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

Ultrasonic

JohnSwift10 said:

Northern_Wanderer said:

I set my boiler temp to 55C, the next morning it took 2 hours to warm to 19.5C in the house. That doesn't please me so I tried 60C and it was 1.5 hours to get to temperature. I've set it to 65C but the weather has warmed up now both in the day and overnight so I can't really compare may daily gas readings at this point to see what savings there might be. I may try again in the autumn if I dare use my heating at all by that point.

I would say that whilst my upstairs temps are fine with lower flow temp, my downstairs is struggling meaning I'd have to up the room thermostat which would seem to be counterproductive.

I guess the low boiler temperature 55C might work if you do have your heating all day and all night but not for me. Like others, I don't want to be continuously woken up by the boiler firing up. I also can't be assed fannying around with the thermostat before I go to bed and want it warm when I get up. I suspect you would need a clever (not a fan of the word smart with these things) programmable thermostat if you really wanted to use a set back temperature at night.

In any case, the manual for my boiler says that the condensing mode should kick in if the boiler output temp is set to 70C or below.

I had exactly the same result when I turned mine down as well.
I also, according to my smart meter, used the same amount of gas as the previous day when the boiler temp was higher.

My conclusion, if it takes 50 kWh of gas to heat your house to a certain temperature and maintain it at that temperature for 15 and 3/4 hours a day it will use that amount  regardless of the temperature of the water in the radiators.

I would rather the house heated up quicker and also the house feels more comfortable with a higher boiler temperature.

The part in bold isn't true due to the differences in boiler efficiency discussed above but the differences are so small (~5%) that it will be extremely hard to detect reliably with testing. This is a separate point to the totally valid one about how long it takes to warm the house up though. 

Ultrasonic

Northern_Wanderer said:

I set my boiler temp to 55C, the next morning it took 2 hours to warm to 19.5C in the house. That doesn't please me so I tried 60C and it was 1.5 hours to get to temperature. I've set it to 65C but the weather has warmed up now both in the day and overnight so I can't really compare may daily gas readings at this point to see what savings there might be. I may try again in the autumn if I dare use my heating at all by that point.

I would say that whilst my upstairs temps are fine with lower flow temp, my downstairs is struggling meaning I'd have to up the room thermostat which would seem to be counterproductive.

I guess the low boiler temperature 55C might work if you do have your heating all day and all night but not for me. Like others, I don't want to be continuously woken up by the boiler firing up. I also can't be assed fannying around with the thermostat before I go to bed and want it warm when I get up. I suspect you would need a clever (not a fan of the word smart with these things) programmable thermostat if you really wanted to use a set back temperature at night.

In any case, the manual for my boiler says that the condensing mode should kick in if the boiler output temp is set to 70C or below.

As you suggest, the logical thing to do to counter the slower warm-up time is to have the heating come on earlier but I'm totally with you on not wanting to be woken up early by the heating as a result.

What I'm giving a go today out of curiosity it to have dropped the heating temperature to 40°C after having initially warmed my lounge up quicker with it set to 55°C. I'll see how this goes in terms of maintaining a comfortable temperature during the day as I'll be home for at least most of it. When you were referring to 'fannying about with the thermostat' were you referring to changing the heating hot water temperature like this, or an in-room thermostat? If the latter I'm not quite sure what you had in mind.

richardc1983

Ultrasonic said:

Northern_Wanderer said:

I set my boiler temp to 55C, the next morning it took 2 hours to warm to 19.5C in the house. That doesn't please me so I tried 60C and it was 1.5 hours to get to temperature. I've set it to 65C but the weather has warmed up now both in the day and overnight so I can't really compare may daily gas readings at this point to see what savings there might be. I may try again in the autumn if I dare use my heating at all by that point.

I would say that whilst my upstairs temps are fine with lower flow temp, my downstairs is struggling meaning I'd have to up the room thermostat which would seem to be counterproductive.

I guess the low boiler temperature 55C might work if you do have your heating all day and all night but not for me. Like others, I don't want to be continuously woken up by the boiler firing up. I also can't be assed fannying around with the thermostat before I go to bed and want it warm when I get up. I suspect you would need a clever (not a fan of the word smart with these things) programmable thermostat if you really wanted to use a set back temperature at night.

In any case, the manual for my boiler says that the condensing mode should kick in if the boiler output temp is set to 70C or below.

