Heating on all the time cheaper than off at night rumour

ISTR hearing that the story began with experiments on immersion heaters and was erroneously extrapolated to central heating which is, of course, completely different.

Some time ago I carried out experiments both in my home and church heating systems. I concluded that there was very little in it in either case, but that it was slightly cheaper to switch off at night. It also depends, of course, whether you're in all day or just the evenings. You'll just have to experiment yourself. The hard bit is finding two periods with identical outside temperatures.

Chris

The rate of heat loss from a system always increases as the temperature difference between inside and outside increases. In fact it's pretty much proportional to that difference. You therefore MUST always use more energy keeping that temperature difference constantly high than letting it cool for a while and reheating it again as needed.

However with a reasonably well insulated house the net extra cost is very low. If we assume in cold weather an 18 degree C difference between inside and outside when the heat is on, 16 hours of heat on during the day and 8 with it off overnight and a 4 degree total drop in temp of the house overnight (i.e. an average of 2 degrees) the net extra cost of keeping the heating on all the time is only about 4%. That's less than you'd save just turning the thermostat temperature down by 1 degree.

For a poorly insulated house the savings would be greater but still not very much. For the saving to be as much as 10% the house temperature would have to drop by 10 degrees C overnight for which you'd probably have to leave all the windows open.

As with most things where some people think one thing and some another the reason is simple. There's not much difference in the two approaches.

The two biggest influences on heating costs are the temperature you set and the insulation of the house. Turning the heat off at night is negligible in comparison to either of those.

except

Yes - it's a load of tommyrot - obviously more energy used keeping it hot all the time.

Now I did work in an oil refinery where if they let the pipes cool it got very expensive, as the tarry gunge set solid, never to move again

AWEM

It's not valid for them anyway.

You have something which, left to its own devices, loses heat over time. The rate of heat loss will reduce as its temperature gets nearer to that of its surroundings (heat transfer rate depends on difference in temperature).

If left to cool, there is reducing heat loss over whatever time elapses. If kept at same temperature, there is heat loss over the same time, but it's constant (-ish) because the temperature difference does not reduce

- i.e. more heat is lost over the same time.

In both cases, that heat loss has to be made up. That's how much fuel/cost will be involved. More heat lost - more cost.

worse if unheated for periods as the walls get damp from rain etc and so become even worse at insulating until they are dried out again - which takes more heat than if they were kept up to temp (and losing heat) throughout same period - even so still seems a bit "mathematical" to me to be relied on.....

Jim

In article , chris French writes

Of course you are right.

Arguing with idiots is seldom satisfying.

I'm confused. Is that a yes or a no to the original question?

tim

I discovered that in my, very leaky, house if I had the heating off whilst I was out all day (or asleep), when I wanted it back on in order to make the room warm enough, quickly enough, I had to turn the boiler temperature up and that this did use more gas.

So, in order to make the room warm enough when I got home (awake), using the normal boiler temp, I set the heating to come hours before and it still wasn't warm enough. I progressively made the "on" time earlier and earlier until it ended up only being off for about 2 hours in the day and 4 at night.

I gave up at that point and it's now on all day, costing me about half a meter unit a day.

tim

ISTR hearing that the story began with experiments on immersion heaters and was erroneously extrapolated to central heating which is, of course, completely different.

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so what was the result of the immersion heater and in what way was it completely different (I agree that it is, I interested in what the tester thought was the difference)

tim

But switching the heating off overnight doesnt see a house lose much temperature. To get the effect you mention it would need to be off a month or more.

NT

I've heard one reasonable idea for when this might be true - in obscure circumstances.

If when cool the walls drop below the dewpoint they will start soaking up water. Wet walls are not as good an insulator as dry ones.

So if when the heating is off your walls start getting damp, then *and only then* it might be worth leaving the heating on. Or more likely, buying a dehumidifier to keep the mould under control!

Andy

In summary - anyone who says that it's cheaper to leave it on, rather than turn it off and then heat it up again later, is talking nonsense. No matter whether it's an immersion heater, central heating or whatever.

That's what I thought, but it wasn't clear

tim

Depends a bit on the house and its construction. Our heating is off over night from 2300 to about 0600. The start time varies as we have a programmable stat with start optimsation. Temp might get down to 15C on a windy night with temps outside down to -5C.

What we don't do is turn the heating off if we go away for a few days, there is this uninsulated 18" thick solid stone wall right through the middle of the building, this holds a lot of heat and if we let it get cool it takes days, literally, before the place is comfortable again. The air will be warm but 32 tonnes (guess 8 x 8 x .5m) of stone has one helluva thermal mass. Advantge is that it keeps the place cool in summer and warm in winter.

If you can do the maths you will find its always cheaper to turn it off and reheat unless you have a very odd tariff. As it cools it loses less heat so it will always lose less heat over the same period when off than when on. This is true for CH and immersion heaters.

The devil is in the detail, and whether your boiler is markedly more efficient at a nice slow trickle of heat than a full bore blast to get a cold house warm

I did a few calcs on this and by and large if your house has a fairly large thermal mass, there isn't a lot on it either way, if its cardboard and rockwool, then the balance tends to tip in favour of only heating it when you need it, as the warm up time is pretty fast. BUT if yor biler is markedly less efficient at flat out heating the balance even there tips the other way,.

i finally gave up worrying about it.

Well if the boiler was 100% efficient, and constant efficiency, yes.

However that's not the case in the real world.

The other issue is diifferential heating: houses coming up to temperature may have some parts hotter than need be, that will lose more heat, depending where the thermostats are.

Quite apart from any considerations of cost etc., IMO it is bad for the health to be subjected to constant temperature. The human organism needs change in all things...

J^n

Which is what I said.