Boiler efficiency

[Tim Lamb]

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?

[harry]

A constant low burn will be more efficient than on-off-on-off. If only by eliminating the post and pre burn "purges" of cold air through the boiler.

[tabbypurr]

Low burn is its most efficient mode.

NT

[Tim Lamb]

Yes but:-)

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

[Fredxx]

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.

[tabbypurr]

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

[Tim Lamb]

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.

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

[GB]

Surely, that's what 'most efficient' means? I know nothing about boiler efficiency. This is purely a semantic point.

[Cynic]

Haven't seen any tripe from IMM for a while. This sort of thread used to attract him like a fly to a lump of shit. Maybe someone killed him?

[tabbypurr]

sounds like someone isn't grasping efficiency

[tabbypurr]

Efficiency is useful power out over power in.

NT

[Tim Lamb]

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

I assume boiler efficiency is non linear. 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.

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

[tabbypurr]

efficiency versus what is nonlinear?

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

[Dave Plowman (News)]

Many seem to call something efficient if it simply works OK. And in that respect an ancient boiler could be just as good as the best modern one.

True efficiency (for most) would be getting the same results for the lowest running costs.

[Tim Lamb]

Efficiency related to boiler output.

OK. Are such examples searchable for a particular boiler?

[tabbypurr]

I expect it is.

I've not even looked. I suspect boiler mfrs probably just quote their best possible headline figure and say nowt else. Anyway efficiency is always going to decline as P_out increases for a given heat exchanger.

NT

[harry]

Boiler losses occur all the time the boiler is hotter than the ambient air.

There are several losses from boilers. All need taking into account.

[harry]

Efficiency is a ratio (usually %) of useful energy output divided by energy input.