Part Thirty Six – Winter Energy Consumption For Heating

This thirty sixth entry was published originally by JSHarris on the 29th December 2014 and received 1,188 views on the closed forum

Just a short entry this time to highlight energy use during this cold spell.

On Christmas Eve, at 15:00, I read the import and export meter readings, left the house heating on to maintain an internal room temperature of 20 deg C 24 hours a day, and went home for Christmas. I left the PV energy diverter on to keep the hot water system running, and the mid-point half way up the height of the thermal store was the same temperature when I got back today as it was when I left on Christmas Eve, 55 deg C.

I got back to the the house late this morning, around 11.45 am, and read both meters again. The import meter recorded that the house had used 29.8 kWh to maintain it at 20 deg C evey day, and the export meter showed that during the same time period the export meter showed that we’d exported 39.5 kWh from the PV system.

So, on average, the house used 7.7 kWh per day to heat, but generated 10.2 kWh per day. On the face of it, this looks like the house is (with no occupants and no DHW demand) actually generating more power from the PV system than it is using to maintain a room temperature of 20 deg C, 24 hours a day, during this cold spell (it was -5 deg C when I got to the house this morning).

I’m honestly chuffed to bits with this incomplete and somewhat dubious data set, as even if it’s in error by 20 % or so, it still seems to show that we have, really, built a true zero, or more probably, negative energy house.

The next step is to get some proper instrumentation in the house to see exactly what the temperatures inside and out are doing, but so far I’m as pleased as punch.

ProDave 29 Dec 2014 08:16 PM:

 “So, on average, the house used 7.7 kWh per day to heat”

 Or presented a better way, an average continuous power to heat the house of 320W.

 Given that it’s feeding an ASHP with a COP of probably about 3, that will translate to about 1KW of heat, which is pretty much in line with your heat loss calculations.

 It’s nice to see practice in line with theory.


 jsharris 29 Dec 2014 09:12 PM:

 It is indeed!

 Given that the weather here has been pretty cold, (sub-zero for a couple of nights, dropping to at least -5 deg C last night, as it was still -5 deg C when I drove over to the site mid-morning), I’m pretty pleased with the overall data so far. It looks like, even with our hot water demand doubling this figure through the year, but with the heating demand dropping through Spring to Autumn, we should be well inside the PH limit, and on target to always export more PV energy than we use from the grid, so will really have a zero energy cost home (or, allowing for FIT, a net income generating home).

 I’m impressed with the ASHP in this cold weather. I watched it carefully this morning and now that I have it turned down to 40 deg C max flow temperature it didn’t go into defrost mode once, unlike when it was set to 45 deg C when it needed to defrost a couple of times a day.


 ProDave 29 Dec 2014 09:59 PM:

 That’s really good about the ASHP performance in this cold weather. It’s been -8, -9 and this morning -10 first thing and not above zero for nearly a week now even in the day.

 House down the road has an ASHP directly driving radiators. I was there this morning fixing a wiring fault and the radiators are just about as hot as you would want to touch (i.e normal ones not large low temperature ones) It would be interesting to know how that unit is coping with the cold air temperature and the high water temperature being asked of it.


 slidersx200 29 Dec 2014 11:55 PM:

 Just wondering if anyone has seen or has any thoughts on the content of this article from March 2014 about the defrost cycle frequency in air source heat pumps.

Looking for ways to achieve the best winter performance (from an ASHP) for domestic hot water production has kept me busy during the post festivities lull and the defrost cycle seems like one key area in which improvement could be made.


 NeilW 30 Dec 2014 10:38 AM :

 I’m starting to think that the trick to this energy efficient house thing is to make sure that the house uses negligible energy in the winter months and then use active cooling to handle the summer overheat – powered by the solar panels.

 Which of course is precisely what you have. You should patent the active cooling slab idea while you still have the chance!


 stones 30 Dec 2014 05:02 PM:

 “jsharris, on 29 December 2014 – 09:12 PM:, said:

I’m impressed with the ASHP in this cold weather. I watched it carefully this morning and now that I have it turned down to 40 deg C max flow temperature it didn’t go into defrost mode once, unlike when it was set to 45 deg C when it needed to defrost a couple of times a day.”

 Great results and good to know what can be achieved / theory matches up with reality.


 jsharris 30 Dec 2014 06:28 PM:

 I hadn’t seen that article about ASHP defrosting, thanks, it makes sense of what mine does. I have a feeling that mine already has some sort of intelligence, as in one of the service menus you can read out the relative humidity. This means there is an RH sensor in there, as well as several temperature sensors, so my guess is that it uses this to control defrost depending on whether or not there is a real defrost risk.

 Today was a good example. -5 deg C when I arrived at the house and the ASHP was still running, pumping heat out of the flow pipe at +40 deg C. It stayed on for an hour or so before the slab was up to temperature for the day and didn’t defrost once. Looking at the defrost water outlet it was bone dry, so my guess is it hadn’t defrosted at all before I got over there. This was probably because the RH was very, very low, because the persistent cold weather we’ve had lately has dried out the air a fair bit.

 I had a visitor around midday who was amazed that the house was very warm (it was around 22 deg C!) and that the heating hadn’t been on since several hours earlier (and the heating is set to only heat the slab to 20.6 deg C anyway). We had a fair bit of solar gain, as it was a bright clear day, and that boosted the temperature a fair bit by early afternoon. When I left the slab was at 20.8 deg C, just from solar gain, so the chances are that the ASHP won’t have to do much tomorrow to warm the house. We’re still exporting more electricity than we’re importing, too, which is nice, but only because this really cold spell is accompanied by very clear skies. I saw over 5kW from the PV a few times today, and 16kWh exported overall, more than double our daily usage from the grid.

 Neil, The trick does seem to be more concerned with cooling than heating. Heating a house like this is really easy, as so little heat is required. One reason the house warmed up this morning is that I was sanding some oak planking that I’d biscuit jointed into a large flat (‘ish) board to make a cupboard under the stairs. I probably used the belt sander for nearly and hour getting the front face dead flat and smooth, and that probably kicks out a few hundred watts. This was followed by loads of hand sanding to get a good finish, so I probably kicked out more than 100W for another hour or so, too!

 I need to get a proper data logger over there and log the internal and external temperatures for a few days, along with the energy consumption and generation, but my gut feeling is that the house performance (in terms of heat loss) is very, very close to the simple heat loss model I did. So close that I’m not sure there’s much merit in bothering with more heat loss detail. The only thing that does need looking at is modelling solar gain and factoring this in to a simple “what if” type modelling tool, as that seems to be the over-riding factor determining the choice of ventilation/cooling system used.


 Alphonsox 30 Dec 2014 09:19 PM:

 Excellent result – and It gives me some hope that heating with E7 isn’t the dumbest idea either.


 TerryE 30 Jan 2015 05:36 PM:

 @Alphonsox, you should do a post on Boffin’s corner on your E7 calcs so we can discuss the pros and cons of this, as I don’t think that Jeremy’s blog entry is not the best place to discuss this and ditto your UFH For New Selfbuild topic. There are a few wrinkles that we could explore, like modelling the diurnal ripple, and how you dump your excess heat outside the winter low months.


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