Gas used running a boiler at different temperatures?

Zither

Hi All,

I’ve just had a new boiler installed which, so far, is a phenomenal improvement over my old one and I’m just testing out boiler temp settings...

So a bit of a vague question but are there any ‘rules of thumb’ about how much more expensive it is to run a boiler at 80 degrees compared to 70 compared to 60 compared to 50?

Now I know a truly accurate answer will depend on how efficient the boiler is, whether it’s condensing, how big the house is, how long the boiler is run for etc. But are there any general rules of thumb like ‘running a boiler at 80 for 1h will use 10% more gas than running a boiler at 70 degrees for 1h’ etc? I’m just trying to work out a good balance between immediate heat and cost efficiency.

Thanks!

Ectophile

Probably not the answer you want, but for a condensing boiler to work best, the return temperature of the water going back to the boiler should be around 55C or less.

So, ideally, you'd tweak the boiler's temperature setting to get a return temperature of 55C or less.

northernsoul

I am just doing some tests on running with different boiler temperatures. It’s difficult because the outside temperature makes a lot of difference to the gas used but here are my results.
21/01/18. Start time 11:30, start of test outside temperature 2.5C. End time 14:30, end of test outside temperature 2.7C. Thermostat target set at 19C. Start room temperature 18C. Boiler temp set at 55C. Stat Temp at end of test 18.5C. Units used over 3 hours 1.12m3 units/ £0.33. Gas per hour 0.37m3 units , gas cost per hour £0.11. Note the temperature never reached the target temp set on thermostat.
Next 3 hours
21/01/18. Start time 14:30, start of test outside temperature 2.7C. End time 17:30, end of test outside temperature 3.0C. Thermostat target set at 20C. Start room temperature 18.5C. Boiler temp set at 65C. Stat Temp at end of test 20C. Units used over 3 hours 1.86m3/ £0.55. Gas per hour 0.62m3 , gas cost per hour £0.18. Note the temperature did reach the target temp (actually reached at 16:45) set on thermostat but my opinion is that the boiler continued to come on has much as before it reached temperature, just to maintain the stat temperature.

A further test for the next 3 hours with the boiler set at same 65C and stat target temp now 21C (started from 20C), which it did reach, again was 0.60 m3 (slightly less) per hour, and the same gas cost per hour of £0.18.

A further test of boiler set at 55C and stat target of 21C, outside temp 4.5C (never achieved 21C, only got to 20C) again was similar to above using gas at 0.33m3 per hour and costing £0.10 per hour.

So, roughly £0.11 per hour at 55C, and £0.18 per hour at 65C, about 63% more.

Note that with boiler set at 55C it never reached target temperature within the time set. It has, sometimes, reached the target temp but after many hours when it warmed up the fabric of the house.

Test are still ongoing but at outside temps of 2C and boiler set at 55C, it doesn’t heat the radiators up enough to reach the target temp. For your interest the (balanced to my preferences) radiators heat up between 5 to 10 degrees (at the top) below the boiler temp (so boiler at 65C equals rads at 55 to 60C). Not run boiler at 75C. I am waiting for temps around 2C to compare as near as like for like results.
My return water temperatures are about 15C less than the flow (water coming out of boiler) from the boiler. So boiler set at 65C, return water temp to boiler 50C.

Terraced house in northern England (as said location hence temperatures make a lot of difference, as does insulation and many other things)
Hope this helps

Norman_Castle

Ectophile wrote: »

Probably not the answer you want, but for a condensing boiler to work best, the return temperature of the water going back to the boiler should be around 55C or less.

So, ideally, you'd tweak the boiler's temperature setting to get a return temperature of 55C or less.

Ideally the boiler would measure the return temperature and adjust accordingly or display the return temperature. Why don't they do this?

I recently adjusted mine to 65 degrees to get a lower return temperature. This gives a more constant room temperature than the previous overheating then switching off for long periods while cooling.

Zither

northernsoul wrote: »

I am just doing some tests on running with different boiler temperatures. It’s difficult because the outside temperature makes a lot of difference to the gas used but here are my results.
21/01/18. Start time 11:30, start of test outside temperature 2.5C. End time 14:30, end of test outside temperature 2.7C. Thermostat target set at 19C. Start room temperature 18C. Boiler temp set at 55C. Stat Temp at end of test 18.5C. Units used over 3 hours 1.12m3 units/ £0.33. Gas per hour 0.37m3 units , gas cost per hour £0.11. Note the temperature never reached the target temp set on thermostat.
Next 3 hours
21/01/18. Start time 14:30, start of test outside temperature 2.7C. End time 17:30, end of test outside temperature 3.0C. Thermostat target set at 20C. Start room temperature 18.5C. Boiler temp set at 65C. Stat Temp at end of test 20C. Units used over 3 hours 1.86m3/ £0.55. Gas per hour 0.62m3 , gas cost per hour £0.18. Note the temperature did reach the target temp (actually reached at 16:45) set on thermostat but my opinion is that the boiler continued to come on has much as before it reached temperature, just to maintain the stat temperature.

