Well as its been chucking it down with rain all day and I've got the flu , I've spent most of my time researching air-source heat pumps , with fairly mixed results . Karenm had previously posted some links (which of course can lead to other links) and with a bit of googling you can end up even more confused than you were to start with , with all the conflicting info . Eventually I found a really good current appraisal of how good(or bad) they are at heating your home and providing you with hot water (link below) , and whether they might or might not be worth investing in , depending on your circumstances . Weather and adequate insulation apart , there are many other factors to consider , particularly that their optimal output temperature is much lower than a normal boiler , so that ideally you should have more/ or larger radiators , larger bore pipes and a faster flow rate , running 24/7 in winter , to get the best out of the system The biggest problem seemed to be that their surveys concluded that most systems in the UK had been incorrectly installed , leading to poor results , so they highly recommended getting a COMPETENT installer ; what chance in Bulgaria then ? Interesting reading though , Tim .http://www.earth.org.uk/air-source-heat-pump.html
Sorry I've gotta feeling that links not gonna work , and anyway you then need to scroll down to their Sources/links - 2011/11/20 NEW Air Source Heat Pumps- Friend or Foe (which is a pdf) ; although there is a lot of other interesting stuff as well .
After quite a lot of looking into this myself there appears to be a fair number of happy users and a fair number of unhappy ones, which ties in with the 'energy trust field trials ' you quoted.
It seems also that the suppliers COP is based in the main on the higher temperatures say 5 and above and there does not appear to be a clarity about performance below 0 and beyond, although there are users with installs that say they are very pleased when it is -15 outside
It is hard though to quantify because when buying something like this you will probably at the same time take steps to improve things like insulation (which is what I intend to do) so the cost benefit then becomes masked somewhat by these other factors.
Having read that thread 900 posts and many links that spun off it I think at this point I would only invest with the aim of using when temps were above 5 OR below that but sunny and I would mount the unit in a sunny location protected from northerly winds.
Installing in this way could see the temperature on a cold -5 day rise ( in the sun) to above 10 degrees which would be very usefull for the heatpump. but before some one says "that will be bad for aircon in summer" човек прави сянка
I am lucky that at the moment I have no aircon so the heatpump cost can be partially offset.
At the end of the day it is living how you want to live also that comes into it, like not having to come back to a cold house after being out all day as the timer will handle things like that.
I would never get rid of the wood burner as I believe this is far better value when the weather gets very harsh.
Among the users on the various forums I read it is the AIR TO AIR installations that in the main seem the happiest and there are more issues with the AIR TO WATER installations
I will report back after my install sometime this month