was up to around 30%, still low, but an indication of the way the MVHR, combined with cold weather, a low external RH and no moisture sources in the house, caused the air to become very dry.

        I was getting a lot of static shocks too – when I took my fleece off in the house I’d get a belt when touching something that was earthed.

  6. Looking at that photo from the low angle, Jeremy, the question that occurs to me is whether you have plans for that retaining wall.

    It looks like a classic for something that will let birds nest, perhaps a climber or semi-climber with year round interest.

    Dare I suggest mixed pryacantha to give you several colours of berry, and which happily leans on walls rather than grows up them?

    Or being south facing where you are could you grow something edible (Himalyan Giant blackberry?)?

    1. My wife has plans to grow things up that wall!

      It gets very warm, as it’s well-sheltered, so gets very little breeze, and it acts a bit like a giant storage heater, rather like the walled Victorian Kitchen gardens in some respects. The wall is rendered double width 215mm hollow concrete blocks, laid in Flemish bond, with steels in the holes, which were in-filled with concrete. Overall it’s around 450mm thick (including the render), just over 3.5m tall and around 35m long, although only about 12m or so of it is behind the garden, the rest is tucked away behind the house and garage.

      One problem is that it has a concrete foundation around 400mm deep that projects forwards into the garden by around 1.5m, and is only a couple of hundred mm below ground level. This means there’s very little soil, so I built two raised beds with some left-over stone, about 1m out from the wall and about 0.6m deep and filled them with some decent manure in the bottom, plus loads of good topsoil. Each is around 3m long, with a gap in the centre where my wife intends putting a covered seat, with things growing over it.

      Raised beds

  7. The other argument might be a long thin greenhouse and make your own wine :-).

    Or a fig tree would be a beauty there. We had one in a similar spot in Derbyshire (foundation of Victorian Greenhouses against the back of a stable block) and it was excellent.

    1. A vine or two has been mentioned, and I suspect would be fine along that wall without a greenhouse. A peach tree grown against it is another on the list, I think. We had a fig tree growing against the wall of our last house, and IIRC it had roots that went down a long way, I’m not sure how one would cope with just a relatively shallow depth of soil.

      Whatever ends up growing up against that wall will probably have to be a mix of things that look nice as well as things that are edible. It it were me then I’d just cover the whole of the wall with edible stuff, but I don’t think that would go down well!

  8. Incidentally, Jeremy, did you follow the House of the Year?

    The winning house in Edinburgh used a 150m deep borehole for a ground source heat pump. I wondered if you had any comments on that.

    >The main heating source for the house is a 150 metres deep ground source borehole connecting to a heat exchanger which feeds >underfloor heating.

    Would that qualify as geothermal ? !

    1. I didn’t follow the “House of the Year” thing, for no other reason than I rarely seem to sit and watch TV now!

      I looked at using a borehole for running a GSHP, but it wasn’t worth it for the tiny amount of heat we need. A cheap ASHP was around 1/10th the price and only about 10% poorer in terms of overall performance, so the GSHP investment would never have paid for itself. There was also the added cost of maintenance – GSHP antifreeze is very expensive and needs replacing every few years.

      It’s not geothermal at that depth, it’s really just another form of solar energy. Rain and surface water is heated by the sun and percolates down to aquifers, taking the heat with it. Our water comes up at a pretty constant, all year around, 8 to 9 deg C, and that’s from an aquifer that’s about 45m down.

      I went down a deep tin mine in Cornwall years ago (Wheal Jane) and at 1400ft down it was around 30 deg C, but most of that heat was from radioactive decay of the uranium, radium etc in the granite, rather than true geothermal. I think you need to get down a a km or two before true geothermal energy (that from the molten core of the Earth) starts to have a significant impact here in the UK. There are exceptions to that in areas where the crust is thin or faulted, and the molten magma is closer to the surface, but I don’t think there are many places like that in the UK (off the top of my head I can’t think of any, but have a feeling there are some hot springs somewhere that are probably geothermal).

    1. Hi Jeremy:

      Hopefully, one last little question. (I tried to insert a screenshot but that didn’t work, so Im going to have to use words).

