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?

Imagine if you wanted to keep your house at 35 degrees. Every time the temp dropped to 34 the heating would have to kick in. You can probably imagine that the heating would be coming on a lot because it’d clearly be a lot of effort to maintain a house that hot all the time.

Or if you wanted to keep it at 52 degrees, and set the thermostat to 52. Can you picture that this would take a lot of heating to maintain a 52 degree house in the UK with our outside temperatures? Even if the house was heated up at the start of the season to 52 degrees, and you “just” needed to heat it back up that one degree from 51 to 52 every time it dipped down.

Basically, the more above the standard outside temp you keep your house, the more often the thermostat will kick in. I bet you can imagine it wouldn’t come on very often if you set the winter thermostat to 12 degrees. And I bet you can imagine it would be on a lot if you set it to 52 degrees. So it’s a sliding scale.

Your sarcasm doesn’t make you look clever when you’re wrong. Insulation is hugely important and makes a huge difference to how much is costs to heat your home, but a house is never going to loose no heat whatsoever. Windows, even double glazed, are one weak point. You open the door multiple times a day. It’s a good idea to open the windows a little to stop moisture building up too.
You really wouldn’t want a perfectly insulated house anyway. A perfectly insulated house would require stopping are exchange or air between inside the house and outside. When you put something that breathes in an airtight space they tend to run out of oxygen…

I'll give it a go with made up figures & dodgy science, to illustrate the point. Say your house loses heat at 1 degree per hour if the air temperature is a little colder outside than inside. Increasing the heating by 1 degree costs £1.

You heat your house to 20 degrees and the outside temperature is 18 degrees, after an hour your house temp has fallen to 19 degrees. To get back to 20 degrees costs £1. This cycle repeats over and over, as your house repeatedly loses heat and regains it.

However, if your house temperature is 17 degrees then the heating never kicks in when the air temp is 18 degrees, costing you nothing.

Now imagine the air temperature outside falls to 10 degrees. Maybe your house loses 5 degrees of heat per hour when the outside temperature is 10 degrees below the inside temperature.

So, if your house is heated to 20 degrees, it's now losing 5 degrees per hour, and after an hour it would be down to 15 degrees. But the heating has kicked in and it costs £5 (£1 per degree) to keep it at 20. Of course, this cycle is continual as the house keeps losing heat.

But if your house is heated to 17 degrees, and the outside temperature is 10 degrees. It is still losing heat, but not as rapidly, as the difference is not as great. At this point maybe the house loses 3 degrees per hour. So after an hour it would be down to 14 degrees. So to replace the 'lost' 3 degrees only costs you £3 (this keeps happening too, but the rate of loss is slower, so the cost is a lot less)

@Cigent several people have said the same thing but I see it's not as obvious as you would like it to be - that the rate at which your house losses heat friends on the difference between the temperature inside and the temperature outside. I'll try another way to picture it.

If you hold a bottle of water with the cap off horizontally and tip it up a few degrees to start pouring some of the water out can you understand that the bottle will empty write slowly add you aren't tipping it up at too great an angle. Now picture what would happen if you tipped the bottle up a lot more - can you see that the water would pour out much faster? The water pouring out is really the equivalent of the heat leaving your house -

the more you tip the bottle up,
the greater the angle of tilt,
the greater the rate at which the water leaves the bottle.

the more you turn up the thermostat in the house
the greater the difference between the inside and outside temperature the greater the rate at which heat leaves your house

So - if you want your house to stay at the higher temperature you will have to put heat back in to the house at a higher rate - the thermostat will keep telling the boiler to turn on to put heat back in because hest is being lost more quickly - because you set the thermostat to 20 and not 17.

You turning the thermostat up to 20 is like tipping the bottle up more - the water (or heat) flows out faster.

friends=depends!

Neither is a tendency to ignore science/experts and to believe your own faulty logic. That's why we have anti-vaxxers and flat-earthers.

I'm sorry that probably sounds a bit rude, but if you're not good at a particular field (be it logic, maths, languages, music, DIY, grammar, whatever) then - as a PP said - at least trust those who are.

I'm short, assuming a real-life house that loses heat (rather than a theoretical one that doesn't), your heating will have to work harder to keep your home at 20° than 17°, therefore it will use more energy, therefore it will cost you more.

