[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.