@cathoderay To be fair - the 28kw was coming from an initial enquiry where the company concerned looked up the EPC for the property - on this basis they more or less declined to quote!!

Not quite sure how these EPCs are calculated but given that you can't let a property  if its does not reach a certain level it seems a bit more emphasis should go into making sure they actually mean something.

Regarding calculating true heatloss from OEM - I must admit my data analytical skills with OEM are not to that level yet - so I am more or less just picking points in time - but I think as long as conditions are reasonably stable this should give a reasonable idea.

My next endeavor will be trying to decide if to invest in some Solar PV and battery storage - so a bit more number crunching will be the order of the day. I do have planning permission and LBC for a small array on our garage roof - can't  decide if Its worth the bother to be honest - I guess a subject for a new topic.

Just a bit more context/personal experience on the importance of air tightness - Our previous property was also a G2 listed stone cottage (gluten for punishment). If I compare this current building with our previous house - whilst both more or less the same basic  construction they were like chalk and cheese in terms of comfort during the winter. Regardless of heat source - oil or heatpump this house has always seemed to be a much easier house to heat - the last house was hard work in the winter time - quite Dickensian at times. What's the difference ? the main one I think was the old place had suspended timber ground floor with a gale running beneath. Also remember taking the wall paper off in the bedroom and the plaster below was literally breathing in and out as the wind blew!! The people who bought our old place did a full back to bones renovation - so presume its better now but has probably lost some character in the process.

Here is the SPAB (Society for the Protection of Ancient Buildings) research on measured in situ U-Values vs calculated (spreadsheet standard tables etc) U-Values for old walls (2012, very recent compared to the age of some of the buildings studied!):

From Table 1 on page 14 I see:

COTTAGE, Selborne, Hants (Upper Greensand), 325mm total wall thickness, in situ U-value 1.45 W/m^2K, calculated is spreadsheet U-value 3.02 W/m^2K, Natural sedimentary rock

which is similar to my property (sandstone, 300mm). In the SPAB report, the calculated value is more than twice the measured in situ value. In my installer calculated heat loss, the U-value used for the walls was 2.78 W/m^2K, possibly getting on for double what it may actually be. Wall losses are a major component of heat loss, these U-values matter. 

Posted by: @travellingwave

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I think if you buy a listed building you have to accept the restrictions that come with it

Absolutely. I'm also in a National Park. But it is a beautiful building in a beautiful setting and I am happy to put up with restrictions, that is part of the deal, until they get silly. My planning office does employs some people with shall we say interesting CVs, I believe they find it hard to get the staff these days given the financial cuts, but we got there in the end. The point I was making is you may have to persevere.  

Posted by: @travellingwave

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@cathoderay To be fair - the 28kw was coming from an initial enquiry where the company concerned looked up the EPC for the property - on this basis they more or less declined to quote!!

Not quite sure how these EPCs are calculated but given that you can't let a property  if its does not reach a certain level it seems a bit more emphasis should go into making sure they actually mean something.

But this sort of nonsense is part of the problem! EPCs are little more than collections of random numbers. And I have to say if a company was prepared to quote on the basis of some random numbers, then they would be off my shortlist pronto. But it all feeds into the old buildings and heat pumps don't work well together old wives tale.

Edit: I see now that I have posted the company was responding to an initial enquiry, but even so they came to a conclusion that was out of touch with the reality. The point remains the same, these companies are not doing anyone any favours.

@cathoderay 
thanks for the SPAB link - very interesting - a lot to digest.

Interesting points for me

1. The methodology of heat flux measurement. In fact I was vaguely aware of this in relation to cooling of chocolate. Wasn’t aware it’s application to building materials but seems sensible.

2. Best correlation between calculated and measured tends to be simple wall build ups- I guess kind of obvious that this would be the case but interesting to see real Data. 

3. I presume similar studies have been done on modern building materials and wall make ups- I have read a bit about the performance gap of modern insulation materials - in particular PIR which is very sensitive to workmanship during installation . In this case opposite problem of under estimating heat loss.

Thinking on about possible ways to measure heat loss - I guess a fairly simple method would be to heat a room with direct electric heaters with a controlled temperature set point and measure electrical consumption over a sufficiently long period of steady state. Probably in combination with external temperature measurement and some analytics you mentioned earlier to compensate external variations.

would anyone bother?? But might be fun to try.