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Author Topic: Guide for Heat Pump sizing  (Read 1421 times)
Countrypaul
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« on: August 07, 2018, 05:28:11 PM »

Is there any sort of rule of thumb or guidance for sizing a heat pump once you know the heat load of your house?  The only information I have found so far seems to suggest that you need 1kW per 10m2 which is making far too many assumptions as it assume all houses are insulated to the same level.

I know our house will need about 8kW when outside it is -3C, we have UFH in a 70mm screed so about 20 tons of screed equivalent to about 10,000L of water for thermal mass purposes, we also have a 430L thermal store. I have had 2 quotes from MCS registered suppliers who both quoted a 12kW ASHP based on the heat load I supplied them with - they both indicated the first step would be for them to perform a full heat load calculation. They both assumed we would apply for RHI, though given that the ASHP may be supplemented by a WBS, I suspect RHI wil not apply and that I might be better off just buying an ASHP and ignoring RHI.

Has anyone any guidance on how to decide the ASHP size? We have assumed that the ASHP would give us enough heat in all but the coldest spells, where we would rely on a boost from another source such as the WBS.  I was hoping to run the ASHP on E7 the majority of the time, though we also have a 3.3kWp PV system - but as always the PV power ain't there when we are most likely to need the heat.

I am not sure on the total amount of heat we are likely to require. I can get the degreedays information and use that to calculate an amount, but the degreedays requires the use of a base figure such as 15C. Where this figure comes from seems to be a black art so the whole heat load calculation then seems to be based on sticking a finger in the air. Any comments?
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bxman
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« Reply #1 on: August 07, 2018, 05:46:18 PM »

When I looked into running an ASHP on E7  I discovered that the COP is related to the air temp at the time that it is operating and the most favourable air temps do not necessarily coincide with the E7 timetable .
cheers Patrick   
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Countrypaul
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« Reply #2 on: August 07, 2018, 06:06:24 PM »

Since E7 is about 1/2 (or even 1/3) the price of "normal" electric the COP would have to also be about 1/2  before using "normal" electric was competitive, and as far as I can tell that is unlikley to be the case. If it gets very cold then we are likely to have to run both during E7 and normal time depending on the size of the HP and that's one reason I have asked the question about sizing. It won't be until this winter I really find out how much heat we actually use. The problem is compounded by the fact my wife always needs rooms about 4C higher than I do, and our two young boys wil happily sit around in just pyjama bottos when its only 16C - there is no way any calculator will deal with this requirement is there?
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eabadger
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« Reply #3 on: August 07, 2018, 06:29:32 PM »

is it a passive house?
the pumps are good we have an 8kw one for towel rads and dhw, but wouldn't heat an old house, i have a client with twin 12kw pumps and his house is not warm and costs a fortune, i am re jigging it this year as original installer has wired incorrectly and some valve stay open when they should close allowing heat to thermo syphon to outside.
wood burning certainly tops them up, but not without work.
i load my floor (200sqm 300mm deep) with heat from wood, keeps home warm 24/7

steve
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1600w PV main array at 24v, excide 2v 1000a forklift cells now x 2, 320w PV secondary array at 12v. Enfield 1944 ex RAF 5.6kw diesel genset (now in pieces, big ends gone), Petter AC1 28v diesel charging set at 2.8kw.
1kw wind turbine.
26kw wood stove back boiler to underfloor heating and dhw
pdf27
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« Reply #4 on: August 07, 2018, 06:45:35 PM »

I know our house will need about 8kW when outside it is -3C, we have UFH in a 70mm screed so about 20 tons of screed equivalent to about 10,000L of water for thermal mass purposes, we also have a 430L thermal store. I have had 2 quotes from MCS registered suppliers who both quoted a 12kW ASHP based on the heat load I supplied them with - they both indicated the first step would be for them to perform a full heat load calculation. They both assumed we would apply for RHI, though given that the ASHP may be supplemented by a WBS, I suspect RHI wil not apply and that I might be better off just buying an ASHP and ignoring RHI.