FFS. I am fairly short, but if course I meant in short.

Oh for a fucking edit button. 😡OF course.

😂 Asking why a piece of logic is wrong is not the same as refusing to believe science. It's asking what the science is, which has now been explained.

But I do understand that some posters cannot help themselves when it comes to spotting an opportunity to be rude and condescending.

Interesting thread

Since the OP has asked about the cost side I would like to add something slightly unintuitive.
You get the biggest percentage savings from turning your thermostat down in autumn and spring. This is because the heating isn't working hard at that time, so 17 to 20 would be maybe half the temperature drop to outside. So your bill would double.
If it's the middle of winter and it's -5c outside, it's going to be super expensive regardless, so that 3 degrees extra, although still costing you similar, it would be negligible in terms of percentage.
So if you're going to bump your thermostat to 25c for a month a year, doesn't matter when you do it but you may as well do it in January when it's cold anyway!

Imagine it as cups of tea instead.

Get 2 cups, with the same volume of water, one at say 60 degrees and one at 80 degrees. Your aim is to keep them at those temperatures by topping up with boiling water from the kettle whenever their temp drops. Do you think you'll use the same amount of water to maintain them at 60 and 80 for, say, an hour, or will you end up using more water to keep the 80 degree one so hot?

I think common sense/experience says you'll need to add a lot more water to the 80 degree one. (If you're not convinced, try it.) You also know from experience that the hotter a cup of tea is, the more heat it gives off. You can physically feel it losing more heat to your hand or the coaster. It's easier to see with an everyday object than with something you're actually inside.

You're tricking yourself with a false comparison that a house at fifteen degrees is roughly the same as a house at nineteen degrees.

More worried as to why you would consider having the heat on in October?
Golden rule in our house, if cold before end of autumn half term holiday, stick a vest on. If really cold, vest, shirt and jumper.

Without adding heat my house will fall below 20 degrees many more times than it would fall below 17 so the heating would kick in more often if the thermostat was set to 20 rather than 17.

Much of the media was critical of the Insulate Britain campaigners but their campaign is a sensible one. The U.K. has some of the worse insulated homes in Europe and a govt should be funding insulating houses with poor insulation. Better for carbon emissions and better for people’s energy bills.

The size of the space that you are heating is also a factor. More energy, and therefore more money, would be needed to regulate the temperature of a large (and probably draughty) space compared to a smaller space.

I live in a place where winter temps will vary from -20 to 5. We have a nice modern heating system in good repair. The house is well insulated and has good windows. At 5, the heating system has no trouble keeping up with whatever temperature we want the house set at. At -20, it struggles to reach the target indoor temperature.

you really cant discount the impact of the outdoor temperature on heat loss from the house.

I put the heating on when it gets cold, which is very much October.

Thanks to everyone for the explanations.

Absolutely this.

Some very good explanations here!

My heating works on the same principle. If I sent it to 17 it would rarely come on at all. So I’d definitely save money. I find though when I changed it from 19 to 18 during the daytime I found it just stayed on longer when it did finally come on, so not sure how much money it saved. I like to be warm, so I’d rather sacrifice other things first than reduce my central heating cost further. I have a log burner so I tend to come down and work next to the fire on my days off when I get very chilly. I am in the office 3 days per week which helps, could up to 4 if I needed to save money as it doesn’t cost me much to get there

Jumping on this to ask - is it very cost effective to switch off a radiator in one of the bedrooms or does it not make much difference?

Everything makes a difference - in this example the energy used to heat the room is effectively funneled into the loop connected to the other radiators (assuming thermostat isn't in the switched off room in question!)

It's not an exact science as radiators aren't 100pc efficient, and the heat will migrate from the hot to the cold room, but if you have ten radiators - and you switch one off - some of the energy you would have used to heat the cold room isn't used

If you do it the other way - only heat the room you are in - the savings can be significant

I thought it meant turn down your target heat by a degree or two, so if you usually aim to have your house at 21 degrees, change to 19 and put a layer on instead?

That's why highly insulated homes use a bespoke ventilation system.
MEV.
PIV.
MVHR.

That's a lot of scienceplaining going on there.....