Part Thirty Nine – Getting Into Hot Water – Episode One

This thirty ninth entry was published originally by JSHarris on the 22nd October 2015 and received 1,066 views on the closed forum


As some will already know, the hot water system has been a bit vexing, with significant over heating problems from the heat loss from the thermal store, severe enough to damage the veneer on one of the doors. The next couple of entries are going to focus on work to redesign the hot water system to reduce the heat losses, yet retain the ability to use excess generation capacity from the PV panels to heat our water and provide a backup system for periods when the solar panels aren’t generating.

After trying several ways to try and reduce the heat loss from our 260 litre thermal store, from reducing the temperature through to adding lots of additional insulation, I finally decided that the whole system needed to change. The catalyst for this decision was the introduction of a new phase change heat storage system to the market, a compact unit that is able to store excess electricity generated by a solar panel installation (or cheap rate E7 electricity) as thermal energy, called the Sunamp PV (see here: ).

The Sunamp PV is around the same size as a combi boiler, has very low standing losses, yet can store about the same usable thermal energy as our big thermal store when it was running at a low temperature to reduce losses. It works very like a 35 kW instant water heater once charged, in that it senses the flow from a hot water tap or shower being turned on and heats up mains pressure water flowing through it. The delivered hot water temperature is controlled via a thermostatic mixer valve, in exactly the same way as that from a thermal store. When the hot water demand stops, the Sunamp PV stops heating. It is designed as a unit to fit in series with the cold supply to a combi boiler, that will allow those people without a hot water tank, but with a PV system, to be able to take advantage of excess generation and store the energy, to offset gas or oil use for hot water.

Because it has low standing losses and accepts a warm water input, the Sunamp PV seems ideal for a low energy house, even though it was designed primarily for use with a combi boiler. In our case we’re going to use this unit in a rather unusual way, as we don’t have a boiler at all, but do have the ability to warm the cold incoming water from our ASHP output. ASHPs are great for efficiently delivering heat energy to warm water, but they have a problem when they are asked to deliver hot, rather than warm, water. It’s very much down to the physics involved in the way heat pumps operate and made worse by the UK climate, that tends to be wet and cool in winter. ASHPs can work efficiently when they are fed with dry air, even very cold dry air, but moist air can cause the external evaporator to ice up. When icing conditions are detected, ASHPs periodically defrost the external evaporator, usually by reversing the cycle and taking heat from inside the house, or the water system, and using it to warm up the evaporator coil to melt any ice. Doing this frequently wastes a lot of energy, and having done some practical experiments with our system I’ve found that if it is set to deliver a warm water output of higher than about 40 deg C then it spends a fair time defrosting in cool, damp, weather and this has a large impact on efficiency and electricity consumption. Our ASHP has a hot water mode, where it can deliver water at 55 deg C if demanded, but if this is used in cool, damp, weather it is disastrous, almost double the electricity consumption, because of the impact on efficiency. It’s little better than using an immersion heater, in practice.

Water at 40 deg C isn’t really usable as hot water for a shower or a bath, especially as this is the peak temperature in a hot water tank, and that temperature will drop as water is drawn off. We need hot water at between 42 and 45 deg C to deliver a shower at 38 deg C, we’ve found, any lower is really too cool for the shower thermostatic valve to work effectively. Any hotter is just wasteful, and going over about 50 deg C increases the risk of scalding. We already have a small (70 litre) buffer tank that is heated by the ASHP to around 35 to 40 deg C and I wanted to be able to use this to pre-heat our hot water, so reducing the amount of additional heating needed to get it to a usable temperature. I also wanted to ensure that the hot water was maintained at a temperature of between 42 and 45 deg C, particularly when having a shower or bath. The diagram below shows the new system layout:

So far I’ve got as far as removing the old 260 litre thermal store (which is for sale on ebay!), fitting the new pipework, pump and plate heat exchanger and putting in place everything needed for fitting the Sunamp PV when it arrives. The photos below show the work partially completed, without the pipe insulation and insulation around the plate heat exchanger and with some of the pipe and cable clips not yet fitted.

