What output was your old boiler and any idea of the ASHP output, also as pointed out the rads shouldn't just warm at the bottom even if the flowtemp is "only" 36C they should be at least lukewarm all over.I live in an old property. I've had internal wall and loft insulation installed. I've had a new ASHP installed last week and my biggest fear seems to be coming true. The house is just really cold. The radiators are warm at the bottom but mostly cold. The temperature in the house won't get above 17.8 degrees and the ASHP is running constantly to achieve that. It's about 8-10 degrees outside currently.
Any tips? (other than putting the old oil boiler back!)
Do the new rads look physically larger than the old, also if you could run with a flow temp of 45C then you will get ~ 33% rad output vs say 20% (of 50deg rad).