On 29/12/2019 22:38:46, Tim Lamb wrote:
In message ,
writes
On Sunday, 29 December 2019 13:01:26 UTC, Tim LambÂ* wrote:
Probably much covered but a fresh issue for me:-)

Viessman modulating system boiler. Reasonably well insulated house.

Underfloor heating controlled by 10 (yes 10) thermostats and DHW given
priority.

In the excitement of moving house, I hadn't given much thought to time
clock settings and left 24 hour operation selected. With the relatively
warm Winter weather I have realised that the boiler spends most of the
burn time on the lowest of the 5 available settings.

Does this matter? I can cheat by limiting the CH on periods to provide a
bigger load. Any thoughts?

Low burn is its most efficient mode.

Yes but:-)

24 hours in low burn at X%Â* or say 5 hoursÂ* at Y% flat out.

Assuming the same excess O2 in flue gasses in each case and the same
flow return temperature, the former, as the heat exchanger will be more
efficient with the slower traversal of flue gasses.

That does depend on a good modulating burner.