This is the plate heat exchanger and pump, connected to the old 28mm pipes that fed pre-heated water to the old thermal store:

The plate heat exchanger is a 20 plate Wiltec unit from Germany, rated at 44kW, to give reasonably performance when operating at the much lower temperatures that this system operates at. The circulating pump is a low energy Wilo unit, that is switched on by a flow switch in the hot water supply pipe. This turns the pump on when more than about 1 litre/minute is flowing from any hot tap. In practice the heat exchanger gets warm from thermosyphon action without the pump running, so preheat is instant, without any lag whilst the pump starts to get warm water from the buffer circulating. This was a happy accident of having the pipes already laid out to get a decent thermosyphon for the thermal store.

This photo shows most of the pre-heat system:

The black thing in line with the outlet from the plate heat exchanger is a solid state flow switch, that switches the pump on and off, It’s wired into the black junction box and is fed with power from the main heating system isolator switch. The system is sealed and pressurised to a very low pressure, as the buffer tank is only tested to 1.5 bar. The expansion vessel is connected to a 1.5 bar pressure relief valve (the lowest pressure one I could find) and also a bottle vent and pressure gauge. I’ve pressurised the system to 0.5 bar and it seems to be fine, with 0.5 bar precharge in the 12 litre expansion vessel. There’s no noticeable change in pressure from cold to warm, but the temperature change is only relatively small, from around 10 deg C to 35 deg C. The discharge from the PRV is via an air gap to a 32mm waste pipe and is visible to comply with the regs, although applying hot water regs to a system at this low pressure and temperature isn’t really sensible, as there’s no possible scalding risk at all.

At the lower left corner the fill pipe is just visible. This runs across and comes up to a filling loop connection point adjacent to the one for the UFH circuit, so I can use the same cold water feed for both. It has a non-return valve and a ball valve in line.

I rearranged the pipework to the Steibel Eltron instant water heater, so that the feed pipe can be easily connected to the Sunamp PV:

This can add up to 9.6 kW of boost heating to the hot water if needed. It is electronically controlled, so only provides as much heat as is needed for the outlet to reach the set temperature. I have it set to 42 deg C and will set the thermostatic mixer valve on the outlet of the Sunamp PV to 45 deg C. This will ensure that for as long as the Sunamp PV can provide hot water over 42 deg C the electric boost heater will stay off, allowing preferential use of the energy from excess PV generation. As soon as the temperature of the water from the Sunamp PV starts to drop, as it becomes depleted, the boost heater will kick in and add just enough heat to keep the water at 42 deg C. It will do this at the 10 litres/minute shower flow rate down to an inlet temperature of 29 deg C, allowing more usable heat to be drawn from the Sunamp PV.

There will be periods in winter when there won’t be any surplus PV generation, so there needs to be an alternative way to charge up the Sunamp PV. In it’s originally designed role, as an adjunct to a combi boiler, this wouldn’t be needed, as the combi would take over and deliver hot water as normal. In our case we don’t have enough boost power available to get the water up to a reasonable temperature if we were to rely on just the ASHP and the electric inline boost heater. To get around this I have added a simple time switch that can be switched on to over ride the excess PV energy diverter:

The observant may note that this is the MkII version of the excess PV power diverter, as I realised the previous version took no account of power used outside.  Now I have a plug in hybrid car, the diverter needs to measure true power import/export at the main incomer.  This meant splitting the circuit, putting the power measurement side down by the meter box and having a wireless link to control the solid state relay that;s now in the black box above and which controls the Sunamp PV heater.

The time switch is set to operate for a couple of hours in the very early morning, before we get up. If the Sunamp PV hasn’t been fully charged the day before, then it will charge at this time using grid power. If it has been fully charged then it won’t need to top up from the grid when the timer operates. There is a switch in series with the power feed from the timer, just to allow the timed boost to be turned off it it’s not needed, which will be all through the summer and for much of the spring and autumn.