      Looking at the detail of Fig.1 (so to speak). In Heating mode, Water @ say 40 deg C arrives at the input of the Thermostatic mixer valve.

      The valve diverts a small amount at 23 deg C to the manifold, from where it flows around the UFH loops.

      The valve diverts the remaining water straight on (so to speak), towards the electrically controlled thermal shut off valve.

      But (and here is my question) between mixer valve and shut off valve it meets the return from the UFH! So there appears to be a short stretch of pipe where the water is flowing in two directions at once?

      I am just trying to understand the detail here so of the plumbing here so I can draw up a layout for me.

      1. Hi Ian,

        The thermostatic valve has no bypass to send the flow water anywhere else, it’s just like a normal radiator thermostatic valve with a remote sensor that’s inserted in the flow manifold (the top one). So, it just blocks the hot water it doesn’t need, and only allows through the tiny bit needed to get the manifold to the right temperature.

        This is why the buffer tank valve opens at the same time as a call for heat, as that gives a parallel path that gives the water somewhere to go. There is a little bit of flow from the return side of the lower manifold to the ASHP return, but very little in heating mode.

        It’s also why the pressure relief bypass is fitted, as that allows water from the ASHP flow to “short circuit” to the return if the pressure gets too great.

        In practice, the pressure relief bypass only tends to operate in cooling mode, when the buffer tank valve is closed and the manifold return valve is slowly opening. By opening, this relief valve prevents the ASHP shutting down from an over-pressure event (the same applies to some boilers).

        1. Aaah! I had wrongly assumed a three way diverter valve!

          So I think I understand the core principles now. (And I understand this is your house blog so I am trying to stay focussed on that – please feel free to edit extraneous ramblings).

          If I were to pare away all the DHW elements, could the system be described like this?
          – there is an ASHP (Boiler heating) circuit individually pumped/circulated
          – there is a separate UFH circuit, individually pumped/circulated (the UFH circulator runs continuously during the day as a modulator)
          – the two circuits are connected by a flow pipe and a return pipe
          – a two way thermostatic valve in that flow pipe set to (say) 24 deg C will open (slightly) when the temperature in the UFH circuit is below 24 deg C. Its function is to control the maximum temperature of the slab.
          – a two way motorised valve *in that return pipe* essentially permits water from the ASHP circuit to enter (and leave) the UFH circuit
          – when heating is called for (by a wall thermostat for example) the two way motorised valve will open *and* the ASHP circulator will run. The ASHP circuit will quickly get up to 40 deg C and warmed water will (dribble) into the UFH circuit, causing the same quantity of water to (dribble) back past the open two way motorised valve
          – when no further heating is required (thermostat reaches its setpoint) the two way motorised valve closes, so no ASHP heated water can flow into the UFH circuit. The two way thermostatic valve might be wide open yelling for heat but no water will flow past it because the two way motorised valve is closed
          – when cooling is called for, pretty much the same thing happens except that the ASHP operates in chilling mode and the two way thermostatic valve is permanently open yelling for heat (ie performs no function)

          1. Hi Ian,

            One of the reasons I chose this manifold was because I knew that the type of thermostatic valve fitted could be adjusted down to about 22 deg C. Here’s a photo that shows it:

            The hot water input is at the bottom, the outlet to the pump inlet is on the right, but there is also a path directly across the return manifold as well. It relies on the pump sucking immediately after the valve to direct water up to the flow manifold at the top. What isn’t clear in that photo is that there is a capillary sense pipe coming out of the back of the valve, connected to a long bulb set into a pocket in the top manifold. This means that the valve is controlled solely by the temperature in the flow manifold. One snag is that Wunda no longer seem to sell this type of manifold valve, which is a great shame, as it’s ideal for a low temperature set up. I think that it should be straightforward to adapt a big (3/4″) remote thermostatic radiator valve though, as that’s all this really is.

            Your description is pretty much spot on!