As you suggest, the logical thing to do to counter the slower warm-up time is to have the heating come on earlier but I'm totally with you on not wanting to be woken up early by the heating as a result.

What I'm giving a go today out of curiosity it to have dropped the heating temperature to 40°C after having initially warmed my lounge up quicker with it set to 55°C. I'll see how this goes in terms of maintaining a comfortable temperature during the day as I'll be home for at least most of it. When you were referring to 'fannying about with the thermostat' were you referring to changing the heating hot water temperature like this, or an in-room thermostat? If the latter I'm not quite sure what you had in mind.

Good test to do, as this is what opentherm control does basically starts off higher then as room temp approaches the set point it backs off on the flow temp. You’ll also have less cycling of the boiler. When the boiler cycles you have unburnt gas in the gas chamber the fan purges it up the flue, this isn’t factored into efficiency ratings but it’s worth considering. 

I think Northern_Wanderer you would find at lower flow temps those noises of the heating system contracting and expanding due to higher flow temps may not wake you up as much with a lower flow temp and using a night set back. It sounds like you have an older type thermostat where you can only set 1 temp at a time possibly. However some people have the boiler in the next room or loft above them and can here the boiler pump and fan etc power up. 

You are right it will take longer to recover to your comfort temp if the flow temp is lowered and the temp of the house drops too much at night, this is why it’s recommended to use night set backs but appreciate this isn’t for everyone. 

Ultrasonic

richardc1983 said:

Good test to do, as this is what opentherm control does basically starts off higher then as room temp approaches the set point it backs off on the flow temp. You’ll also have less cycling of the boiler. When the boiler cycles you have unburnt gas in the gas chamber the fan purges it up the flue, this isn’t factored into efficiency ratings but it’s worth considering. 

I suspect I'll have far more cycling of the boiler actually as I'd normally have the heating switched off for chunks of time whilst I'm at home whereas with the very low flow temperature I'm intending to leave it on all day. 

richardc1983

Ultrasonic said:

richardc1983 said:

Good test to do, as this is what opentherm control does basically starts off higher then as room temp approaches the set point it backs off on the flow temp. You’ll also have less cycling of the boiler. When the boiler cycles you have unburnt gas in the gas chamber the fan purges it up the flue, this isn’t factored into efficiency ratings but it’s worth considering. 

I suspect I'll have far more cycling of the boiler actually as I'd normally have the heating switched off for chunks of time whilst I'm at home whereas with the very low flow temperature I'm intending to leave it on all day. 

True in that respect, you in it in shorter periods to begin with, but still no harm in experimenting is there as may be better if you are at home to leave it on low. 

What boiler do you have, you may be worth finding out it’s minimum turn down. 

Spies

I set mine to 16c fallback but left everything else the same on Weds, I used 10kw more gas than Thu and Fri having changed setback back to 14c, I turned boiler flow temp down for yesterdays test and it saved practically nothing so I'm not convinced my testing method was accurate.

Weds 40kwh, Thu and Fri 30kwh

Ultrasonic

richardc1983 said:

Ultrasonic said:

richardc1983 said:

Good test to do, as this is what opentherm control does basically starts off higher then as room temp approaches the set point it backs off on the flow temp. You’ll also have less cycling of the boiler. When the boiler cycles you have unburnt gas in the gas chamber the fan purges it up the flue, this isn’t factored into efficiency ratings but it’s worth considering. 

I suspect I'll have far more cycling of the boiler actually as I'd normally have the heating switched off for chunks of time whilst I'm at home whereas with the very low flow temperature I'm intending to leave it on all day. 

True in that respect, you in it in shorter periods to begin with, but still no harm in experimenting is there as may be better if you are at home to leave it on low. 

What boiler do you have, you may be worth finding out it’s minimum turn down. 

I checked the manual before doing this but I have a Vaillant EcoTec Pro 28.

richardc1983

Ultrasonic said:

richardc1983 said:

Ultrasonic said:

richardc1983 said:

Good test to do, as this is what opentherm control does basically starts off higher then as room temp approaches the set point it backs off on the flow temp. You’ll also have less cycling of the boiler. When the boiler cycles you have unburnt gas in the gas chamber the fan purges it up the flue, this isn’t factored into efficiency ratings but it’s worth considering. 

I suspect I'll have far more cycling of the boiler actually as I'd normally have the heating switched off for chunks of time whilst I'm at home whereas with the very low flow temperature I'm intending to leave it on all day. 

