On Tuesday, 3 November 2020 18:32:31 UTC, Tim Lamb wrote:
It is that time of year...

I was a bit taken by surprise when someone recently suggested that
condensing boilers work best at 65 deg. C. My thought was that it does
not leave much room for DHW at 60deg. to avoid bacteria.

My flow and return piping is insulated so there should not be much loss
tank coil to boiler.

The other question I have related to weather compensation. My system
boiler has provision (not fitted) for this but no other arrangement for
reducing the set temperature when running the underfloor heating. The
manifolds have local thermal control valves but it seems daft running
the boiler at 65 when 55 would be ample.

Could I fit a suitable fixed resistor in place of the Weather
compensation detector (assumed to be a thermistor of some sort)?

This weekend is *move the by-pass valve* event!

The purpose of "weather compensation" is to anticipate increased heat load by means of an outside temperature sensor.

So if it get cold outside the heating is boosted before there is any fall in temperature indoors.

The amount of "boost" needed depends on the house construction/insulation.
May or may not be adjustable.