Has anyone any guidance on how to decide the ASHP size? We have assumed that the ASHP would give us enough heat in all but the coldest spells, where we would rely on a boost from another source such as the WBS.  I was hoping to run the ASHP on E7 the majority of the time, though we also have a 3.3kWp PV system - but as always the PV power ain't there when we are most likely to need the heat.
One issue is that the RHI requires the heat pump to be sized sufficiently large that it can be the sole source of heat - essentially they're required to prove that you never need to use the wood stove, and the calculations assume that you never use it for working out subsidy payments.
Easiest way to deal with E7 and PV is to get a heatpump with an "SG Ready" contact, and rig up some sort of box to fire a relay whenever there is cheap power available. Pretty easy - drive one off a simple timer and the other off an output from the PV system triggered whenever export goes above a certain level. The nice thing about the SG Ready contacts is that you can program the heat pump to do whatever you want when it is on - typically turn up the thermostat by a degree or two. That will heavily bias the system to working when the power is cheap, but won't have any significant comfort impact when it isn't since the system will still fire up whenever it gets cold inside.
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Countrypaul
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« Reply #5 on: August 07, 2018, 07:32:16 PM »

The house is not a Passiv house, it was orginally built in 1956, then extended in 1968. We have renovated and extensively remodelled it which includes putting in a huge amount more insulation than there was, however there are parts where there is some thermal bridging for example. The whole top floor (dormer bungalow) is made of SIPS as is a rear extension and the rebuilt garage/1968 exension. All the SIPS parts have been carefully taped (SIGA Sicrall/Risson/Corvum) etc. The roof has 150mm PUR , the SIPS walls 170mm PUR and the old walls 70mm PUR plus 50mm rockwool (<20% total ground floor), north windows tripple glazed, S,E,W windows double glazed etc. MVHR system, only 3 pipe penetrations, 2 through the wall for MVHR and one through the roof for soil pipe vent, pipes insulated. Ground floor Beam and block, DPM, 120mm PUR, VCL, 70mm screen with UFH, screed separated from walls by 50mm pur (external walls) and 25mm PUR (internal walls) We have not had a air leakage test though I am considering it.

The WBS is likely to be a Dunsley Yorkshire with back boiler, we are limited by conservation area etc.

Yes all the limitations on RHI is one reason I'm thinking of just ignoring it. If you have a WBS with BB plumbed into the TS then again that makes life with the RHI much more complicated. A 8kW ASHP can be had for <2K whereas a 12kW installed under MCS/RHI was quoted at 14K. Installation is simple (so I'm told) even though it is likely to be 15M from the TS (most of the pipe in a conduit under the house).

I'll have to remember about the HP need a "SG Ready" contact - what does SG stand for in this context? 
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pdf27
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« Reply #6 on: August 07, 2018, 07:49:12 PM »

SG = Smart Grid. For a future renewable grid we're going to have to be able to switch big electrical consumers (i.e. electrical heating) on and off remotely. Typically they have 4 options:
  • Inhibit - turn off the heat pump for a short period of time (say 30 minutes - a short enough time that you won't notice the temperature dropping) to ease off a peak on the grid.
  • Normal operation
  • Boost 1 - slightly increased power draw, so when power is cheap - typically turns the thermostat up a bit
  • Boost 2 - too much power on the grid, turn everything on to soak it up - typically turns on the internal immersion heater and uses it to heat the hot water tank.

Normally they're controlled by 2 sets of contacts giving 4 options (00, 01, 10, 11).
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Countrypaul
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« Reply #7 on: August 07, 2018, 08:05:12 PM »

Are they volt free contacts do you know? Presumably, if so, they could also be driven by an Arduino or RPi.
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pdf27
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« Reply #8 on: August 07, 2018, 08:10:40 PM »

Are they volt free contacts do you know? Presumably, if so, they could also be driven by an Arduino or RPi.
I suspect it may vary by manufacturer. I've got a Panasonic manual, and for that there are 3 terminals and you need to make or break the connection between the input and common pins to give a signal. Running that off an Arduino or RPI should be easy with an interface board, but I'm not sure if it can do it natively - I've never got one to play with. I'd guess it's a 5v signal and a few mA of current.
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