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We have a combi boiler running on 47kg LPG bottles in our annexe. It's a fairly modern, condensing, job. In the interests of being as using as little gas as possible, I turned down the heating setting on the boiler from the standard flow temp of 68deg to about 55deg.
Now, I'm thinking that this was possibly counter-productive, because with a lower flow temp, the rooms will take longer to warm up, so the boiler will run for longer, so using more gas. Am I right? Should I leave the boiler setting at the standard 68deg?
The annexe is modern and well-insulated, and all the rads have TRVs.
Lots of variables, depending on efficiency of heat exchanger, length of pipe runs etc.
I'd take an empirical approach to monitoring and take it from there.
All depends on wether the boiler is running at 100% duty cycle. If it is able to heat the water up faster than the radiators / HW tank can absorb the heat then it will switch off, leaving just the pump running. It then waits for the water temp to drop below 55 before it fires back up.
The lower the temp you set the more likely it will be on 100%. As you set higher temps, I would expect it will probably switch off more as it can heat the water faster than the radiators can sink the heat.
All depends on your system.
So at 68 it may have a duty cycle of say 5:2 whereas at 55 it might be 5:3 (for on:off).
So at 68 it may have a duty cycle of say 5:2 whereas at 55 it might be 5:3 (for on:off).
Gotcha. So, at 68 it's on less, but using more gas (presumably, given that it's getting the water to a hotter temp?) when it's on, and at 55 it's on more, but using less gas each time?
FWIW, the temperature setting dial on the boiler has a little 'e' for "Efficiency" (according to the handbook) at the 68deg setting, so should I just set it there and leave it to work as designed, and stop trying to be clever...?
I’m open to ridicule, but is there also something about legionnaires? Aren’t hot water supplies supposed to be kept above 60 degrees to kill it off properly?
Gotcha. So, at 68 it’s on less, but using more gas (presumably, given that it’s getting the water to a hotter temp?) when it’s on, and at 55 it’s on more, but using less gas each time?
Does depend wether it can control the burner temp. Our 30 year old boiler is either full burn or off, so all it does it adjust the on/off ratio depending on how many rads are drawing heat.
I’m open to ridicule, but is there also something about legionnaires? Aren’t hot water supplies supposed to be kept above 60 degrees to kill it off properly?
The CH circuit is closed and probably full of Fernox preservative etc, so not really a risk.
I think legionella is only potentially an issue on the dHW side where it is feeding a HW storage tank: it needs to be kept above 60degC then. If instantaneous HW then the legionella risk is negligible.
My old thermodynamics lecturer once claimed it was better to turn up the boiler temperature to maximise radiation - radiated heat can make a room feel much warmer than it is, in the same way that a cold wall can do the opposite.
The e setting is for condensing mode. Basically your boiler can recover some of the energy that would have gone up the flue IF (and its a big IF in some cases) you can get the RETURN TEMP to the boiler below 55degC. Turning the flow temperature down to 68 (marked e) will achieve that in the vast majority of well sized systems. condensing mode - approx. 92-95% efficient, non-condensing ~85% efficient.
If you find that the house isn't warm enough at this flow temperature then you'll have to make the flow temp hotter, potentially increasing the return temp above the magic 55degC, losing about 7-10% of efficiency, or increase your radiator sizes.
Very (like almost non-existent) little chance of legionella issues with a combi boiler, regardless of the settings on the boiler.
Our boiler has a detent and mark on the heating system temperature dial. Reading the manual it suggests keeping the heat at or below this level for maximum economy. It doesn't have a lower limit.
We also installed an external temperature sensor a year after the boiler was installed - and the boiler now seems to modulate the heat it puts out relative to the external temperature, up to where we set that temperature dial.
It has saved some money on bills.
The e setting is for condensing mode. Basically your boiler can recover some of the energy that would have gone up the flue IF (and its a big IF in some cases) you can get the RETURN TEMP to the boiler below 55degC. Turning the flow temperature down to 68 (marked e) will achieve that in the vast majority of well sized systems. condensing mode – approx. 92-95% efficient, non-condensing ~85% efficient.
Completely forgot about this. Ours is too hold to do this.