Tim Lamb used his keyboard to write :

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.

Modern boiler have separate temperatures for CH to HW. 65C is the
maximum for efficient condensing, ideally it needs to be less than that
- I have mine set for Max 60C for CH, Max 80C for HW production.


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

OK


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.

Weather compensation and a clever control system, means the boiler
output can go up to the maximum set, but it certainly does not have to
hit the max temperature. It would only heat the water to what ever
temperature it decides is necessary to achieve your desired
temperature.

My system throttles the boiler back, as it near the set temperature, it
predicts the input power needed, so it does not over shoot.

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

What would be the point of that? The boiler needs the outdoor ambient
temperature detector, to improve it output Kw setting, to better match
the houses heat loss.