I took the photos above earlier last week, and since then have added all the insulation, tidied up the wiring runs and added pipe clips. I’ve also added a ply base board on the floor to support the Sunamp PV and make the installation look a bit neater.

What has surprised me is how effective the plate heat exchanger system is. It provides instant warm water that is perfectly OK for hand washing without needing any other heat input. I had the electric boost heater out of circuit whilst I was setting things up, so only had the plate heat exchanger providing warm water, and even my wife reckoned that the warm water it provided was fine for most things, just a bit too cool for a shower. I think it would be perfectly OK to feed basin taps directly from this part of the circuit and reserve the really hot water supply for the kitchen tap, utility room tap and shower/bath. Not really practical for our system, as I can’t separate out those runs very easily, but it might be worth thinking about if anyone else wants to do something similar.

I should have some more photos in the next entry, showing the completed installation, with everything tidied up.


 Alphonsox 24 Oct 2015 02:26 PM:

 Looks very interesting – Could I please request a higher resolution version of the diagram (PDF maybe) it’s a bit too blurred to read.



 jsharris 24 Oct 2015 02:49 PM:

 Good point, I didn’t check that the jpg was still readable after being re-sized by the forum software, so I’ve now added a PDF copy of the drawing that should be easier to read.


 Alphonsox 24 Oct 2015 02:52 PM:

 Perfect – Thanks


 ProDave 24 Oct 2015 03:33 PM:

 Is there any form of temperature sensor in the Sunamp? (or could one be fitted?)

 If so, you could automate the timed boost and only activate that if the sunamp core temperature was under a pre set value.


 jsharris 24 Oct 2015 04:33 PM:

 It’s got some in-built intelligence, so will only charge if it needs to charge, which means that there’s no real need to do anything than use a time switch for boost charging. The Sunamp PV has two power connections, one always on, one from the PV diverter. If the time switch applies power to the PV input then the unit will only take power if it needs to. It could have lost around 300 Wh between the last PV charge during the day and the time that I’m going to set the time switch to activate, around 4 am. I don’t know the threshold at which the Sunamp decides an energy top up can be used, but suspect it could be low enough to take some power to make up for the small losses over that time. I can’t say I’m going to lose any sleep about it using around 5p worth of electricity to top up on nights when it doesn’t really need to, especially as I’m pretty certain I can turn the timed boost off for around 6 months or more of each year. The annual running cost penalty of just having such a simple system will be less than £10, and I can live with that.

 When I get some spare time I may look at doing something a bit more sophisticated. I’ve already realised (as this is the weekend the clocks go back) that I have ordinary 5 time switches around the new house, all of which need changing when the clocks change. I already set up the data loggers I have using an MSF radio code receiver, and have been thinking of building a centralised programmable timer, with an MSF receiver to keep the real time clock accurate, that transmits unique on and off signals to remote power switches. It looks to be easy to do, but will have to wait until I have time to fiddle about with non-essential stuff like this. I’ve searched high and low for radio code clock time switches, to no avail. Given the availability of very cheap clocks with built in MSF receivers I’m a bit surprised that no one seems to have picked up on making time switches or programmers that use the same clock system.

 I could improve on the simple time switch for the Sunamp PV by using a combination of time and measuring the previous days PV charge into the unit to determine whether or not to boost and how much to boost by. Something along the lines of this bit of pseudo code:

 IF Sunamp PV state of charge < X AND if time of day >Y AND <Z THEN activate Sunamp PV boost charging UNTIL state of charge >= X


 ragg987 25 Oct 2015 11:43 AM :

 Love these schemes you come with with, jsharris.

 It strikes me that you will use the Sunamp like a storage battery for PV – albeit converting the excess electrical energy to heat. Why not use electrical storage instead?

