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As for heating, the rads will have been supplied based on the much higher flow and return temperatures of the oil boiler - so to run them at much lower temperatures needed to make the heat pump even vaguely efficient means they can only provide a relatively small proportion of your heating demand during mild, mid season conditions - so it's a balance point as to where the heat pump goes off and the oil boiler fires. If the heat pump can't output more than the oil boiler return temperature, then it's adding nothing if operating as a parallel system. The alternative is to use the heat pump down to a specific external ambient and then fire up the oil again
It can work, in this fashion, but would be unusual, and I most certainly would not get tied into making any top up payments due to lack of performance, unless the contract works is also looking at controlling the boiler flow and return temperatures and looks to swap out the heat emitters
For the capital costs involved, you'd be better off improving insulation and airtightness
Going by the Sutherland Tables they are still much more expensive to run than oil. I was going to post a link but apparently they charge money to look at their data.
According to the latest energy cost comparison figures (July 20) kerosene is running at about half the cost of ASHP, so as OMS says spend your money on improving insulation instead.
As for your oil system , not every house is super thermal effiicienct and I know lots of people who find stuffy modern houses too much , and have the windows open with the heating on, which has always seemed a bit nuts to me . If you are able to recover the heat in your flue as part of the design , then that should be efficient , even if you dont recover the flue heat , you will need to see how it works in a few winters .
The main problem with air source heat exchangers is in hot climates , where they want cooling , but if you do it , have oil burner serviced /cleaned and try to think where you can put your monitoring points , if you have large wood beams , keep any air flows away from joints as it will dry them out .
its so unusual , you will have to think of it as a venture to look and see, the main thing with oil is that in the future it may treated differently as a fuel , there are some bio ethanol plants and if the biodiesel plant comes along that would replace oil , however undoubtably at 90% efficiency the gas boilers we have now seem to offer the best for wet circulating systems .
If things start getting hot enough in the summer for the British start installing air conditioning, that's when solar PV would come into its own.