[phicoh]

It is worse for heat pumps. For condensing boilers the return water temperature needs to be low enough, but the system can easily be set to 60 degrees.

For heat pumps, 40 degrees is about the max for efficient operation. So radiators need to be quite a bit bigger compared to what you need for a condensing boiler.

[ZeroGravitas]

It can be set at 60, but it's not at it's full efficiency until 45 degrees.

So if a new system got installed and sized for running at 60, then it's losing 10-20 percent efficiency.

https://www.theheatinghub.co.uk/articles/turn-down-the-boile...

> The correct temperature setting for heating on a combi boiler is 'as low as possible', but as a guide most older homes can run their heating systems at 60°C and newer homes at 50-55°C. They start to reach their very highest efficiency potential at 45°C flow temperature or lower, but this can be too low for older properties.

> Ultimately how low you can turn your flow temperature down will depend on the size of your radiators, how well insulated your home is and your thermostat temperature.

....

> A vast skills and knowledge gap (that dates back to 2005 when condensing gas boilers become mandatory) means 99% of installers do not understand how condensing gas boilers work and therefore cannot set them up to run as they were designed to. Installers have been let down and so have UK households. We have simply not benefited as much as we should have from this leap forward in boiler efficiency.

[phicoh]

Then you are talking about something else. If the return temperature is below 40 degrees then the condensing mode kicks in. This can typically be achieved if the system is set to 60 degrees. Most radiators for this type of system work well with a temperature drop of 20 degrees. Obviously the flow should be adjusted accordingly.

If you go lower, efficiency may increase by tiny amounts, but in most practical setups the system is likely to become unstable and you will lose more than you gain. This is especially true if the system is also used to provide hot tap water. For hot tap water, in particular for a shower, the system needs to be able to heat a lot of water quickly.

[ZeroGravitas]

From the link I provided:

> NB turning down your heating flow temperature is separate to setting your hot water temperature, but we cover that too - see section 11

[phicoh]

Yes, but that does not mean that the actual heater can run in a low power output mode for a sustained period of time.