 Also, any indication of the costs involved – e.g. 260l water storage @ £yyy vs Sunamp at £xxx?


 ragg987 25 Oct 2015 11:56 AM :

 Google search example of PV Lithium battery:


2 x 3.2kWh units give >12 kW

>6000 cycles to 90% DOD

<£4k for both


 jsharris 25 Oct 2015 12:17 PM:

 Yes, it is very much like a very efficient battery, but for storing thermal energy rather than electrical energy. It’s a fraction of the cost of a battery with the same power delivery capability, though, and has a potentially longer life. It can store 5 kWh of thermal energy, lasts for at least 10 years (may well last a great deal longer as there’s nothing much to degrade) can deliver a very high peak power (35 kW) and costs around £1700. It’s also compact, 300mm deep, 700mm high and 530mm wide, about the size of a wall-mounted combi boiler and about the same peak heat output.

 The closest battery system is probably the Tesla Powerwall. The 7 kWH model is a bit bigger than the Sunamp PV, at 180mm deep, 1300mm high and 860mm wide, has the same 10 year guarantee (but the lithium batteries it uses are known to have a calendar life of only around 12 to 15 years), can only deliver 2 kW continuous and 3.3 kW peak output (so no use at all for instant water heating) and is likely to cost around £2500 or so (retail prices haven’t been confirmed in the UK yet).

 A battery system can’t come close to the power delivery of this thermal storage system and is significantly bigger, so isn’t really suitable for hot water generation, as it would still need a big hot water tank because of the low power output.

 In terms of water storage versus the Sunamp PV, then our thermal store cost around £900, was 1900mm high and 550mm wide and deep (with the additional insulation needed) and weighed about 280kg, versus about 80kg for the Sunamp PV. In terms of standing heat losses, the thermal store (with the extra insulation) had measured losses of about 2.5 kWh per day, the Sunamp PV has measured losses of 0.607 kWh per day. Both have about the same power output to heat water instantly. So, the cost is around £800 more for the Sunamp PV, for a very much smaller unit, that is less than 1/3rd of the weight, with heat losses that are less than 1/4 of those of the thermal store. The heat losses from the thermal store meant that more direct electricity was needed for boost to compensate, so there is a running cost saving of around 1 to 2 kWh per day, at a guess, which could add up to £50 or so per year; not enough to justify the change on cost saving grounds, but useful all the same.

 My primary reason for changing from a thermal store is to get rid of the high heat losses. These were causing overheating problems in summer, and resulted in the services room, and the adjacent bedroom, getting very hot indeed (it got to over 40 deg C in the services area during hot weather this past summer). The space saving is also extremely useful, as it’s allowed me to reinstate the 100 litre pressure tank that I had to remove when I added the additional insulation to the thermal store (because there then wasn’t room to fit both).


 ragg987 25 Oct 2015 12:36 PM:

 Thanks for that, jsharris, seems to make sense in your scenario.

 Is your services room part of your MVHR circuit? I will likely face a similar scenario with my build, but intention is that our services room will have a MVHR extract. I was hoping that the excess heat in the room is extracted from the room and re-distributed in the winter (heat-exchange to cold incoming air) and dumped outside in the summer (summer bypass). Was also intending to have low-temperature UFH slab in this space, expecting it to distribute the heat as per your experience.

 Sorry if I have hijacked your thread !


 jsharris 25 Oct 2015 12:58 PM:

 Yes, the services area is in the ventilated space, and that creates more energy consumption in summer by needing additional cooling. The winter benefit of waste heat is tiny, as a fair bit of the time its a nuisance as much as it is in summer. In our case, the spare bedroom next to the services area gets some solar gain early morning (it faces East) and it never seems to need additional heating, so the heat leaking from the thermal store was a nuisance all year around, just a much bigger nuisance in summer.

