Combi Boiler 60 Degrees

spookyelle

I've set my combi boiler flow temperature for central heating to 60 degrees (as per Martin's recent advice). I've now found that my living room takes much longer to heat up to the desired temperature (set on a thermometer in the same room) because the radiators don't get as hot. Makes sense, but now I'm wondering how this could possibly be more energy efficient, as the boiler needs to continue pumping hot water (albeit at 60 degrees) to the radiator for much longer in order to get the room up to temperature. Wouldn't it be better to heat the water more, but cut off sooner?

Hope that makes sense!

BUFF

What make/model of boiler do you have? & what controls (programmer, room 'stat, TRVs)?
Your boiler probably starts off at full burn & then after a period modulates down.
Assuming that you have a condensing boiler then the lower the return temperature below the dew point then the better it condenses & the more efficient it becomes (but we are talking a few % not massive amounts) so burns less gas for the same heat transmitted to the heating circuit.

Gross simplification but the only real variable is the extra electricity for the pump running longer.

If your rads were correctly sized for e.g. a 75C flow then your rads are now undersized for a 60C flow which is why your room is taking longer to heat.

Ultrasonic

An important factor is how little power the water pumps in central heating systems use. From a quick Google most seem to be in the 40-60 W range. 

[Deleted User]

The lower the flow temperature, the lower the return temperature, and the greater the boiler efficiency. Yes, undersized radiators may be an issue in some rooms.

Ultrasonic

BUFF said:

If your rads were correctly sized for e.g. a 75C flow then your rads are now undersized for a 60C flow which is why your room is taking longer to heat.

No matter what size the radiators are it will take longer to heat a room with a lower flow temperature, so the fact this happens doesn't specifically indicate a problem. The issue is more if the speed a room heats up becomes too slow for an individual, or if in the coldest weather it's possible to achieve the desired temperature at all or not.

@spookyelle I experimented a bit with dropping the flow temperature to my radiators last winter and ended up with it on 55°C. I tried going as low at 50°C but that was too low for me. Just try 60°C and see how you get on. Don't expect huge efficiency savings though as @BUFF said. How big will depend how high you had your heating flow temperature set previously but it's likely to be < 5%.

QrizB

spookyelle said:

Wouldn't it be better to heat the water more, but cut off sooner?

Probably not. Let's try a short thought experiment.

Imagine that, with a flow temperature of 80C, your radiators can dissipate 12kW. Let's also imagine that it takes an hour to get your house warm at that output, so it supplies 12kWh.

With a flow temperature of 80C your boiler flue gases will not be condensing and so you might achieve 85% efficiency. At 85% efficiency, your boiler will need to burn (12 / 0.85) 14.1kWh of gas in order to supply 12kWh to the house. If gas is 10p/kWh, that will cost £1.41.

Now imagine that we turn the flow temperature down to 60C. Your radiators will be cooler and their output will fall, to 8kW. It will now take 90 minutes to supply the 12kWh your house needs to warm up.

With a flow temperature of 60C your boiler flue gases will condense and you might increase the efficiency to 90%. You will now need to burn (12 / 0.9) 13.3kWh of gas, at a cost of £1.33. You have saved 8p.

But ... you've had to run the boiler for 30 minutes longer. If the electrics of the boiler and its pumps use 100 watts, that an extra 0.05kWh of electricity. At 34p/kWh you've spent an additional 1.7p on electricity. so your saving is more like 6p - still a saving.

k_man

If the longer time to heat the room/house due to lower flow temperature, then causes you to turn the heating on earlier to reach the target at a specific time (and there is a thermostat, rather than just a timer to stop heating), then there will be a slight increase in heat lost from the room/house during the heating up phase.

This will dip into the 6p saving, but how much will depend how well insulated the house is.

k_man

k_man said:

If the longer time to heat the room/house due to lower flow temperature, then causes you to turn the heating on earlier to reach the target at a specific time (and there is a thermostat, rather than just a timer to stop heating), then there will be a slight increase in heat lost from the room/house during the heating up phase.

This will dip into the 6p saving, but how much will depend how well insulated the house is.

Or is this mitigated by the following (although if so it would not take a lot longer to warm up):

BUFF said:

What make/model of boiler do you have? & what controls (programmer, room 'stat, TRVs)?
Your boiler probably starts off at full burn & then after a period modulates down.
Assuming that you have a condensing boiler then the lower the return temperature below the dew point then the better it condenses & the more efficient it becomes (but we are talking a few % not massive amounts) so burns less gas for the same heat transmitted to the heating circuit.

Gross simplification but the only real variable is the extra electricity for the pump running longer.

If your rads were correctly sized for e.g. a 75C flow then your rads are now undersized for a 60C flow which is why your room is taking longer to heat.

Although if so, that complicates @QrizB's sums somewhat.