I have not fitted a heat pump yet. I have, however researched them. Here are some of the things I have found out:
1) They work well in a new-build - you will be able to design the house, its insulation and radiators / underfloor heating to suit the lower flow temperature of a heat pump.
2) Before selecting a system, do some due diligence on repairability and parts supply. They have one at the stables where we keep our horses in one of their rental houses that has been offline for nearly a year waiting parts making the house unlettable. They are seriously considering ripping it out and fitting a new replacement - at substantial expense - because they cannot get parts for the broken one.
3) There are high flow temperature heat pumps available now - but I haven't found one that quotes efficiency at the higher flow temperature.
4) You will have to get used to a different way of working compared to gas boilers. Most people run the gas boiler when they want heat and have it turned off for big chunks of the day/ night when not needed. Heat Pumps are more effective when run almost continuously.
5) People I have spoken to who have fitted them as part of a planned upgrade including insulation and other elements won't go back to gas.
6) My brother has researched heat pumps as a replacement for an elderly oil fired boiler and concluded that the cost of upgrading insulation and pipework in his 60 year old house combined with the cost of a heat pump makes it untenable for him.
7) When I did my research, ASHPs were 300% to 500% efficient compared to 90% for a typical gas boiler. At our current rates (Octopus), we pay 22.6p/kWh for Electricity and 5.62p/kWh for gas. That means that an ASHP needs to be at least 360% efficient to break even on our tariffs.
8) Average flow temperature ups 37.5oC compared with 70oC for a gas boiler.
| 
