There is a bit of a myth about using ASHPs (or any heat pump) in an old thermally leaky house. The thermal load, is the thermal load, a heat pump does not care why the load is what it is. What it really comes down to is sizing correctly. With a traditional boiler, gas, oil, electric, you work out your maximum load, add a small contingency, they go and buy the nearest size to it, knowing that it can run at full power when it is demanded. A heat pump, and especially an ASHP, do
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