            Sorry about the images thing. It may be possible to manually insert links to images hosted on an image site using image tags, as that’s all the hosting software I’m running (WordPress) seems to do,

  9. Hi Dan,
    I’m working on a way to add a menu bar with stuff that can be downloaded. I think I can do this relatively easily, but may well have to use the work-around of uploading the file with a “safe” extension, like .txt, and asking people to just change it after download to .xls


    I’ve uploaded it and added a new small “downloads” menu at the top right. The Excel version of the master spreadsheet is now there ready to download. If you have any problems, please let me know.


  10. Glad it worked OK – I did try it in Libreoffice as a download from the other house, as a check that I could pull the proper file from the server (always a challenge to check things when the machine you’re uploading from is the same you’re trying to test a live site with!)

    The menus will probably change a bit over the next few weeks, as I try and organise things a bit more coherently. I am not, by any stretch of anyone’s imagination, good at this web stuff. I have a test machine (a Raspberry Pi 3) set up with an exact duplicate of this site (a LAMP stack with WordPress and the same plugins) and so can sort of play around to see if something works or not, but it always good to have confirmation from someone else when it does.


  11. Jeremy,

    We have now got to the point in second fix where we need to finalise the design of our water and heating solution. You and I have bounced ideas between us and usually converged on a set of common conclusions. My latest thoughts were to have an even more stripped down system than you last documented, and on reading this I find that we are again converging. Some key variations are:

    – PV. We don’t have this because of Planner objections, so we are using Green tariff E7 only.
    – ASHP. Same potential planning issues so I will defer any ASHP for at least a year.
    – Buffer Tank. I am using a 10 tonne concrete one under our feet, aka the slab.
    – Single measurement point. Yes, though in our case not the room stat, but the temperature of the water return from the slab UFH loops.
    – SunAmp. Again yes, but a duplex in parallel, because I want to support higher flow rates.
    – Flat Plate Exchanger for SunAmp. Again yes, but coupled to the slab UFH circuit. This also preheats *both* DCW paths for the DHW: through the SunAmp and to the TMV after the SunAmp.

    This last one merits further explanation. Even without a 35°C buffer, I still think that there is a net benefit in preheating the DCW feed into the DHW to slab temperature as opposed to the ~4-6°C feed from the nearby reservoir (in the winter). For a target mix temperature of 45°C, this drops the delta contribution from the SunAmp from 40°C to 25°C which roughly increases the effective capacity of the SunAmps by 60% or so. In the winter all this heat is being supplied by E7 off-peak tariff electricity anyway. In the summer this cools down the slab slightly which isn’t a bad side-effect.

    My heating strategy is simple: to dump a “block” of heat into the slab in the off-peak period by a 4kW inline heater in the UFH loop, timed to complete just before end of the period. The heating time is based on 2-day smoothed external temperature, and the measured slab temperature in the run up to the heating period. I also have the option to do a mid-day top-up at peak rate if the slab temperature falls below a preset threshold, but as you found, it may be easier to accept the ~1°C daily sawtooth.

    Anyway, I’ll post the details to BuildHub and would welcome any comments, here or there 🙂

  12. Terry, I’m glad our thoughts are converging! Some comments, really from our perspective, and so they also reflect the views of our planners (remembering we’re in an AONB, next to a Grade II listed building, etc).

    First off, I accidentally “cheated” with the planning application. At the time I submitted it we were going for a GSHP, so that’s what’s in the D&A. It didn’t even cross my mind that I should have asked the planners about the ASHP, but it did get noticed later, after I’d installed it, and the view was that it was such a trivial change that it wasn’t even worth noting. Our planners signed it all off when we got our final conditions cleared, with the ASHP installed and clearly documented, without any fuss at all.

    The reason I mention this is that the slab is a good heat store, but a lousy buffer for the ASHP. It takes so little heat energy, that even with our small ASHP modulated right down to its lowest output (which is around 400W input power, measured), I can’t get a discernible temperature difference between the flow and return. The recommendation is for a delta T of around 5 deg C, but I can’t get even 1 deg C delta T at the low flow temperature I have to use to keep things stable.

