Posted by: @rob-of-york

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The HTC value is found by averaging over lots of degree-hours, however, the details of exactly what happens each hour or each day are unnecessary for the calculation (a big advantage in this approach), only the average indoor and outdoor temperatures for the month are needed along with the energy going in from various sources. (Caveat that outdoor temperature is always below indoor).

I get the general principle. My contention is that you need to sample often enough to get true average, since infrequent sampling may miss observations that should contribute to the average, but will be missed out with infrequent sampling, if that makes sense. For example, here are my hourly OAT measurements from the first half of August this year, with two mean daily temperature values (°C), one calculated using hourly samples, the other using two samples, midday and midnight. The differences aren't huge, but they are there nonetheless:

Thus, I suggest, while the HTC can use averages over long periods, those averages do need to be based on enough samples to ensure the average reflects the reality of what actually happened, and that means the data need to be collected at relatively short intervals. In other words, you have to start with hourly samples, even if you end up with a single average covering a whole month.

But I think we agree on this, that is, you need enough underlying samples. All I am doing is emphasising the point that the long term averages used to calculate the HTC must be based on frequent enough underlying sampling to ensure the final averages reflects what actually happened.

The other point is whether you need to calculate the heat delivered from other sources. Given that with a steady IAT, the heat loss = the heat delivered by the primary heat source (boiler, heat pump) + the heat delivered by all the other heat sources (people, electrical appliances, solar gain), then if you measure the heat delivered by the primary heat source, that is it not the case that that value is the amount of heat the primary heat source needs to deliver? 

I'm still working on/thinking about this and I also want to run the HTC method on my Jan 2026 data (I have all the necessary variables) to see what results I get, but have some other things I need to attend to right now. I'll be back later!

Posted by: @jamespa 

Whilst I don't like determining things based on a single measurement point because you don't have a tool to spot errors, I also recognise that you have to use the (best) info you can reasonably get. 

I couldn't agree more with the point about not trusting single measurement points. Best to repeat the calculation over multiple months if possible.

I originally did the calculations for my house based on the Dec-Jan period in 2014/5. If I break it down into separate calculations for Dec-24, Jan-25, and Feb-25, the heat loss works out at 6210W, 6110W, and 6220W respectively, and the HTCs are 259W, 255W, and 259W respectively. Weighted average 6170W and HTC 257W. (The solar gain was based on 10 kWh per sqm incident in Dec and Jan, and 20 kWh incident per sqm in Feb). The average outdoor temperature changed considerably over these three months, 7.3C in Dec, 2.8C in Jan, and 5.0C in Feb. Indoor averages were near constant at 18.5C, 18.2C, and 18.3C. 

Since these figures are all very much in the same ballpark that gives me good confidence that the real heat loss in somewhere in the range 5900W to 6400W. (These are the outliers with the solar gain adjusted +or- 5 kWh per sqm from the defaults. The rest of the data are either measured by the smart meter or good estimates of occupancy, so only a small variability is likely there).

With good quality input data, I think this practical approach is a really good way of getting a handle on heat loss values.