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

r2015

So my conclusion is very similar costings for a 12 hr warm house until bedtime .Vs twice a day timed with a cold spell between . As my wife and I are retired will take the warm all day .

I've found that as well, I don't have a gas fire but I leave my heating on all day and off from 23:30 till 08:00.

I also have a smart meter and if I went out at night and switched the heating off, at say, 18:00 and on again at 22:00 when I came home, the amount of gas used between 22:00 and 23:30 was the same as used between 18:00 and 23:30.

Some nights the IHD does not even register any gas usage for an hour.

So now I don't bother turning it off when I go out for a few hours.

richardc1983

Are you all using opentherm boilers with the likes of Nest, Hive or Honeywell Lyric?

System

Beware of Opentherm and Honeywell controls. My experience of both is that when there is a difference in the actual and requested temperature of more than 2C, there will be an Opentherm demand for maximum boiler heat. Honeywell call it their ‘fuzzy logic’. This makes sense when heating first thing in the morning but not if you pop out to the shops for a couple of hours. My gas recordings (15 minute usage) show that it is cheaper to leave the CH on. (other houses, boilers and controls may give a different result)

richardc1983

Hengus wrote: »

Beware of Opentherm and Honeywell controls. My experience of both is that when there is a difference in the actual and requested temperature of more than 2C, there will be an Opentherm demand for maximum boiler heat. Honeywell call it their ‘fuzzy logic’. This makes sense when heating first thing in the morning but not if you pop out to the shops for a couple of hours. My gas recordings (15 minute usage) show that it is cheaper to leave the CH on. (other houses, boilers and controls may give a different result)

All opentherm controllers work on the same principle. If you run at a low flow temp all the time you'll never achieve setpoint. I limit my boiler to 60c max so that when opentherm asks for full output it won't go above 60c, currently 0c outside and opentherm on my Honeywell lyric controller ticking along at 38c house is cosy.

Bricks

malc_b wrote: »

Most people have problems with the idea that the house energy need is so constant but if the house is still warmer than outside it is still leaking heat to outside. Where is that heat coming from? It is from the house structure and eventually you have to pay that heat back.

The big variable here is how well the house is insulated, isn't it?


If it's not well insulated then yes, the heat is leaking away from the thermal mass to the outside whilst the heating is off, and then the boiler has to work hard to 'repay' that to get the temperature quickly back up.


It's different if the house is well insulated though, because in the same time period the thermal mass will have leaked much less energy away and then it's potentially perfectly feasible for the temperature to be brought back up whilst the boiler's in condensing mode.


Everything will also depend on the thermal mass of the house, which can vary widely, depending on its construction and the position of the insulation. A house with internal insulation will have very little thermal mass, for example.

malc_b

Bricks wrote: »

The big variable here is how well the house is insulated, isn't it?


If it's not well insulated then yes, the heat is leaking away from the thermal mass to the outside whilst the heating is off, and then the boiler has to work hard to 'repay' that to get the temperature quickly back up.


It's different if the house is well insulated though, because in the same time period the thermal mass will have leaked much less energy away and then it's potentially perfectly feasible for the temperature to be brought back up whilst the boiler's in condensing mode.


Everything will also depend on the thermal mass of the house, which can vary widely, depending on its construction and the position of the insulation. A house with internal insulation will have very little thermal mass, for example.

Actually the better the insulation the more percentage heat loss at night when the heating is off. Think of tent. The moment you turn the heat the tent is cold so heat lost to the outside falls to zero straight away. If heat the tent 12/24 then cost is 50% of 24/24. Now consider a super insulated house in which the temperature drops next to nothing over night. The heat lost at night (heating off) is same as heat lost during the day (heating on) because the inside to outside temperature is constant. Hence the more you insulate a house the less you save in percentage terms from turning the heat off overnight.