    The low flow temperature is the other key issue. From practical experiment, I’ve found that small changes in flow temperature cause a big swing in slab temperature, one that I found difficult to control. By the time the slab temperature sensors had responded, the water in the UFH was already too hot. When the heat input was then turned down, or off, the slab temperature would continue to rise for some time, and the house temperature is very sensitive to small changes in slab temperature, I found.

    I must have spent a couple of hundred hours or more in trying to perfect a slab temperature control system, and in the end it was highly complex, using PID control of the UFH heat input (by PWM’ing the UFH thermal actuator) and using the outside temperature rate of change to try and predict how much heat needed to be put into the slab in order to maintain a steady temperature.

    In the end, thoughts of my mortality caused me to have the major re-think I had. I was sitting writing the umpteenth iteration of controller code, when the thought struck me that when I’ve shuffled off, very few people would be able to fix the system, or even understand how it works. If I’m honest, I’ve changed loads of things because of future maintenance considerations. For the same reason, I’ve also avoided anything that’s likely to have a short supported lifespan. People who opt for sophisticated, smart phone app controlled, house systems worry me, as I doubt many of those systems will be supported in 5 years time.

    Getting back to the slab, I can see a potential flaw in your strategy of “dumping a block of heat into the slab”, because of the extreme sensitivity I’ve noticed between slab temperature and house temperature. For example, in the very cold spell in December, the house was sluggish to warm up when the heating came on at around 08:00. The flow temp into the slab was the usual 23 to 24 deg C, so I manually turned it up to about 28 deg C, and went off to do some work. Around 11:00, I felt a bit warm, noticed that the heating was off and the ASHP flow temperature had dropped to ambient, a good indication that the ASHP had been off for over an hour. The room temperature was around 22 deg C, and the slab temperature was about 22.5 deg C and rising. It carried on rising for the next couple of hours, until by around 14:00 the room temperature was over 23 deg C, and getting a bit too warm. Sadly I didn’t have the new version of the data logger working then (it proved a spur to me to get it modded and re-fitted) and so I don’t have recordings to prove what happens.

    The reason for this is that the original data logger was integrated with the temperature control system – I used some of the same sensors for both. When I took out the home made controls and fitted the simpler room and tank thermostat system I didn’t put back all the data logging I had originally. It’s now all back and has been running for a week or so, so as soon as we get some decent outside temperature changes I should be able to see pretty much exactly what happens. I’ll write a blog post on what I’m now doing in terms of data logging, just because I’ve found that it has been absolutely invaluable in working out how best to control things. I will admit to being more than a little upset that the control strategy I thought would work well turned out not to!

    Getting back to your ideas:

    Yes, there is very definitely a benefit in pre-heating the cold water going in to the Sunamp, we’ve found. Yes, having two Sunamps would be a significant advantage, not because of any flow rate concerns (just one can dump over 30kw into the water flowing through it, more than a lot of combi boilers) but because having 10kWh of stored thermal energy, with such low losses, gives a much better buffer when it comes to occasional high DHW demands. I plan to add two more thermal batteries to our Sunamp to get ~10 kWh of storage, for just this reason. Bear in mind that even a 10 or 15 deg C preheat means that the Sunamp flow rate can increase a lot and still maintain a high DHW temperature.

    So, in essence I agree with most of what you suggest, with the exception of the points above. The slab temperature thing is not simple to understand, but I “think” I know what might be happening. I only have five slab temperature sensors, and rather stupidly (with the benefit of hindsight) I placed them all at the same depth, and they are all at least 100mm horizontally from any UFH pipe. Because of the need to bring UFH pipes closer together through doorways, and because I couldn’t easily fit any sensors at these “pipe congestion points” , I know we get relatively small regions of the slab that get a degree or two warmer than the rest, when the flow temperature is increased. We have 6 doorways like this, with one, in particular, having around 80% of the UFH pipe coming through it. That’s the door between the kitchen and utility room and it seems no accident that those rooms are the ones that are always the hottest. The fact they are MVHR extract rooms doesn’t help, as they don’t have MVHR outside air coming in to them directly, but instead are fed with slightly warmer air from the rest of the house.

