Fairly big with a number of rooms with fiddly calculations because of what is next to them. I have done these calculations a few times for myself and loads of times for others.
I didn't include going round measuring up in this (actually that's time consuming) . Doing a rough sketch of each floor takes a few minutes. I then look up the U values for everything and write them down. Once that is done, I can very quickly calculate the loss or gain through each surface of each room as a simple multiplication. These are then added up for each room, factored to allow for boiler temperature etc. Finally the numbers are added to complete the job.
A pocket calculator is useful. Another way is to use a spreadsheet to do the multiplication and summation.
If one is methodical it is pretty easy.
.andy
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