It is possible to design a system or even to by luck end up with a system where the boiler is always in condensing mode. It depends on how much the radiators are oversized. So lets assume 20C drop across the radiators, 75C out from boiler, 55C return, just on the limit for condensing mode. Radiators are 70% efficient now from their spec'ed figure so they need to be 1.4 bigger (100/70). Let's ignore the 5% saving overnight and call the heat loss constant. And assume we heat 16/24, radiators need be 1.5 bigger (24/16), so 2.1 total. Or assume 4+4/24, radiators need to be 3 x bigger, 4.2 overall.


By bigger I mean based on calculating the radiator sizes on the usual assumptions, 20C in, -1C out, no allowance for running cooler, no allowance for running less than 24/24. Obviously if you build those in the X bigger figure is less. But either way you end up with the same size radiator at the end.

Bricks

malc_b wrote: »

Actually the better the insulation the more percentage heat loss at night when the heating is off.

Greater percentage - but of a much smaller overall amount, compared to a badly insulated house, right?


The way I see this, to work out whether it makes sense to keep the heating on for a given time period when it's ok for the house to cool down a bit (so, say, during the working day when people are out, or overnight) is to look at how long in advance you'd need to put the heating on, in order to reach the target temperature at the target time, if the boiler is just functioning in condensing mode. So, in a badly insulated house, it might be that effectively you need to start heating again 15 minutes after people leave the house in the morning, so you might as well just leave it on all day. However in a well insulated house, because it will not cool down a lot and because you are not fighting a high leakage rate, the heating can come back on 30 mins before people get home and bring the temperature up in time.


It might be, in that well insulated house, that were you just to leave the heating on (on a thermostat) all day, then the energy used would not be a lot more - however, it would still be more, so assuming the control system allows it, it still makes sense to turn it off for a period.

Anthorn

There is more than one myth in the debate:
(1) Keeping the heating on all the time is cheaper than turning it off and turning it on again.
(2) If the heating is turned on when we need it and off when we don't need it it's more expensive because we have to heat from cold.

We need some common sense here: If we keep the heating on all the time we keep the property warm all the time but when only heated when we need it means the property gets cold. That latter fact is relevant to certain groups such as the elderly.. But keeping the heating on all the time costs more than only using the heating when we need it. That is pretty much obvious.

Personally, I have the central heating set to a timer and have the main thermostat in the lounge and thermostatically controlled radiators in other rooms such as bedrooms and bathrooms which are on all the time when the central heating is on.

In conclusion and recap,. it's a matter of comfort and also necessity for some people. We can have our homes warm all the time provided we can afford to pay for it

orrery

I can't really see that it makes that much difference (subject to the level of insulation, of course). My house is mainly brick, even the inside walls, so I'm always amazed at how long it takes to heat up the structure again.


After a week's absence in winter, even with the heating on for a whole day prior to return, it is still noticeably cold and the temperature falls significantly again overnight (as the structure still isn't warmed up).


Leaving the heating on for the week would have to boiler coming on infrequently to hold the normal temperature.


So, I'm sure that it is cheaper not to have it on all the time but I doubt it makes much difference for a week's absence (although for longer, it may well be more significant).

richardc1983

Anthorn wrote: »

There is more than one myth in the debate:
(1) Keeping the heating on all the time is cheaper than turning it off and turning it on again.
(2) If the heating is turned on when we need it and off when we don't need it it's more expensive because we have to heat from cold.

We need some common sense here: If we keep the heating on all the time we keep the property warm all the time but when only heated when we need it means the property gets cold. That latter fact is relevant to certain groups such as the elderly.. But keeping the heating on all the time costs more than only using the heating when we need it. That is pretty much obvious.

Personally, I have the central heating set to a timer and have the main thermostat in the lounge and thermostatically controlled radiators in other rooms such as bedrooms and bathrooms which are on all the time when the central heating is on.

In conclusion and recap,. it's a matter of comfort and also necessity for some people. We can have our homes warm all the time provided we can afford to pay for it

What you're not understanding here is that its not on all the time full blast.

Another way to look at it is having a lightbulb on at 10% brightness 12 hours a day will use less energy than a lightbulb on at 100% 6 hours a day.