@ian33a — I'm not so sure about the numbers in the last spreadsheet excerpt you posted. If your average daily oil use is 6.58L, and if an average heating season is 150 days, then that gives a total of 987L pa, yet you say your annual use is 2402L. Over a full year that is 6.58l per day, but you are not (I take it) running the heating all year round! 2402L over 150 days is 16L per day. 

There is a similar problem with the January 2026 use of 367L. Over 31 days, that works out at 11.8L per day, and this is a period in which (I think) you also had the heat pump running. However, if the 11.8L use per day is correct, then on average you were delivering 11.8 x 10.35 x 0.7 = 85.5 kWh per day / 3.56 kWh per hour to the house from the boiler. On a 50:50 split that means a total of 7.12 kWh per hour, but on a 20:80 boiler:heat pump split the heat loss then becomes 35.6 kWh per hour. Neither figure is credible, the first is too low, the second is way way too high.

I think without knowing the split for historical data you are on a hide into nothing here. Playing 'what if it was X:Y' games is really just playing with a random number generator.

I wonder, how do you know the oil use for Jan 2026 was 367L? I ask, because accuracy matters here. If you know it was 367L, and you also know how many kWh the heat pump used, and the COP, over the same period (to work out the energy delivered to the house by the heat pump), then you could make a stab at determining the relative split over January 2026. But you are still no closer to a heat loss at design conditions. If you have a mean OAT for your house for Jan 2024, then you might be able to say when the mean OAT is whatever it was, then the mean heat loss is whatever it was. But that is just a single data point. Some may say that is in fact enough, because in fact you do have another data point, the heat loss is zero at a mean OAT of 15.5°C (or thereabouts), and so you can draw a line with a gradient and see what the heat loss is at the design OAT.

It may be worth trying that on an out of interest basis, but I would still be very sceptical of any result. What you need is a range of kWh values (hourly or if not daily) over a range of matching hourly or daily mean OAT values that you can plot to (a) check the plot is credible and if it is then (b) see what the kWh delivered is at the design OAT. As always, the constant IAT requirement applies.

The underlying problem here is the loss of information inherent in using averages. It's a bit like knowing on average parents have 1.5 children. Apart from the fact there is no such thing as a 1.5 child (unless possibly you count some very weird congenital abnormalities), the average tells you nothing about how many children real world parents have. The range of possibilities that could give rise to an average of 1.5 is large, and you have no way of knowing which one reflects reality. In the same way an average for January 2026 loses the very range of KWh / OAT values that just might yield enough information to let you get closer to the real heat loss at design temperatures.                     

Posted by: @rob-of-york

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I'm sure you could easily adapt it to oil usage, assuming that you have a reasonably accurate measure of oil use during that period.

It's an enticing idea, but the problem is knowing the oil usage over the reference period to sufficient accuracy. All I had was a sight gauge on the side of my tank, and yes, I could measure the height of the column, and do the calculations, but I suspect the measurement accuracy of a tape measure held against a column is a very long way from perfect. Over a day, probably not worth doing. Over a month? Perhaps. I no longer have an oil tank, and back when I did, I didn't know how useful it might be in the future to have monthly readings, so I didn't take them. But if someone reading this does have an oil tank and oil central heating, and has the interest, then it is certainly worth a try. 

These days there may also be other devices (ultrasound based? float based?) that can measure oil use reasonably accurately. If such devices exist, and prove accurate, then we may be onto something.  

@cathoderay I started to record oil usage per month on a regular basis about two years ago. Before that it was somewhat haphazard in frequency but everything was accounted for on an irregular cadence. 

I'm first to admit that the accuracy is suspect - it's always difficult with a large tank. However, by measuring the height of the oil with a dipstick on the first of every month, it gave me something. I also found that it was pretty accurate if done carefully. I could cross correlate a tanker delivery (which I assume was a properly calibrated amount) with what I measured, irrespective of the amount of oil in the tank before and afterwards and to within just a few litres.

I didn't bother with a more frequent cadence. Once a day was impossible, especially when the tank was half full. Every week, perhaps more helpful, but a chore and not done.

Obviously, I have no more granularity than monthly so cannot account for cold and warm days and how the oil usage will vary depending upon OAT. All I could do was establish a trend on a monthly cadence.

I do agree though, extrapolating to a shorter cadence if fraught with issues and my earlier experiments don't prove anything conclusive.

I had looked at other methods but none of them are accurate - an electronic gauge which comes with the tank has just ten graduations. I didn't want to spend a fortune on other methods so gave up!