Here is an installation we completed nearly six years ago in a 326-sqm solid stone, single glazed Manse in the Scottish Borders, that only had insulation in the loft.
We installed a 22kW MasterTherm UK inverter heat pump on a direct heating design (no buffer or system separation) with internal temperature control. We upgraded the radiators only, and the majority of the radiators are fed by 10mm microbore.
The system works faultlessly, maintaining a constant temperature of 20C within the building. As you can see in the main photo, the outside temperature is -15C, well beyond the duty point design temperature, and it is still operating without a hitch.
An analysis of the bills over the six years has shown that the predicted reduction of 40% over the existing oil system promised at sale has been achieved. This is a substantial energy and carbon saving over the existing technology.
This is how well-designed and installed heat pump installations actually can work. Unfortunately, these results are not common in our industry due to the poor design practices stipulated by most manufacturers and merchants.