Energy myth-busting: Is it cheaper to have heating on all day?

Ultrasonic

Does this apply equally to all boilers I wonder?



Source: https://www.heatgeek.com/condensing-boilers-efficiency/

richardc1983

Not on a sealed system such as a combi boiler. Your hot water also isn’t stored in your boiler. It gets heated inside and then sent on demand. Your hot water you could turn this down to 50c too and be comfortable, You would never get in a bath or shower at 50c it’s too hot. 

You may find that your small size rads may impact on how low you can go but I would certainly try 55 to 60c… Toy with the idea of running the heating for longer periods at lower flow temps. You may find the other rooms heat up more evenly. All you can do is try it, if it doesn;t work you can go back. When i had my new boiler I put new bigger rads in which double the heat output at the lower flow temps. The boiler is always in condensing mode and tells me that it’s running at efficiency level of 91% since I got it.

Ultrasonic

If the above graph is generally applicable then the unknown for most I'd have thought is how much lower the return temperature would be for a given set temperature. 

The differences don't look huge though if a 20°C drop is a reasonable figure to work with? Efficiency at 70/50 is about 90.5% vs maybe 94% at 60/40 (3.5% increase) and something like 96% for 50/30. The sort of differences that are collectively a good idea environmentally if everyone does them but not changes that are going to make an individual notice a dramatic change in energy bills. If I'm reading this right.

richardc1983

Ultrasonic said:

Does this apply equally to all boilers I wonder?



Source: https://www.heatgeek.com/condensing-boilers-efficiency/

If it’s a condensing boiler the figures will be the same as it’s based on the dew point temperature which is not related to the boiler but an atmospheric factor. E.g water will condense on an ice cold glass of water but the surface of that glass needs to be at the dew point before it will condense etc. 

richardc1983

Ultrasonic said:

If the above graph is generally applicable then the unknown for most I'd have thought is how much lower the return temperature would be for a given set temperature. 

The differences don't look huge though if a 20°C drop is a reasonable figure to work with? Efficiency at 70/50 is about 90.5% vs maybe 94% at 60/40 (3.5% increase) and something like 96% for 50/30. The sort of differences that are collectively a good idea environmentally if everyone does them but not changes that are going to make an individual notice a dramatic change in energy bills. If I'm reading this right.

I find it impossible to get a 20c difference between flow and return. As the rad temp drops the difference between flow and return closes in. I get 10c usually when ticking over. My boiler efficiency I've just posted. You won't be getting a 20c drop when your houses is upto temp as the rads will either be cycling off or the house is warm enough and the boiler clicks off. As this happens the gap closes and your return may be less than 5c. 

Ultrasonic

richardc1983 said:

Ultrasonic said:

If the above graph is generally applicable then the unknown for most I'd have thought is how much lower the return temperature would be for a given set temperature. 

The differences don't look huge though if a 20°C drop is a reasonable figure to work with? Efficiency at 70/50 is about 90.5% vs maybe 94% at 60/40 (3.5% increase) and something like 96% for 50/30. The sort of differences that are collectively a good idea environmentally if everyone does them but not changes that are going to make an individual notice a dramatic change in energy bills. If I'm reading this right.

I find it impossible to get a 20c difference between flow and return. As the rad temp drops the difference between flow and return closes in. I get 10c usually when ticking over. 

So this will mean the potential improvements are even smaller than what I just posted I think.

I'll investigate tomorrow whether my boiler is 'smart' enough to show me the return temperature but from memory I don't think it does.

richardc1983

Not quite, if your boiler is flow at 70 and your return is at 60 or 65c when trundling along and coming on and off, you are not in condensing mode. For you to get in condensing mode you need to get the flow temperature down. So for example if you set it to 50c then even if it was flowing back to the boiler at 48c the boiler will be in condensing mode. It;’s likely that it will be lower and at say 40 or 45c and thus condensing even further. 

Ultrasonic

richardc1983 said:

Not quite, if your boiler is flow at 70 and your return is at 60 or 65c when trundling along and coming on and off, you are not in condensing mode. For you to get in condensing mode you need to get the flow temperature down. So for example if you set it to 50c then even if it was flowing back to the boiler at 48c the boiler will be in condensing mode. It;’s likely that it will be lower and at say 40 or 45c and thus condensing even further. 

As per the graph though, it's not like there is some immediate huge jump in efficiency between not condensing and condensing.

70/60 looks like about 87% efficient vs 60/50 being about 90.5% efficient (and you suggested dropping temperature may reduce the differential so this may be an overestimate). That's a 3.5% difference between the two.

Do you have an answer to my Legionella question?

richardc1983

Ultrasonic said:

richardc1983 said:

Not quite, if your boiler is flow at 70 and your return is at 60 or 65c when trundling along and coming on and off, you are not in condensing mode. For you to get in condensing mode you need to get the flow temperature down. So for example if you set it to 50c then even if it was flowing back to the boiler at 48c the boiler will be in condensing mode. It;’s likely that it will be lower and at say 40 or 45c and thus condensing even further. 

As per the graph though, it's not like there is some immediate huge jump in efficiency between not condensing and condensing.

70/60 looks like about 87% efficient vs 60/50 being about 90.5% efficient (and you suggested dropping temperature may reduce the differential so this may be an overestimate). That's a 3.5% difference between the two.

Do you have an answer to my Legionella question?

Ah yes, I see what you mean now. That’s why you try to get the flow temp as low as you possibly can to get the closest you can get to the max rated efficiency of the boiler of 50 degree flow 30 degree return. 

I answered it on one of the other posts you may not have seen:

Not on a sealed system such as a combi boiler. Your hot water also isn’t stored in your boiler. It gets heated inside and then sent on demand. Your hot water you could turn this down to 50c too and be comfortable, You would never get in a bath or shower at 50c it’s too hot. Now if you had a hot water cylinder it’s different and hot water must be stored at 60c or if not then a weekly cycle where it is raised above 60c for a period to kill anything off. 

You may find that your small size rads may impact on how low you can go but I would certainly try 55 to 60c… Toy with the idea of running the heating for longer periods at lower flow temps. You may find the other rooms heat up more evenly. All you can do is try it, if it doesn;t work you can go back. When i had my new boiler I put new bigger rads in which double the heat output at the lower flow temps.

Ultrasonic

richardc1983 said:

Not on a sealed system such as a combi boiler.

Why is it of zero concern? Whilst it is a sealed system water still comes out, in small amounts when radiators are bled and potentially more significantly if there is a leak or repair work happening. 

Yes I understand it's not an issue for the hot water, which I already have at a lower temperature.

I may experiment with dropping the heating temperature to 60°C, although that probably won't make a big enough difference to confidently detect.