True in that respect, you in it in shorter periods to begin with, but still no harm in experimenting is there as may be better if you are at home to leave it on low. 

What boiler do you have, you may be worth finding out it’s minimum turn down. 

I checked the manual before doing this but I have a Vaillant EcoTec Pro 28.

Found it. It’s not the best in turns of minimum output of 5.4kw in Non Condensing mode and even worse when in Condensing mode of 6.9kw minimum output. It has a turn down ration of 1:5. So you could even say that at 6.9kw when in condensing mode it’s not able to modulate further. When it’s in non condensing mode it can go down to 5.4kw but then that’s less efficient due to not condensing. You set the output range via the flow temp on the boiler OR via an opentherm or weather compensated controller but I know Vaillant doesn’t have opentherm they have their own version of it which ties you into their controller. 

Ultrasonic

richardc1983 said:

Ultrasonic said:

richardc1983 said:

Ultrasonic said:

richardc1983 said:

Good test to do, as this is what opentherm control does basically starts off higher then as room temp approaches the set point it backs off on the flow temp. You’ll also have less cycling of the boiler. When the boiler cycles you have unburnt gas in the gas chamber the fan purges it up the flue, this isn’t factored into efficiency ratings but it’s worth considering. 

I suspect I'll have far more cycling of the boiler actually as I'd normally have the heating switched off for chunks of time whilst I'm at home whereas with the very low flow temperature I'm intending to leave it on all day. 

True in that respect, you in it in shorter periods to begin with, but still no harm in experimenting is there as may be better if you are at home to leave it on low. 

What boiler do you have, you may be worth finding out it’s minimum turn down. 

I checked the manual before doing this but I have a Vaillant EcoTec Pro 28.

Found it. It’s not the best in turns of minimum output of 5.4kw in Non Condensing mode and even worse when in Condensing mode of 6.9kw minimum output. It has a turn down ration of 1:5. So you could even say that at 6.9kw when in condensing mode it’s not able to modulate further. When it’s in non condensing mode it can go down to 5.4kw but then that’s less efficient due to not condensing. You set the output range via the flow temp on the boiler OR via an opentherm or weather compensated controller but I know Vaillant doesn’t have opentherm they have their own version of it which ties you into their controller. 

I'm afraid I've read what you've written but I don't actually understand what any of that means in practical terms. What are you saying I should or shouldn't try?

I was naively just going by the statement that the max. flow setting range was 30 - 80 °C. Before seeing your post I had been nosing at a bit more of the installation manual and was wondering if the anti-cycling time set may be an issue, or indeed a clue to not try to push this too far.

richardc1983

Ultrasonic said:

richardc1983 said:

Ultrasonic said:

richardc1983 said:

Ultrasonic said:

richardc1983 said:

Good test to do, as this is what opentherm control does basically starts off higher then as room temp approaches the set point it backs off on the flow temp. You’ll also have less cycling of the boiler. When the boiler cycles you have unburnt gas in the gas chamber the fan purges it up the flue, this isn’t factored into efficiency ratings but it’s worth considering. 

I suspect I'll have far more cycling of the boiler actually as I'd normally have the heating switched off for chunks of time whilst I'm at home whereas with the very low flow temperature I'm intending to leave it on all day. 

True in that respect, you in it in shorter periods to begin with, but still no harm in experimenting is there as may be better if you are at home to leave it on low. 

What boiler do you have, you may be worth finding out it’s minimum turn down. 

I checked the manual before doing this but I have a Vaillant EcoTec Pro 28.

Found it. It’s not the best in turns of minimum output of 5.4kw in Non Condensing mode and even worse when in Condensing mode of 6.9kw minimum output. It has a turn down ration of 1:5. So you could even say that at 6.9kw when in condensing mode it’s not able to modulate further. When it’s in non condensing mode it can go down to 5.4kw but then that’s less efficient due to not condensing. You set the output range via the flow temp on the boiler OR via an opentherm or weather compensated controller but I know Vaillant doesn’t have opentherm they have their own version of it which ties you into their controller. 

I'm afraid I've read what you've written but I don't actually understand what any of that means in practical terms. What are you saying I should or shouldn't try?

I was naively just going by the statement that the max. flow setting range was 30 - 80 °C. Before seeing your post I had been nosing at a bit more of the installation manual and was wondering if the anti-cycling time set may be an issue, or indeed a clue to not try to push this too far.

There is nothing you shouldn’t try you just need to experiment and see what works best :