On Tuesday, 31 December 2019 11:49:12 UTC, Tim Lamb wrote:
In message ,
tabbypurr writes
On Monday, 30 December 2019 09:27:45 UTC, Tim Lamb wrote:
In message ,
tabbypurr writes
On Sunday, 29 December 2019 22:42:41 UTC, Tim Lamb wrote:
In message ,
tabbypurr writes
On Sunday, 29 December 2019 13:01:26 UTC, Tim Lamb 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% or say 5 hours at Y% flat out.

Sure, a house takes longer to heat up from cold. What point were you making?

From a total gas consumption POV I was wondering if running a boiler for
long periods at low output/high efficiency is better than drawing the
same total heat output but over a shorter period.

sounds like someone isn't grasping efficiency

I'll happily try to understand an explanation. Maths was never a strong
point:-(

I assume boiler efficiency is non linear.

efficiency versus what is nonlinear?

Given that a fixed amount of
energy is required over a 24 hour period, and that boiler losses only
occur during firing, my uncertainty is whether a shorter firing period,
but at reduced efficiency, would use less gas overall.

If you need say 24kWh dumped into the house over the day, you could run the boiler (making up rounded guesstimated efficiency figures) thus:
1. 4kW output for 6hrs at 90% efficiency, thus consuming 24kWh/90% = 26.66kWh
2. 24kW out for 1hr at 80%, thus consuming 24/80% = 30kWh of gas.

In practice the efficiency difference is probably greater than that.
Losses when off are not zero but relatively trivial.


NT

This seems fundamental to matching boiler and set up to the expected
load.

Off must use less gas:-) The house will be kept heated 24 hours anyway.