combi boiler setting

JEN22

What do people do in winter. Do you leave the combi set at say 19 to come on and off all the time or switch it off totally between certain hours?


Whats more economical?

Grenage

It depends on the occupation and insulation. My last place was quite well insulated and we worked 9-5. I left the thermostat at 18C 24/7 and worked out about the same cost as having it on in the morning and evening.

benten69

Been reading about this the last few days as I've just has a new combi fitted & Nest Thermostat put in. Every website and study I have read says leaving it on a low temp all day will cost you more money and use more energy. As the boiler is always coming on and off to maintain temp. No different than setting it at 25*c all day. Once it has initially reached 25*c it will use the same energy to maintain that temp as it would to maintain 20*c all day.

Best and most economical way is to turn it on when you want it, and turn it off when you don't need it, such as being out of the house.

dggar

benten69 wrote: »

........As the boiler is always coming on and off to maintain temp. No different than setting it at 25*c all day. Once it has initially reached 25*c it will use the same energy to maintain that temp as it would to maintain 20*c all day........

.

Interesting idea there. Can you link to any study that backs this up.

Say the outside temp is 15'C
In one case the boiler has to supply energy to maintain a temp difference of 10'C. in the other only has to supply energy to maintain a temp difference of 5'C.

no1catman

Sorry, but I just can't see that that is logical, a higher temp., will cost more.

I leave mine set at 12.5c overnight, so very rarely comes on, comes on for a few hours in the morning - 19.5c, with a standby temp of 17.5c, back up again for a few hours lunch-time at 19.5, down to 17.5 until 5.30pm when back up again till about 10.00pm.
As it gets colder in the winter, then the hours up to 19.5c get longer.

Seems to work, as according to my Energy supplier, we use less gas than comparable semis.

benten69

dggar wrote: »

Interesting idea there. Can you link to any study that backs this up.

Say the outside temp is 15'C
In one case the boiler has to supply energy to maintain a temp difference of 10'C. in the other only has to supply energy to maintain a temp difference of 5'C.

I'll go back through my search history and try to find it for you, but either way the temp difference doesn't matter. Think of it this way

Temp set to 25*c, the house gets to this temp, boiler turns off, temp goes down to 23*c, boiler comes on, then gets to 25*c and goes off, etc, etc, etc.

So it is constantly keeping the same volume of air (house) within a 2*c range and is having to come on and off constantly to do that. The exact same thing happens at 20*c, or 18*c, or even 15*c, assuming the house would be lower than these temps with the heating off anyway.

So having it on constant will cost more money, regardless of temp.

no1catman wrote: »

Sorry, but I just can't see that that is logical, a higher temp., will cost more.

As above, it will cost more to reach the initial higher temp, but not to MAINTAIN that temp. Saying higher temp automatically means more cost is like saying turning the thermostat up will make the house heat faster....it wont.

Grenage

No, it will cost more to maintain a higher temperature. The greater the temperature difference between the inside and out, the more rapid the loss. To consider otherwise is madness.

Alex1983

Benten69 I understand your logic of once it’s up to temperature then maintaining that temperature probably doesn’t make much difference if it 21, 22, 23 or what ever but I think the big factor you may be not thinking about is the outside temperature if it colder outside the house will lose the inside temperature quicker meaning the boiler will work harder to keep the house at the higher set temperature, the bigger the difference the more it will cost.

My opinion is a new modern boiler with low water content heat exchanger heat so quickly that it has to be cheaper to heat the house when you want heat not all the time maintaining a constant temperature. Old fashioned boilers with cast iron heat exchangers is probably a different matter.

I go to hundreds and hundreds of houses a year and nearly every one will have a different opinion on the most economical way of running there heating, I don’t think there will be huge amount of difference in annual running cost weather you use a timer or constant 20 deg. I’m sure people will say I’m wrong and this is the better way and then the next will say it’s better the other, if it worries you that much try it both ways and see which way you use less units but there is so many variables I think you’ll find it hard to get a definite answer.

[Deleted User]

benten69 wrote: »

I'll go back through my search history and try to find it for you, but either way the temp difference doesn't matter. Think of it this way

Temp set to 25*c, the house gets to this temp, boiler turns off, temp goes down to 23*c, boiler comes on, then gets to 25*c and goes off, etc, etc, etc.

So it is constantly keeping the same volume of air (house) within a 2*c range and is having to come on and off constantly to do that. The exact same thing happens at 20*c, or 18*c, or even 15*c, assuming the house would be lower than these temps with the heating off anyway.

So having it on constant will cost more money, regardless of temp.

The power consumption is proportional to the difference between the inside and outside temperature, so for example, if it's 15C outside and it takes 2kW to heat the house to 20C, it will then take 4kW to heat it to 25C. The ratio of the temperature difference to the power is known as the thermal resistance, in this case the overall thermal resistance of the building would be (25-15)/4000 = 0.0025K/W

The 2 degree range you refer to is just the hysteresis on the thermostat, the greater the hysteresis the slower the boiler will switch on and off. The ratio of the on time to the total time is the duty cycle, and this will increase as the power consumed increases. In the example above, if the boiler was 20kW then the duty cycle from the roomstat would be 2/20 = 10% at a temperature of 20C, and 4/20 = 20% at 25C.

[Deleted User]

JEN22 wrote: »

What do people do in winter. Do you leave the combi set at say 19 to come on and off all the time or switch it off totally between certain hours?


Whats more economical?

It's cheaper to switch off. The problem with switching off at night is not that it doesn't save money, it does, the issue is comfort.

When the heating comes back on again the air temperature warms up quickly, but your comfort is dependent on the temperature of the walls too, and they take much longer to warm back up. That's why the house seems to take ages to warm up if you come back after a holiday at Christmas.

If you react to the slow warm up by turning the stat up, then switching off overnight will save you less than you expected.

Strider590

benten69 wrote: »

No different than setting it at 25*c all day. Once it has initially reached 25*c it will use the same energy to maintain that temp as it would to maintain 20*c all day.

This is incorrect, not far wrong, but incorrect none the less.

Your forgetting temperature differential, the larger the difference between the temperature it is at and the temperature it is fighting against (outside temp), the more energy will be used.

If your home is 25C, whilst outside is 5C and you turn the heating off and monitored the temperature, you would see a steep drop from 25C and a much slower drop from 15C down to 5C.

As for OP, as I work 8am til 5pm, I personally find the best way is to have heating on from 4pm until 10pm, then heat just the landing/bedroom with my modified (fitted a DIY digital thermostat with remote sensor) oil filled radiator, from 2am til 6am.
Theory being that I won't be downstairs for long enough to feel the cold from midnight til 5pm.