    I think that, if I were in your position, using E7 (and we were looking at doing the same at one point in the design process) then I would look at having something other than the slab as the primary heat store. A buffer tank is the obvious choice, as you only need to store a kWh or two, and with E7 you’re not temperature limited, so could shove the buffer up to 60 to 70 deg C in cold weather (effectively just use a weather compensated thermostat for the tank, best with rate of change sensing, I think). The Rolls Royce solution would be another single Sunamp dedicated to the heating. That would easily take care of the period between E7 charge periods and have very low losses, plus it would take up very little room. It’s output could be easily mixed down, with no worries at all about maintaining temperature differentials, as I have with the ASHP. The big snag is, of course, the price!

    Can I suggest that we have a chat on the phone in a week or two, when I’ve got a decent amount of data from the new logger? I think it’d be easier to understand how we’re both thinking with a chat, as I think there are some rather unintuitive things going on with the way our house responds to changes. I’d never have believed that a simple room thermostat could work so effectively, for example, if I hadn’t felt (and measured) things myself. BTW, in practice, the less than 1 deg C daily variations we get are unnoticeable. At our old house I’ve found that around a 2 deg C change is generally the point where one or other of us notices it’s got a bit chilly. I try and run our old house at 20 deg C, but in cold weather have to run it at 21 deg C, because otherwise it “feels cold”, because we have a lot of glazing, that although reasonably good (around 1.6 W/m².k) is no where near the thermal reflectance of the glazing at the new house, so your body senses the increased radiative loss a lot more, and the room temperature needs to be warmer to compensate. When the room temperature drops to about 19 deg C is when we both decide the time has come to press the heating button!

  13. Jeremy, Picking up your points.

    – UFH temperature drop down. Be wary of the 5°C guideline as this is really for a conventional high output UFH system. With 3 parallel of loops of PEX at a maximum recommended flow of 1½ m/s, this gives a flow rate of 0.9 l/s or 3.2 m³/hr. So a measured Δt of 1°C equates to 0.9×4200 W = 3.8kW. In my case 3kW inline heater case, the loop temperature will rise to whatever is needed for the slab to absorb the 3kW. It’s difficult modelling this precisely but I estimate that the return temp will start at slab ambient increasing by roughly 1°C / hr as the (initially) radial propagation from the pipes warms the slab in the proximity of the pipes. So I will need maybe 3-4hrs of heating during off-peak in the Jan low point. Of course dropping the UFH flow rate will bump this Δt proportionately (I will probably trim my rate down say to 1m/s or lower) and the time varying return temperature.

    In Jeff’s case he is pumping his slab direct from his ASHP for a fixed period at a set coolish (~25°C) preset temperature so here his dynamic is flipped and his ASHP will supply whatever his slab will absorb at that temperature.

    – Measuring slab temperature. For the reasons you discuss, I haven’t even bothered with in slab sensors. My slab temperature sensing mechanism is to measure the (input and) return temperatures on all three loops (three because it’s cheap to do so and this will help me balance zones flows) and use a time smoothed average return temp. This temp will rise during on-periods and only fall during off-periods reaching close to equilibrium after maybe 6 hrs. It will then track the general heat loss of the slab into the house environment (and ground losses, of course) so maybe drop another 1-1½°C over the next 18hrs.

    PID control isn’t effective in this sort of regime. I was thinking of a simpler daily iterative tracker scheme trimming the overnight dump to try to achieve the same end temperature at end off-peak+4 hrs (i.e. midday). So adding, say an extra 10 kWh overnight per °C off target at midday (remembering this variance will be typically nearer 0.1°C). This should track to a stable optimum within a few days. (If it proves to be hunting, then I could use a MLE with a target term based on the measured outside temp, but I suspect that this complexity is entirely unnecessary.)

    What I want to do is to start with a data-informed control policy and play around over the first year or two, then strip it back to the simplest possible control regime. (I also don’t want to drop Jan in it if I drop dead or start going gaga.)

    – System response to step (e.g 28°C) demands. What you describe is exactly what I would have anticipated, so I don’t see this as a potential flaw in my strategy. But live instrumentation will verify this or otherwise. (BTW, Jan has asked for a temperature control in the hall, and I’ve put my foot down on this one: “there’s no point in changing the set point temperature by more than about ¼-½°C per day, or you’ll just set the house temperature bouncing around for days”).

