Thanks @old_scientist ! I suppose expanding it is just as easy as that, if the physics of it is linear (which I have no idea about)!

I concur in principle with @old_scientist however there are some points to consider

1. your house has a heat input other than the heating, due to waste heat from electrical appliances the occupants.    Roughly you can assume that your entire electricity consumption is dissipated as heat (and thus contributes to warming the house), and then add 100W per occupant-day (so two occupants each present half the time = 100W).  This will typically work out at 600-1000W which, with a 2.9kW load, becomes significant when extrapolating and needs to be factored in. 

Alternatively its fairly commonly taken to be the case that heat loss is proportional to the difference between OAT and 15.5C, not 20-21, the 4.5C difference accounting crudely for this effect. See degreedays.net for as bit more on this approximation.

2. I would be very wary indeed about relying on such a small data sample.  In practice there is a lot of scatter because of solar gain, wind and the fact that houses take two days or more to 'stabilise'.  Personally I would want several months worth of data and would plot daily consumption vs degree days or average OAT

In conclusion I think you are (almost certainly) right to conclude that ~6kW is an overestimate, but I wouldn't rely on ~3kW without collecting more data.  Also dont forget to think about DHW reheat time.

Thanks @jamespa ! That's great info. We also have an Air-to-Air HP (Mistubishi installed for Air Conditioning mainly). I have been taking readings, but that was the only day I had things set to use exclusively gas and kept the temp steady in the house (as opposed to midday and night setbacks). 

Regarding heating from other sources, it certainly appears to be the case. For example, yesterday it was a pretty constant 13 degrees outside. I turned the gas heating off and from 9am to the following 9am (on 22nd Feb) the house stayed at 21 degrees inside. I just 0.9 kWh for some heating of the open plan ground floor lounge leading up the the end of my 7p/kWh night slot plus running for about an hour before breakfast time.

So, with a temp difference of 8 degrees (without much wind) almost no heat loss in noticeable in 24 hours.

I'll dig into the info at degreedays.net which seems like something I need to know. Thanks for the referral to that!

according to degreedays.net, that 16th date (at Gatwick observation station) was about 14.3 heating degree days for the 15.5 base temperature. The 21st was about 3 HDD.

It does match my memory that if it's generally about 15 degrees outside we would need no heating on.

@sussex-greenish

I like your logic.

Where does the 65.33kWh come from?  I guess your LPG bullet has a flow meter on it and you've looked at the calorific value of propane.

If you're boiler is operating at 37C flow temperature your return temperature must be lower than that so you're well into the condensing zone for burning propane.  Assuming 85% efficiency seems reasonable to me.  Since you're in the condensing zone on your boiler you can use the "higher heating value" for propane.  The higher heating value (sometimes called Gross Heating Value) assumes you're condensing the water produced by combusting hydrocarbons.

https://www.engineeringtoolbox.com/heating-values-fuel-gases-d_823.html

Once you know how much energy you've put into the system, the energy transferred out of the system can be calculated with

Q = U.A.DT

Q = Energy per unit time (power)

U = Heat transfer coefficient

DT = difference between hot and cold

There's a lot of assumptions going on here, like you're house is a sealed box with no draughts.  OR...the draughts though your house never change.  We're also ignoring any solar gain, or extra heat from cooking or living in the house.

If your energy input is over a long enough time period all of the above effects are averaged out.  But you do need to keep a constant DT over the time you're measuring the energy.

If you re-arrange the above equation to

Q / DT = UA

you can calculate UA then use that number in the original equation ( Q = UA.DT ) to calculate the energy per unit time at a different DT.

So long as the DT doesn't change too much, e.g. 18C to 20C and the weather is the same (not huge winds or driving rain) the heat transfer coefficient wont change too much and you can estimate the heat loss.

The big uncertainty in this analysis is "how much energy did I put into the house", which is down to how you got that 65.33kWh number.

