In an earlier contribution to this discussion,
405 TD Estate wrote:

While far from an expert I believe condensing boilers work more
efficiently at lower temps though of course your lowest temp is
governed by the coldest room or maybe water temp if not seperate and
not enough for a shower or bath. I think that is why they should be
set as low as possible. If you set the boiler to a lower temp with a
thermostat the water may not be hot enough to heat the room. if you
set the Temp to Max there will be enough heat but you will waste gas.
I would set the boiler temp as low as i could get away with, and even
consider changing it from spring/Autumn (little heat needed) and
winter (lots of heat needed) would keep things as efficient as
possible.

I don't profess to be an expert on condensing boilers, but my understanding
is that many of them will modulate the heat input as necessary by turning
down the flame, depending on the temperature of the return water. This being
the case, the flow temperature will presumably be lower when there is less
heat demand (e.g. in milder weather). If this is so, I'm not exactly sure
what the role is of the boiler stat - but it may perhaps govern the maximum
flow temperature, in which case it would only come into operation
occasionally.
--
Cheers,
Roger
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