I’ve updated the blog on the heat loss calculation I did for my own house. Worthwile reading I hope for anyone with a terraced property with solid stone walls built before the year 2000, and interested in getting a heat pump installed. Feedback most welcome!
https://trystanlea.org.uk/roombyroomheatloss
Quick summary
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Default CIBSE/MCS assumptions for pre-2000 air change rates, unheated neighbours, design outside temperature and stone wall U-values result in a calculated heat loss of 7.5 kW for our house! This would typically result in a 8.5 kW or 10kW heat pump being installed. We are seeing a lot of Eco4 installations in our area (North Wales) installing this size of heat pump as a result.
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In reality the highest cold weather heat demand that I recorded, on the 1st of December 2023, was 2.9 kW over 24 hours with an outside temperature of -1°C and an average living room temperature of 20°C. I extrapolate this and other measurements to indicate a measured heat loss of ~3.3 kW at a design outside temperature of -1.4K (± 0.6 kW). Our 5kW Ecodan with a real world maximum output of 4.2-4.5 kW during heavy defrost conditions has been plenty sufficient for our requirements.
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With better input assumptions for air change rates, stone wall U-values, heated neighbours, and design outside temperature (based on local weather data) it’s possible to reproduce the measured heat loss in the heat loss calculation. Some additional over-sizing to cater for unheated neighbours and strong wind storms may be reasonable but it’s worth thinking it through carefully.
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There’s also a section on choosing design flow temperatures and why I think a design flow temperature of 35-40C is required for a SCOP of 4 or above if the system is sized based on an accurate heat loss.