[sokoloff]

A building’s heat loss (the rate of energy needing to be re-added to maintain thermal equilibrium) increases with decreasing outside temperatures and decreases with better insulation.

A heat pump’s max output in heating mode decreases when the outside temperatures are low enough (whereas a gas boiler has a roughly constant max output and it’s quite inexpensive to size a wall-hung combi for 150K BTU/hr [44 kW] if needed).

Buildings that are fine on the coldest design day with a gas boiler may need more output than a heat pump can provide on that day.

Adding insulation can reduce this gap, which is why you’ll often find a heat pump project needs insulation, while a boiler replacement like-for-like would merely benefit from additional insulation.