ASHP Installer sizing calculation - how do they do it?!

[drcarrera]

Hi

 

I'm trying the decide whether to replace our oil fired boiler with an ASHP, the main drivers being that the current boiler and tank will need replacing soon, the oil tank takes up a lot of room, and I'd like to have a modern more ecologically friendly system. However, I can't tally the sizing requirements I've been quoted with my own figures.

 

It's a five bedroom house, built in 2004, with a total floor area/volume of around 300m2/700m3 and a footprint of 150m2 (some of it is single storey, some 3 storey). My own usage figures, using the very helpful spreadsheet on here, based on average temps etc come out to around 25KWh per year which tallies quite well with the known oil usage of 3000-3500 l per year.

 

However, the worst daily heat loss figures I'm calculating are 5000W average and 7000W peak. The two quotes I've been given so far calculate 14KW and 17KW!

Even adding what's needed to heat a large DHW tank on top of space heating I don't quite see how they're so high. The 17KW one has recommended a  hybrid set up with 16KW ASHP and oil boiler which seem a bit excessive. 

 

Am I mis-interpreting the daily heat loss figures from the spreadsheet?

[ProDave]

Can you post a copy of your spreadsheet here?

 

the 25kWh you quoted, is that per day?  It's certainly not per year as you stated?

 

3500L of oil per year would equate to 36,225kWh of heat total.  Lets guess 30,000kWh of that is for heating, the rest is DHW.

 

Assuming you heat the house half of the year, that will be using 164kWh per day on average for heating.  Lets assume on the coldest of days you will need twice that (and obviously less when it's mild) so a worst case of 328kWh in a day.

 

assuming 20 hours a day heating, that's 16.4kW average.

 

So I would say the larger units you have been quoted are only just adequate and a 5kW unit would be woefully inadequate.

 

Also how is the heat conveyed to the house? radiators or under floor heating or both?  If radiators I would definitely say don't swap to an ASHP.

[SteamyTea]

57 minutes ago, drcarrera said:

Am I mis-interpreting the daily heat loss figures from the spreadsheet

I think you are. kW is the power you need, kWh (or more likely MWh over the year) is the energy.

 

Thank of energy, the kWh, as how much fuel is in your car, power (the kW) is how much you press the accelerator.

[pdf27]

7000W peak on a 300m² floor area is ~23 W/m². That's about twice the power consumption of most Passivhaus buildings, which suggests the model should be giving you something in the region of 30 kWh/m²/year, maybe 25 kWh/m²/year at a stretch. That's ~9000 kWh of heat per year - equivalent to about 900 litres of oil. You're using 3-4 times this amount, which suggests that your spreadsheet is seriously under-estimating your consumption. Based on that heat pump sizing in the region of 15 kW doesn't feel unreasonable.

[drcarrera]

Many thanks for the replies. 

Sorry, the kWh was a typo – should of course be MWh (as a physicist I should as least have got that right!).

I’ve revised that figure a bit but work it out as follows: We use around 5250l of oil per year but have an Aga, which uses around 2500l per year. So 2750l on heating and DHW which is around 28.5 MWh. The boiler runs at around 93% efficiency which gives an output figure of around 26.5 MWh per year (in practice it's probably less than this as the boiler's quite old). Obviously there's a degree of space heating from the Aga as well which based on estimated latent output could be as much as 9 MWh but this is very localised . Incidentally, heating is UFH to most of the ground floor and radiators elsewhere.

 

My back of a fag packet heat loss calculations for a temperature delta of 20 degrees are as follows :

 

		Area		u value		Loss
Walls		212.3		0.35		1486.1
Windows		47.0		2.00		1880
Doors		9.1		3.00		546
Roof		149.9		0.16		479.68
Rooflights		3.0		1.20		72
Floor		140.2		0.25		420.6
					Total	4884.38

 

I haven't included ventilation losses but it still seems way too low. U values are largely based on buildings regs when the main house was built in 2004. I've attached the spreadsheet as well. I'm obviously inputting something incorrectly.

Heat loss calculation.xlsx

[SteamyTea]

35 minutes ago, drcarrera said:

My back of a fag packet heat loss calculations for a temperature delta of 20 degrees are as follows

How much are you loosing from the UFH to the ground?

Bigger ∆T, large area and possibly poor insulation levels.

