One answer here is to sort out insulation - to the point where you don't need any "central heating" - on the few really cold days of the year just use a fan heater. Maybe reducing your Oil Usage by - what? - 90%? might be enough, and just keep the existing until it is defunct. The longer that is the more chance that better Tech will be available ... maybe V2H EV and just "burn" the power when you need heat will be the most cost effective solution in 10 years - when the whole North Sea is covered in Wind Turbines and your car has a 100kWh battery, and you only drive 50 miles a week ...
Passive House energy requirement is of the order of 15 kWh/(m²yr) for heating
For EnerPHit (retrospective upgrade to a property that cannot achieve Passive House) that is 25kWh/(m²yr)
(depending how far North you are within Europe as a whole)
I'd be interested to hear how your current oil fuel usage compares with that.
I think retrofit of existing houses, to those levels, is just plain unaffordable ... Wifee and I built a Passive House extension; main part of the house is basically "1960 poured concrete" and as such already very air tight, and as such probably more easily "upgraded" than other recent-builds, yet we decided it was far too hard.
(In our case no South facing aspect, which the extension does now have, and also a risk that the concrete floors, which contained the heating pipes, might well contain asbestos pipe lagging, and no wish to undertake the effort [
cost + move-out-for-months] that would have been needed to sort that out)
I'm also interested in "how little" can be done to insulation etc. to get worthwhile benefit. Over the first two decades we lived here we filled the cavity, insulated the loft far more, added double glazing, and some other improvements. And then we had damp problems - not the sort that grow mushrooms on the ceiling! but condensation on the inside of windows, and the fact that air quality wasn't great (in the old days people would have just aired-the-house and thrown all the windows open in mid-Winter. An expensive option these days). We could have conquered that by raising the thermostats - but having done all that work the pay-off was that the house was comfortable to live in with same/lower thermostat setting - and commensurate fuel reduction.
So after adding Passive House extension, and discovering how fabulous the air quality was, we retro fitted MVHR to the old part - that took care of the condensation and any poor air quality (the sorts of things where previously air in a corner or a spare bedroom didn't really get circulated was solved by MVHR forcing air into the room - in a controlled fashion - that's far more effective than just living in a draughty building!)
My understanding is that "doing a bit" and then "A bit more" (as many DIY'ers will have done over the decades) doesn't work anything like as well as getting the building up to Passive House or EnerPHit standards. Also building regs have increased recommendations - the loft insulation depth has doubled every decade for the last umpteen decades ... so DIYers have re-done that job (or paid to have it done) numerous times, rather than once and right-first-time and spend-the-money-once.
My experience of Passive House is the absence of draughts. Part of the design calculations is that the inside window glass surface temperature must not get so cold (I think 4C below room temperature) that the air falls (convection) which then causes occupants to sense a draught .. .and wack the thermostat up, or just be uncomfortable. If we go away in mid winter for a weekend and turn everything off the house loses about 1-2C in that time ... if we invite a dozen friends round for an evening meal in November we will likely have to open a window to get rid of the excess heat (better still remember to jack the MVHR up to Max before they arrive, but I seldom remember to do that, nor (much to Wifees chagrin) put the Climate on in the car when I get up from the table in a restaurant ...)
Also once you get the building air tight you can have the benefits of MVHR. Wifee and I have not had a single winter cough / cold in the had dozen years we've had MVHR - even though working in an office with people around us coughing and sneezing all winter (Covid apart)
For anyone not in their forever home I can't imagine why they would spend the money. People say if PV "I might move" ... multiple that cost by 10x and then decide how significant a move might be! House is still the same footprint - adding a conservatory at least increases the footprint, and that is included in any sale price. "Same footprint, but really good insulation" ... not sure that gets you much for a 2nd hand house.
One option would be to get a
PHPP calculation on your house, and then see what various changes might achieve - PHPP allows what-if calculations like "Double the loft insulation", "Add triple glazing", "Wrap the whole house" ...
