Overnight Thermostat Setting Thoughts

[Deleted User]

In terms of energy efficiency ( kWh used and £££ cost over the whole day) is it best to set the overnight thermostat unit (I have a wireless, moveable ESI model) setting low enough so that the gas boiler doesn't fire up at all during the night?  I've never tried this before but am considering an overnight thermostat setting of maybe 16C in the bedroom (currently it does go down to maybe 14-15C) and other rooms are invariably lower by 1-2 C here in the NE.

This morning saw me wake up to a very chilly 12.5-13C in most parts of my bungalow with the bedroom a little higher.  Boiler fired up at about 0820 for a couple of hours before it reached 17C (thermostat setting) and that will remain the ambient house temperature for the rest of the day with the CH firing up intermittently going up to a tropical temp of 17.5-18C during the evening.

Currently using 40-50kWh/day gas with present settings.

lohr500

I'm not sure about this either!!

I have our heating set to to 13C overnight on the assumption that the boiler will only kick in if we have very cold external temperatures which end up cooling the house down significantly.

It then raises to 16.5C from 7am so we can wake up to a reasonable warm house.

Looking at the Hive App which tracks the internal temperature, the thermostat has never hit 13C or fallen below throughout the whole of the current cold snap. So an overnight 13C setting seems to work well for us.

I see no point in keeping the house at a higher temperature all night when not needed. Yes, it will use more energy to bring it back from 13 to 16.5 initially, but my gut feel is that this will be less than maintaining 16 to 17 all night.

Petriix

My Netatmo thermostat has an 'auto-adapt' setting where it figures out when to turn the heating on in order to achieve the desired setpoint temperature at the specified time. My partner made me turn this off yesterday because the boiler disturbed her at 4:30am. This morning it was 13.5 degrees in the kitchen when I got up.

Ultimately the lower the temperature gets by the morning, the more you've saved by not heating overnight. 

Olly_J

Petriix said:

Ultimately the lower the temperature gets by the morning, the more you've saved by not heating overnight. 

That's not strictly true, as you have to put most of the energy back in that has escaped overnight to reheat it back to the previous temperature in the morning. Hence what you save is the difference in heat loss per hour at say 18c, compared to the heat loss at 13.5c, which is probably only a few KW, which isn't to say it isn't worthwhile, but there would not be a lot different between having a setback to 16c instead.

The added benefit to having a setback of say 2c is that because you don't need to try and increase the temp of the thermal mass of the property by such a large amount, you can run your boiler at a lower flow temp i.e 50-55, which leads to better condensing efficiency

Petriix

Assuming that your boiler is correctly configured, there are no circumstances in which maintaining a higher temperature (and higher than the outdoor temperature) will use less fuel. It's basic physics. 

BUFF

A lot of this is going to come down to your individual house & heating system.

As an example a recent report found that using a 2C setback in their tests actually resulted in lower boiler efficiency although they were not sure why https://media.nesta.org.uk/documents/UoS_Nesta_Energy_House_Initial_Report_-_FINAL_2022-10-08.pdf

vic_sf49

Guess what experiment I ran this week... yep... different overnight settings on my thermostat.

Outside temp was about the same for each night (-2).

Night one t'stat = 14 degrees. The boiler didn't fire up overnight.

Night two t'stat = 16.5 degrees.  Boiler fired up 7 times, for varying lengths of time. 

After checking timings, temps, and usage (kWh) from my Hive thermostat/app and the Bright App, the cost difference was 15p for the two test periods (1700 - 1000 on consecutive days).  That was 9p gas (0.84kWh) and 6p* electric (0.62kWh).

My bungalow was close to the same temp to start with on both days, but there was a 0.7 degree difference. With hindsight, I'd have got these the same, but as the temps at the end stayed at that 0.7 degree difference, I won't lose sleep over this.

* the only other variable would have been whether my fridge/freezer worked a bit harder one night, and whether my solar panels skewed the results, although both mornings were dark and foggy, and producing nothing to write home about.

I was surprised the resulting usage/costs were so close. Yes I could keep that 15p in my pocket, but the warmer overnight temperature was definitely more comfortable when up and down during the night time. 

Edited to correct some maths. Bright app has changed some figures as I'm looking at this again 2 days later, and has now included some usage that wasn't showing previously.

Gas usage was 8.7kWh / 0.89p more the 2nd night, which is far more logical. Cba working out the electric again.

InvertedVee

Olly_J said:

Petriix said:

Ultimately the lower the temperature gets by the morning, the more you've saved by not heating overnight. 

That's not strictly true, as you have to put most of the energy back in that has escaped overnight to reheat it back to the previous temperature in the morning. Hence what you save is the difference in heat loss per hour at say 18c, compared to the heat loss at 13.5c, which is probably only a few KW, which isn't to say it isn't worthwhile, but there would not be a lot different between having a setback to 16c instead.

The added benefit to having a setback of say 2c is that because you don't need to try and increase the temp of the thermal mass of the property by such a large amount, you can run your boiler at a lower flow temp i.e 50-55, which leads to better condensing efficiency

Very well described, at least for well-insulated homes with occupancy throughout the day.

InvertedVee

BUFF said:

A lot of this is going to come down to your individual house & heating system.

As an example a recent report found that using a 2C setback in their tests actually resulted in lower boiler efficiency although they were not sure why https://media.nesta.org.uk/documents/UoS_Nesta_Energy_House_Initial_Report_-_FINAL_2022-10-08.pdf

Fascinating report, thanks for the link.

Important to note that the Energy House used in the test represents a typical 2-bed end terrace EPC band D with typical UK boiler and basic controls. It says, on p26...

BUFF

As I said "A lot of this is going to come down to your individual house & heating system."

It's a shame that they didn't repeat the experiment with e.g. a Class 4 thermostat rather than a Class 1 & then again ideally with a Class 6 or 7 but obviously resources (time & funding) are an issue.
& it's also a shame that they couldn't work out why setback actually reduced boiler efficiency in their case.

macman

You can prevent the boiler firing during the night just by setting the timer suitably. Mine goes off at 10.30am and on at  about 7am. Most modern programmers/boilers have a frost 'stat setting which will kick (typically at 5C) in if the temp drops to a very low level, but you can adjust this up.