[SteamyTea]

38 minutes ago, drcarrera said:

Obviously there's a degree of space heating from the Aga as well which based on estimated latent output could be as much as 9 MWh

Is that latent or sensible?

 

[drcarrera]

2 hours ago, SteamyTea said:

How much are you loosing from the UFH to the ground?

Bigger ∆T, large area and possibly poor insulation levels.

As the house was designed and built with UFH I'm hoping there's a reasonable amount of insulation. It certainly seems to work well enough.  The UFH covers around  95 m2 of the total (i.e. the entire footprint of the main, 3-storey house) although 30m2 of that is usually off as its the kitchen. For the calculations I've assumed constant 8 degrees under-house temp with floor u of .25.

[SteamyTea]

You could try testing it from the temp difference between the flow and return, and at what rate the room heat up.

 

2 minutes ago, drcarrera said:

For the calculations I've assumed constant 8 degrees under-house temp with floor u of .25.

I that a temperature difference of 12°C?

 

Should it not be the slab temperature minus the ground temperature?

[drcarrera]

46 minutes ago, SteamyTea said:

You could try testing it from the temp difference between the flow and return, and at what rate the room heat up.

 

I that a temperature difference of 12°C?

 

Should it not be the slab temperature minus the ground temperature?

I guess it should be but just went with what the spreadsheet had as default. I suppose I could  assume a slab temp of 35 (assuming that's the UFH temp). But then one could also argue the heat loss behind a standard rad will be far greater as well as it will be way higher than room temp - then the calculations start getting too complicated! 

Anyway, I've just increased the floor temp delta to 27 degrees and the difference it makes isn't so significant as to make the end result for the whole seem any more correct.

Edited March 17, 2021 by drcarrera

[J1mbo]

Ventilation losses are probably another 3kW or more for a dwelling of this size. The fabric might not perform to spec, especially if it's dot-and-dab on thermalite and not sealed at the top. DHW may well be using 750 - 1,000 litres per year.

[tonyshouse]

When designing heat pumps you can slightly under size to save on capital cost that is only needed for a few days every year and top up on those days with direct electric heating or enjoy being at 18.5 instead of 20 for those few days that don’t necessarily happen every year. 

[SteamyTea]

11 hours ago, drcarrera said:

But then one could also argue the heat loss behind a standard rad will be far greater as well as it will be way higher than room temp

But the wall area is a lot smaller than the floor area, and convection currents will limit the temperature.

 

11 hours ago, drcarrera said:

I've just increased the floor temp delta to 27 degrees and the difference it makes isn't so significant as to make the end result for the whole seem any more correct.

That is only changing the temperature, not the floor insulation.  Though as you say, you only have the building regs when the house was built to go on.

8 hours ago, tonyshouse said:

When designing heat pumps you can slightly under size to save on capital cost that is only needed for a few days every year and top up on those days with direct electric heating or enjoy being at 18.5 instead of 20 for those few days that don’t necessarily happen every year. 

Trouble is that you also run the risk of lower CoP at other times.  So under sizing is a high risk policy.

[drcarrera]

11 hours ago, tonyshouse said:

When designing heat pumps you can slightly under size to save on capital cost that is only needed for a few days every year and top up on those days with direct electric heating or enjoy being at 18.5 instead of 20 for those few days that don’t necessarily happen every year. 

We do have a log fire in the lounge for the few very cold evenings when the UFH takes a while longer to get things comfortable so that would be feasible - even more so if we keep the Aga.  Also, we currently don't tend to heat the whole house at once as the office and "day" room are on a separate circuit and this goes off before the UFH comes on, although of course this would probably change if going to a lower temperature system.

 

But to go back to the original question...

So lets now say worst case scenario with heat loss on the few nights a year when the temp drops to -5 is around 6kW. Add 4 kW for ventilation losses. 2kW for DHW (unlikely - there are only two of us) Let's add 25% if the house was built by cowboys with disregard for buildings regs. 

Hey presto! We now have 15kW!  Is that really how the ASHP suppliers would have calculated it?  If so it rather makes all the measuring and calculations rather pointless! 

I still think there's something fundamentally wrong with my heat loss calculations, though.  I'm also beginning to wonder whether the Aga uses a much larger proportion of the oil than I think. 

So in light of all this, perhaps the oil/ASHP hybrid system as recommended by one supplier would be the best way to go. I would have rather just had an ASHP, though.

Edited March 18, 2021 by drcarrera