A further test for the next 3 hours with the boiler set at same 65C and stat target temp now 21C (started from 20C), which it did reach, again was 0.60 m3 (slightly less) per hour, and the same gas cost per hour of £0.18.

A further test of boiler set at 55C and stat target of 21C, outside temp 4.5C (never achieved 21C, only got to 20C) again was similar to above using gas at 0.33m3 per hour and costing £0.10 per hour.

So, roughly £0.11 per hour at 55C, and £0.18 per hour at 65C, about 63% more.

Note that with boiler set at 55C it never reached target temperature within the time set. It has, sometimes, reached the target temp but after many hours when it warmed up the fabric of the house.

Test are still ongoing but at outside temps of 2C and boiler set at 55C, it doesn’t heat the radiators up enough to reach the target temp. For your interest the (balanced to my preferences) radiators heat up between 5 to 10 degrees (at the top) below the boiler temp (so boiler at 65C equals rads at 55 to 60C). Not run boiler at 75C. I am waiting for temps around 2C to compare as near as like for like results.
My return water temperatures are about 15C less than the flow (water coming out of boiler) from the boiler. So boiler set at 65C, return water temp to boiler 50C.

Terraced house in northern England (as said location hence temperatures make a lot of difference, as does insulation and many other things)
Hope this helps

Thanks! That’s some amazing experimentation! Will be interested to hear about your results at 75 degrees too.

7p more between 55 and 65 is fascinating. I didn’t think it would be that much different. Thanks so much for posting! I know there are so many variables to heating a house but really helps to see ‘what sort of difference’ running a boiler at different temps.

macman

It should make no difference. It takes X kwh of energy (a fixed amount) to heat the house from Y Centigrade to Z Centigrade, if you increase the boiler flow temp it will simply do it faster.
The only difference is that if you do it faster, the heat loss will be lower, unless you have 100% efficient insulation.
This does not take into account the greater efficiency of the boiler when running in condensing mode of course.

shortcrust

In my experience the difference is tiny. I had mine at 65 last winter but have had it at 80 this time. There's a massive difference in how quickly the house warms up in the morning but my gas usage is pretty much the same. I've no idea what it cools down to by when it returns to the boiler. I've got one of those infrared temperature guns things so might have a look and report back! The clouds of vapour that pour out of it do suggest there's not a whole load of condensing going on.

Zither

macman wrote: »

It should make no difference. It takes X kwh of energy (a fixed amount) to heat the house from Y Centigrade to Z Centigrade, if you increase the boiler flow temp it will simply do it faster.
The only difference is that if you do it faster, the heat loss will be lower, unless you have 100% efficient insulation.
This does not take into account the greater efficiency of the boiler when running in condensing mode of course.

Yeah I agree - in a perfect system it would use the same amount of gas. I just didn’t know if there were other things that meant, for example, that a boiler might waste more gas the higher the boiler output temp is set at. After getting new boiler I’m on a bit of a budget so keen to save as many pennies (whilst staying as warm) as possible!

Zither

shortcrust wrote: »

In my experience the difference is tiny. I had mine at 65 last winter but have had it at 80 this time. There's a massive difference in how quickly the house warms up in the morning but my gas usage is pretty much the same. I've no idea what it cools down to by when it returns to the boiler. I've got one of those infrared temperature guns things so might have a look and report back! The clouds of vapour that pour out of it do suggest there's not a whole load of condensing going on.

Haha yes I’ve also been wondering about how much vapour these condensing boilers are supposed to create. At 80 mine also kicks out a lot more vapour than my old system! Maybe time to turn it down to 60

Would be v interested how you get on with IR temperature gun - was thinking about doing the same... when I buy one (or I could just spend the money it would cost on actual heating haha.

northernsoul

The problem with a comparison between one year and another, one day and another, is that there are so many variables. Even if outside gets to a temperature of 2/3C (what I am waiting for my next test at boiler temp of 75C), there is wind (which blows into my old house) and sun warming up the fabric of the house (like now). To be somewhere near the same conditions I want 2/3C between 11:30 and 17:30, cloudy (no sun to warm up the house), no wind, and need a room increase of 1/1.5C from 18.5 to 20C (easily to achieve start temp using the heating).
The tests do not compare keeping (maintaining) to the same temperature using boiler temps of 55C and 65C because at 55C it never got to target temp, so was never maintaining the target temperature, unlike at 65C, which did reach target temp and therefore maintained it afterwards.
So difficult to get laboratory conditions at home.

northernsoul

I use a IR temperature gun to measure temps.