Clean, check, and set again
Coil and fan cleaned, condensate clear, refrigerant circuit checked on pressures and temperatures, filters and strainers cleared, system pressure and expansion checked.
A heat pump is only as good as the system it is asked to heat.
It makes a lot of warm water rather than a little hot water. Put that into a house designed around a 70 degree oil boiler and radiators sized for it, and the house is cold and the running cost is worse than what you replaced. That is where most of the bad heat pump stories come from, and none of them are the fault of the unit.
So the survey comes first, the emitters and pipework get assessed honestly, and you are told what needs upgrading before anything is ordered.
They work below zero. Output drops as it gets colder, which is why the design margin matters, but the unit does not stop.
Radiators usually grow. Not always all of them. The survey says which, and it is cheaper to hear it now.
Running cost depends on the design. A well designed system beats oil. A badly designed one loses to it. The difference is the flow temperature.
Some houses are not ready. If yours is one of them we will say so, and quote for the work that would make it ready instead.
Coil and fan cleaned, condensate clear, refrigerant circuit checked on pressures and temperatures, filters and strainers cleared, system pressure and expansion checked.
Usually the flow temperature, the weather compensation curve, or a system that was never balanced. Almost never the thing the owner has been told to replace.
Coil sizing, reheat times and the weekly sterilisation cycle set correctly, so the hot water is not being made by the immersion heater without anybody noticing.