Depends what you designed the system for. Not enough info.
If it's a condensing boiler, you set it as low as possible which
still heats the house adequately. The perfect temperature is just
low enough so the room thermostat never quite has to switch off.
This will change depending on outside temperature though, and you
will also need to raise it initially to get a cold house up to the
right temperature (the more sophisticted boilers do this adjustment
If it's a non-condensing boiler (which are not legal to fit anymore
except in a few very specific circumstantes), then it should be set
similarly to the above, except the return water temperature must be
at least 60C in order to prevent condensation and corrosion in the
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