    – Number of SunAmps. Remember that we are a three person household and all three of us like to have the occasional bath, so the per day demand can be quite peaky. I decided 2×SunAmpPV in parallel would be better than 1+expansion. This was toss-a-coin stuff.

    I think that it’s a good thing that we have different views on the buffer tank decision. There’s no “correct” answer here. If I get my approach working stably then it will be one template solution, yours will be another. Both will have strengths and weaknesses that we can document for others to evaluate / use. Given the hard data, I will still have the option to add an ASHP and/or a buffer tank in future. (Andrew mentioned that they are considering releasing a smaller PCM box optimised for buffer tank use, and this might be available in these time-scales).

    Let me do the write up on the forum and I’ll email you the link when I’ve done so. I’d really like to have a chat, once you’ve had a read. Regards Terry

    1. The “5 deg C drop between flow and return” thing is a perfect example of the way that conventional heating engineer rules-of-thumb are wholly inapplicable to a low energy house! I’d not been as diligent as you in working out the heat input this gives, but as you’ve shown, it’s absolutely massive, way, way more than a low energy house needs. One issue I’ve had, that may well be relevant, is that the ubiquitous DS18B20 temperature sensors often have a temperature offset error. It’s small, and as far as I can tell pretty linear over the -5 deg C to +50 deg C range, but it can easily cause problems if you’re using two of them to measure Δt, for example. I’m now in the habit of calibrating them, against an ancient NPL standard mercury in glass thermometer I’ve had for years. I’ve not seen offset errors of more than about 0.4 deg C so far, but that’s enough to cause problems if you’re measuring UFH flow and return temperatures.

      I suppose I could have used better sensors, but these one-wire ones are so easy to use, and have a slightly better than 0.1 deg resolution, that they seemed a good choice.

      Your comments of control strategy match my experience, but not being a control engineer and working from 4o+ year old, half-remembered lectures on it from uni, meant that I spent a lot of time trying to understand the system response. Part of my problem was my scepticism regarding heating that dated back to the very start of the planning for our build, where part of me just could not believe the heating requirement numbers that were coming out of all the heat loss models! I was very much an applied scientist, and spent around 20 years of my career measuring things to try and find out why the stuff we were working with behaved as it did. It gave me an advantage in knowing how to measure stuff, but I wish I’d spent more time paying attention to control theory lectures.

      You’re absolutely spot-on to say there is no “correct” answer to these conundrums, every house will behave differently as will the occupants.

      Today was an interesting example of the high thermal capacity our house has. We had some big new furniture delivered, and it was around 6 deg C outside, and 21 deg C inside. During the delivery we had the French windows wide open, along with most of the internal doors. I started off thinking in the way I would at our old house, and going around shutting doors to try and reduce the heat loss, but gave up, as it was too much of a nuisance. The only thing I did do was turn the heating system off, as I didn’t want the momentary air temperature drop to cause it to kick in. When the delivery was completed, around 30 to 40 minutes later, I shut the outside doors. The house had dropped to around 15 deg C, but within ten minutes was back up to 20.7 deg C, without the heating coming on! The house has continued to warm up by about another 0.2 top 0.3 deg C since then, just from incidental heat gain plus the heat stored in the floor slab. It was an impressive example of how a pretty low mass house can still have a high heat capacity and a long thermal time constant!

      I look forward to reading your write up, no need for a link, I do still read the forum, albeit using a VPN so my IP probably doesn’t show in the logs……………… That’s not deliberate, I use a VPN most of the time now, really just because I don’t like the way our government (and that of other countries) wants to collect data on my web browsing habits. I’m not up to anything nefarious, but have a view that reading stuff on the web should be like reading a book or magazine, and I’d rather not have the government “looking over my shoulder”. If I’m feeling particularly awkward I’ll use TOR, simply to wind up the data collection bods, as I KNOW they focus on those that use TOR, and the more innocent TOR users there are the more of their time will be wasted.

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