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Harvest Thermal - ASHP with a smart thermal battery

 
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Air Source Heat Pumps (ASHPs)
Last Post by JamesPa 4 months ago
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JamesPa
 JamesPa
(@jamespa)
Illustrious Member Moderator
Joined: 4 years ago
Posts: 5476
17/06/2026 8:25 am  

Posted by: @springswood
↑

Out of curiosity I dusted off my 40+ year old physics knowledge and tried to measure the heat capacity of my house. More of a very rough estimate. The equation is blessedly simple:

Heat Capacity (Joules per degree C) = Time constant (seconds) x Heat Transfer Coefficient (Watts per C). 

Which all puts my heat capacity in the range 1.5-3.4 kWh/C.

Thanks for this, I didn't realise the equation was that simple. 

I tried to get an estimate of mine a different way by looking at measured temperatures/gas consumption and lag times (I would have to look back to remember exactly how) and came up with 15kWh/C (+/- 5kWh/C).  The method I used would have taken 4 cats, cooking and people into account, but has other problems.  I came up with a similar value for another house on which I did some analysis and got to about 10-14kWh/C. 

Reversing that back my heat loss at design OAT of -2 and an average IAT of say 20 is 7kW.  Various heating sources are probably equivalent to say 3C temp difference (roughly 1kW), thus the heat transfer coefficient is 7000Wh/19C = 370Wh/C.  If my heat capacity is in fact 15kWh/C = 54MJ/C then it would take =140,000s to cool by 37% (7C) = 40h.  Im pretty sure that's quite a bit longer than it actually takes, even though the rate of cooling slows down as the house gets cooler.  Thus my current estimate of heat capacity is probably wrong.  That said I dont allow the house to cool to 13C very often (or at all), and it may well take longer than I think!

I do have data logging (because I have a heat pump and home assistant).  Heat pumps measure everything and home assistant logs much of it) so could perform the experiment with the specific objective of measuring heat capacity.  Im very tempted to do so next season.

 


This post was modified 4 months ago 4 times by JamesPa

4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.


   
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