Why does an ASHP provide a more comfortable environment than a gas boiler?

Air Source Heat Pumps (ASHPs)

Last Post by AndrewJ 41 minutes ago

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AndrewJ

(@andrewj)

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Topic starter 23/09/2025 1:58 pm

I heard this a lot: having a heat pump creates a home environment that is a consistent warmth throughout, because it's always on, and is a lot more comfortable than the heating provided by a gas boiler which is an on/off system.

With my old boiler, I used to have a schedule set as follows: 05:30 - 23:00 21c; 23:00 - 05:30 19c. It was run from a Hive thermostat which measured the temperature in the living room and requested heat from the boiler as needed. I.e. when the temperature is < 21c then the boiler fires up; when it's >21c then it shuts down (notwithstanding an amount of hysteresis)

I now have a Cosy 9 which has a Primary Pod in the Living Room and an app schedule that is essential 00:00 - 23:59 22c (I may well play with a setback period but it isn't the point of this post.) This primary pod is used by the Cosy 9 controller to call for heat from the Heat Pump as needed. I.e. when the temperature is < 22c then the heat pump fires up; when it's >22c then it shuts down.

In both these cases the heating system is controlled by a thermostat in one room which calls for heat, the implication being that different rooms will experience a different air temperature based on the difference between their heat loss and the living room's heat loss.

My expectation, I guess from a general misunderstanding of what "always on" meant was the following. The house has a heat loss of 7.5kW @-3c and the system has been designed on that basis with individual radiators sized to the installed room's heat loss. The ASHP runs on Weather Control and is always on. Therefore the heat pump is continuously providing hot water around the system at a temperature determined by the outside temp and the requirement to provide enough heat into the house such that it is balanced at the required temperature (22c say) given the heat loss. It does this by adjusting the flow temperature as needed. In this scenario no thermostat is required just some adaptation of the weather curve to suit the particular buildings and actual vs design differences. Each room has a radiator sized for its particular heat loss and thus an overall temperature is maintained across the house (give or take a small amount of 0.Xc degrees.) This "fails" (if it's the right word) at edge cases where the outside temperature is too warm for it to reduce the flow temperature enough so would either turn off or would require an intervention in the house, e.g. opening windows or putting on a jumper.

So, on the basis that it seems the heat pump is also run as an on/off system, how is it actually different to a gas boiler in terms of thermal comfort?

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