Regards

Bob

Hi @bobtskutter ! Thanks for your reply -- I'll have to take some time to digest it, some new concepts in there for me. The kWh figure I arrived at this way:

Read meter 24 hours apart, got gas consumption in cubic metres for that time period: 2.39 cubic metres

Convert to Litres (m³ x 3.85): 9.2 L

Convert to kWh as per value Calor publishes (L x 7.1): 65.33

(Note the calorific value is stated to be 95 MJ/m³)

but an AI gives me a slightly different value (maybe due to Calor rounding that L x 7.1 figure):

To calculate the kWh from 2.39 cubic meters of gas with a calorific value of 95 MJ/m³, we need to follow these steps:

1. Calculate the energy in megajoules:
   2.39 m³ × 95 MJ/m³ = 227.05 MJ

2. Convert megajoules to kilowatt-hours:
   227.05 MJ × 0.2777778 kWh/MJ = 63.07 kWh

Therefore, from 2.39 cubic meters of gas with a calorific value of 95 MJ/m³, you would get approximately 63.07 kWh of energy149.

I think your original calculation was based on propane LIQUID.

You measured propane GAS volume, then converted it to LIQUID volume, then worked out the total energy released.  That's one way to do it, it's not a wrong way or a correct way, it just depends on what data you have for the calorific value.

The differences you're seeing are due to rounding off to different different numbers of decimal places.  Plus there will be a difference between the calorific value of the propane that Calor / FloGas / WhoEver sell (because it's not pure propane) and what you find in a text book (which will be pure propane).

What ever method you use to calculate the heat input into your home, it's going to be inaccurate.  You might be able to measure the volume of gas fairly accurately, but you won't be able to measure the efficiency of your gas boiler, so you assumed 85%.  Try not to get too hung up on the number being 63kWh or 65kWh of propane burnt.  Instead think of it as being around 2kW to keep your house at a constant temperature at the inside / outside temperature on that day.  i.e. it's not zero (you do have some heat loss) and it's not 5, or 10 or 20 - it's a low number so your house appears to loose a small amount of energy at those weather conditions.

Regards

Bob

Ok... I did another one, still not using the DegreeDays concept, but using another snapshot. From 11:55pm on March 3rd to 8:55am on March 4th I used 29.52 kWh of gas. At 90% efficiency that's 26.568 kWh of heat. Timespan is 9 hours so 2.952 kW each hour.

Average OAT was -1 C. IAT 19. So thats 147.6 watts per degree. If we measure to a standard of -2 degrees to +16 degrees (house equilibrium point) that's 18 degrees, so 18 x 147.6 = 2.657 kW. That's my house's heat loss.

Maybe right?

Posted by: @sussex-greenish

↑

Ok... I did another one, still not using the DegreeDays concept, but using another snapshot. From 11:55pm on March 3rd to 8:55am on March 4th I used 29.52 kWh of gas. At 90% efficiency that's 26.568 kWh of heat. Timespan is 9 hours so 2.952 kW each hour.

Average OAT was -1 C. IAT 19. So thats 147.6 watts per degree. If we measure to a standard of -2 degrees to +16 degrees (house equilibrium point) that's 18 degrees, so 18 x 147.6 = 2.657 kW. That's my house's heat loss.

Maybe right?

Quite possibly not, unfortunately.

Houses have a significant heat capacity (thermal mass) which means that measurements over short periods of time can be significantly distorted by the preceding conditions.  Also any particular day can be further distorted by solar gain, wind etc; 2.6kW is small enough to be affected quite significantly by these (if the loss were larger, perhaps less so). 

I would personally not feel comfortable rely on less than several weeks worth of measurements to determine house loss, and I would want to plot them vs degree days or average OAT so that I could get a feel for the scatter and thus likely error in the measurement.

Here is the data I used, measured daily over two years (I have a smart meter so this is relatively easy).  Each point is one period of 24 hours.  If you take the data for 9 degree days (OAT 7.5C - a pretty typical mid season OAT) there is a factor of 2 difference between the lowest consumption day and the highest consumption day.  You don't know which of these you have measured.

For the avoidance of doubt I remain convinced that this is a good method (possibly the best we have) for determining the loss, particularly of properties where the fabric construction isn't uniform (ie much of our housing stock which has been subjected to miscellaneous fabric upgrades of varying effectiveness), but you need enough data to deal with the scatter effectively.