To think keeping thermostat at 20 will cost the same as keeping it at 17?

[Cigent]

I know it's said that turning the heating down a few degrees will save money, but how?

If I'm, say, October I set the heating to 20 degrees and leave it like that, and if it falls to 19 degrees it automatically kicks in until the house is up to 20 then switches off, how would that cost any more than keeping it at 17 and it kicking in when it dropped to 16 to heat the house one degree? In both cases it's only kicking in to heat the house by one degree, so how does keeping it lower cost less money?

I understand that if I turn it on when the house is at, say 15 degrees, then it would cost more to get to 20 than 17, but once it's there surely it would be the same to keep it there?

F I’m 99% sure you’re wrong but very tired and a bit pissed so just placemarking for some more eloquent mnetter to explain the answer to us both!

The temperature is likely to fall below 20 degrees, so the heating will come on more often. It's less likely to get as cold as 17 degrees so will click on less often.

Because rate of heat loss is dependent on the difference between the inside and outside temperature. The difference between 20degrees and outside temp will be more than between 17 degrees and outside temperature so heat loss will be faster and heating will need to be on for longer to keep it steady!

The cost may be the same but the frequency will be different- it will kick in more often if set at 20 than if set at 17. And therefore cost more.

Because the outside is colder the 20 degree house will loose heat faster than the 17 degree house. So the heating will have to kick in more often to reheat by that one degree. You'll notice it if you listen. The higher you have your thermostat, the more often your boiler kicks in.

There are more days when the natural temperature is sub 20 than days when it’s sub 17. So your heating will kick in way more often if you set it to 20 than to 17.

Also if the natural temperature is say 16 the heating will need to stay on longer to make your house temp reach 20 than to reach 17.

At 20 degrees the heating will come on far
more often, in order for your house to be that warm. At 17 degrees it will come on less frequently and not stay on so long, so will be much cheaper.

Damn. I was with the op until the rest of you rocked up with your common sense.

You're also not accounting for the energy required to maintain at 20degC prior to this. In October, the outside temperature is not going to be 19degC so you need to heat the whole house the extra already.

You aren’t allowing for the different rate of heat loss from the house to the outside.
The greater the difference in temp between the inside and the outside, the steeper the gradient. If the outside air temp is 17C, you
need expend no energy at all to keep your house at 17C - there will be no temp gradient and no heat loss to the exterior.
But if you want to have the house at 20C, the house will (unless hermetically sealed) keep losing heat to the exterior, which you have to burn gas to compensate for.

Your house does not exist in a vacuum. The outside temperature matters. If your house never lost any heat then it wouldn’t make a difference whether you heat 19 degree air to 20 degrees or 16 degree air to 17 degrees but houses do lose heat.

Because the temperature outside will be lower, so the boiler will have to work harder/ longer to get it to a temp.
So if outside is 14, it will click on for maybe half an hour to get it up to 17, compred to an hour for 20 (random times, I don't know). And when the temperature drops, it has to click on sooner for a higher temp, so if it's been 18 outside it might have to click on to raise it to 20, where it wouldnt need to at 17.

In short, yes more expensive, as it on more often & longer.

If only there were some way we could stop houses losing heat......

Unless you keep your heating on all the time, it's going to fall by more than one degree overnight. Say it falls to 15 degrees, it's going to take a lot more energy to get it to 20 than to 17.

Thanks everyone - I do believe I must be wrong, because it's said so frequently, but I still struggle to understand how.

From October - February the outside temperature won't rise above 10 degrees or so, so whether it's 17 or 20 it will be much colder outside, so would it really come on that much more frequently at 20? Would it not be that when the house reached 17 then cooled to 16 it would do say at the same pace as it reaching 20 and cooling to 19?

Why not trust the experts and scientists if you do not understand it yourself?

This is what I do. Just set it to 20 and leave it, so it's 'on' all winter then when the weather gets nicer I just wind it down to 11 so it never clicks on in the summer.

I don't think blind acceptance and unwillingness to learn are particularly good traits.

If you've got a smart metre you could run an experiment usage over 2 weeks at each different temperatures. It wouldn't be completely accurate as you can't control the outside temperature and the relative difference. I don't know the why, but our heater does come on more when the temperature is set higher.

You will save money and energy usage if you have your heating lower during winter.

Now the question is - should you leave it lower during the day and a bit warmer in the evening or just turn it on when you need it.

This is why insulation matters! If your house were perfectly insulated it would stay at 20 degrees. Most U.K. houses are very “leaky”. So when the outside temperature is lower your boiler has to keep working hard to maintain 20 degrees inside, as it’s constantly replacing lost heat/energy. Keeping it to 17 degrees uses less power as the temperature difference isn’t as high.
Of course with proper insulation you’d be warm and use less energy…

That's an excellent idea. I'm going to do that.

It comes on more because it is needed to get to a higher temperature?

And when it reaches that temperature, the gradient to the outside is greater so it loses energy more?

A bit like how your body works to maintain its internal temperature.

I think the explanation will still be the one PPs have said. The higher the temperature differential the faster heat is lost, so the more times the heater comes on.

See, next time a teenager asks why they should study physics as they'll never use it, then you can tell them!