 Since taking the thermal store out the temperature has dropped a lot up there and it’s now about 20 to 21 deg C without heating or cooling, which seems OK. Our cooling system doesn’t have to work too hard, either. With the thermal store working we’d often see MVHR extract air temperatures up around 25 to 26 deg C, now they are down to around 21 to 22 deg C (with no heating yet) and that is allowing more reasonable fresh air temperatures and avoiding the need for the cooling heat pump in the MVHR to come on as much.


 joe90 26 Oct 2015 10:42 AM :

 Jeremy, this is fascinating, in my case we are not sure we will have PV due to the drastic cutback in FITS, so the sun amp may not be appropriate for us,it is interesting that you say the warm water from the buffer/ASHP was sufficient for basins etc, this occurred to me and I may well do this in my new build. Correct me if I am wrong but you say you don’t have enough boost power from the ASHP and inline water heater?, this is what I was thinking of doing as the hot water load for us will be fairly small. I thought the inline water heater was capable of providing enough hot water from cold or warm water?


 jsharris 26 Oct 2015 11:17 AM :

 I’d be more than happy to just use the ASHP boosted water for hand washing. It comes out of the taps at around 30 degC, so isn’t hot, but is warm enough to not be uncomfortable. It’s not warm enough for dish washing, though, so you’d need hotter water for the kitchen tap, but you may be able to get this from using a boiling water tap with the optional thermostatic mixer valve to supply the hot tap, so only needing a cold supply to the kitchen.

 The electric boost heater we have is limited to 9.6 kW, and that isn’t enough to run a decent shower, even with pre-heated water being fed to it. There are a couple of problems that arise, the first being that the pre-heat temperature from the ASHP buffer drops off fairly quickly when the hot water flow rate is high. The main problem is that 9.6 kW just isn’t enough to heat water to an adequate temperature for the sort of flow rate a good shower or bath needs.

 If you think about combi boilers, which are really just instant water heaters when in hot water mode, then you need around 25 to 30 kW to deliver a decent hot water flow. Ours (in our current house) can deliver hot water at 28 kW, and it needs that much power to heat water from cold mains temperature to the 45 deg C we have it set to. Our mains water here drops to around 5 deg C in winter, so the combi needs to raise the temperature by about 40 deg C at 10 litres per minute, and that takes all of the 28 kW available.

 The 9.6 kW electric boost heater can only give a 13.7 deg C temperature increase at 10 litres per minute. To get a decent temperature increase, say around 30 deg C, the flow rate would have to be restricted to around 4.57 litres per minute, which is pretty feeble (and is why electric showers are fairly feeble, as they are rarely more than around 12 kW).

 You could use a bigger instant electric water heater. The largest single phase version I’ve seen is 12 kW, though, and that would need it’s own consumer unit, just to take the size of cable needed and the RCD/MCB/RCBO. The largest RCBO I could get to fit our Crabtree CU was 50A, OK for the 9.6 kW heater but not for a 12 kW one.

 If you can tolerate showers that are around the same as an electric shower, and pretty slow bath fill times, then you could possibly get away with the 12 kW heater and pre-heat, but remember that the pre-heat isn’t going to last that long when you’re drawing water at high flow rates, unless the buffer tank is pretty large. The buffer will start to be recharged by the ASHP as soon as it cools, though, so you could have another few kW of “instant” heat coming from the ASHP, and that might well boost things up a bit. In our case the ASHP can add around 6 kW of additional heating all the time that the buffer is having heat drawn from it, so that increases the instant water heating power available from just the ASHP buffer and electric boost heater to around 15.6 kW, enough to give a 22.3 deg C temperature increase at 10 litres per minute, or to give a 40 deg C temperature increase at 5.57 litres per minute.


 joe90 26 Oct 2015 02:29 PM:

 Jeremy, a very comprehensive reply, I thank you. It does appear however that my “outline” plan is flawed, bugger.


 TerryE 27 Oct 2015 10:45 PM:

 Jeremy, thanks again for doing the leg-work. I will fork this solution, but with a few changes since I am starting with a clean sheet. My immediate thought is to collocate all of the “hot” components in a single facilities cupboard which has, say, an additional 10cm PUR on sidesbase and top to minimise losses from hot pipework heat exchangers, etc. I don’t think that we will be having PV in our first installation (planners!), but the Sunamp PV seems to be a far better fit to our usecase than even a minimally configured Sunstack, and it leaves the door open for a later PV install.


 recoveringacademic 29 Oct 2015 08:37 AM :

 Just in time Jeremy, just in time. Thanks for the effort and hard work you put in.     


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