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Adventures in heat loss calculations

Rob of York · 2026-08-29T15:49:23Z

I thought I’d post my recent adventures in heat loss calculations, in the hope that they may be of interest to others embarking on the same. Over the last few days, I’ve done some heat loss calculation for my house, a 1970's 4-bed detached (146 sqm), which has as much insulation as we could reasonably add without it turning into a full retrofit. As a stubborn and pragmatic scientist, I decided I’d work out the heat loss as far as possible myself from the basics. I used 24 degrees as the design temperature difference (i.e. -3C outside and 21C inside). I calculated the heat loss in two ways and also tried out a simple estimate: Theoretical: based on the attributes of the house (room sizes, construction materials and their U-values etc.). Practical: based on the energy going into the house that kept it significantly warmer than outside during December 2024 and January 2025. (I used those months because our only heating system then was a gas boiler). Quick estimate: using the Michael De Podesta Formula 1. Heat loss (theoretical): 1a. Thermal conduction: I worked out the areas of the various building components (floors, ceilings, walls, windows etc) on a room-by-room basis and either looked up their U-values from the rdSAP 10 manual or worked them out from the U-values for the individual materials used and their thicknesses. (The open university web pages were useful in that respect, seeand the next few pages). From the various U-values, areas, and the temperature difference, I worked out the heat loss. The overall thermal conduction losses came to about 3720 W. 1b. Thermal Bridging: This takes account of the joins between walls and roof, walls and floor, windows and walls etc. which tend to be areas of high heat loss. rdSAP 10 accounts for this by increasing the U-values of everything in 1a above by a fixed amount depending on the age of the property, e.g.by 0.15 for my 1970's house. The overall thermal bridging losses came to about 1180 W 1c. Ventilation Losses: These are given by a simple formula: 0.33 x temp difference x house internal volume x ACH. Where ACH is the number of air changes per hour. Appropriate values for ACH are an area of some debate as they can vary widely for two ostensibly similar houses. As I’ve taken care that my house is well sealed, I used a value of 0.5 for ACH, however, I currently have no hard evidence for that value. The overall ventilation losses came to about 1340 W Total theoretically determined heat loss 6240 W Thermal conduction was 60% of the total, thermal bridging 19% and ventilation losses 21%. 2. Heat loss (practical): I have a semi-professional weather station in my back garden. The outdoor and indoor units record temperature every 30 minutes, hence I could easily get average indoor and outdoor temperatures for Dec 2024 and Jan 2025 and the average difference in temperature over each of those months. I then worked out the total heat energy going into the house from the following: Gas consumption figures, assuming 0.9 boiler efficiency (condensing except when heating hot water). Subtracted gas consumption for hot water heating, since the water is still hot when it drains out of the house. Electricity consumption figures, since all use of electricity within the house eventually turns into heat. Two people in the house at 100W each for an average of 16 hours per day. Solar gains from 10 kWh per sqm incident energy on east and west facing windows over each whole month. (Double glazed unit g-value of 0.49, glass to frame ratio of 0.7, overall window area measured). Correcting from the actual difference in average indoor and outdoor temperatures gave an input energy of 147.8 kWh per day to achieve a 24-degree temperature difference. Total practically determined heat loss 6160W 3. Michael De Podesta Formula The Michael De Podesta Formula is simply annual gas usage divided by 57.3 multiplied by the 24-degree temperature difference. Total heat loss based on Michael De Podesta Formula 6031W Thoughts: The first thing that struck me was that the figures from the two completely different heat loss calculations are surprisingly close. As they are worked out completely differently, this gives me some confidence that they are about right. Further, the simple Michael De Podesta Formula also gives a value that is within 5%. The ACH value has the most uncertainty and varying that has a big effect. I don't know if there is anything I can do to reduce that uncertainty short of paying for an air tightness test. There is quite some heat that goes into the house from sources other than the heating system. This amounts to about 570W from non-heating electricity use, people burning calories, and solar gain through the windows even at the dullest time of year. If I just want the maximum demand on the central heating system, then it looks like this is about 5500 to 5700W, not including hot water heating. Comments welcome!

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Air Source Heat Pumps (ASHPs)

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cathodeRay

(@cathoderay)

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09/09/2026 6:04 pm

@jamespa and @rob-of-york — thank you both very much I think I have now got it well enough to retain it! The light bulb paragraph for me (I am better at abstract conceptual thought using words rather than maths) is this one

Posted by: @rob-of-york

↑

What do the new data points look like on the graph? Answer, exactly the same as the old data points, but the scale on the y-axis has shifted by 1kW. (Note this assumes that none of our original data points required less than 1kW from the primary heating system). The regression line now hits the y-axis at -1kW. The primary heating system puts out 1kW less at any given time relative to the situation without the additional source of heat. However, nothing here changes the slope of the graph. If the temperature difference increases by 1C, then it is the primary heating system that has to provide the extra power equal to HTC. The additional source of heat does nothing different, it just sits there delivering 1kW.

In particular "but the scale on the y-axis has shifted by 1kW" (even if perhaps it is the regression line that shifts, not the scale on the y-axis, is the line drops rather than the axis scale rises) and then "However, nothing here changes the slope of the graph", ie the HTC remains constant, whatever the value of PO.

I even managed to sail past this point without noticing its importance in my chart with the second dotted line! When I added the line, I was careful to make it parallel, ie not change the gradient, somehow appreciating, but not understanding, that HTC, the gradient, stays the same.

Midea 14kW (for now...) ASHP heating both building and DHW

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