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

orrery

Cardew wrote: »

The EST have a fixation about standby consumption.

It really is an easy one, this. If the device that you are worried about gets in any way warm on standby, then disconnect it.

Any modern power-brick should be stone cold when disconnected from whatever it powers.

Remember - making lots of small improvements ..... will only ever gain us a small improvement.

tichtich

Mercy wrote: »

I was answering the question in the first post - is it cheaper to leave the heating on all day or words to that effect.

Not really. You were just comparing on-all-day with one particular off-some-of-the-time regime, one which involves wastefully turning the thermostat up to 24 degrees. If you compare two inefficient regimes (as you're doing) then you're only finding the best of a bad bunch, not the best overall.

As HappyMJ suggested, your best bet would be to leave the thermostat on the temperature you actually want, but set the timer to come on an hour before you get home. If one hour turns out not to be enough, increase this warm-up period until it's long enough. However long the warm-up takes there must still remain some interval between switching off (when you go out to work) and the start of the warm-up period. During that interval you will be saving money.

P.S. To be fair, the question at the start of the thread ("Is it cheaper to have heating on all day?") doesn't specify an alternative. Cheaper than what? Sure, it's cheaper than some wasteful alternatives. It's cheaper than burning £5 notes. But the question is sensibly interpreted as comparing on-all-day with the most efficient practical alternative, the one which is generally recommended, i.e. setting the timer to come on a while before you get home. This was the alternative mentioned in MSE's Myth-busting FAQ.

Cardew

malc_b wrote: »

People are again saying is obvious from the physic when it isn't. I explained this in previous post but before I get stamped on let me say:

FACT 1: Heating 24/24 a house loses more heat than 16/24 (or xx/24)
FACT 2: A house losing more heat is NOT the same as using more fuel

Consider a house at 20C with outside at 0C and say it is losing 1kW through the walls etc.. So you have a 1kW fire on and the house stays at 20C. Then you turn the fire off. What happens? Does the inside of the house fall immediately to 0C? Obviously not. So the house is still losing just under 1kW. If we say the heat is off for 8hrs and in that time the temperature drops to 10C inside the loss starts at 1kW and ends up at 500W, call it an average of 750W (it's not a straight line fall so assuming straight line gives ~10% error so close enough for an example). Hence for those 8hrs this house loses (8x750W) 6kWh of energy. This comes from the house structure and has to be replaced when you next heat the house. Hence:

FACT 3: When CH is switched off the house continues to lose heat from its structure for many hours/days. When the CH is switched on this energy debit has to be repaid.

That is the physic of the situation so what does that mean for the CH? Having the CH off longer means that the average house temperature is lower so the total loses are lower. But the shorter you run the heating the harder the CH has to run when it is on as you have to both balance the standing loss and replace the energy in the house structure. which is why longer CH can mean less fuel used which proves fact 2.

The post above has reported just this effect. When the house is heated 24/24 to 16C it is more comfortable at a lower temperature and uses less fuel. The house structure is warmer, no cold walls to create draughts and make you feel colder.

Also, going back to the 1kW house, if heated 24/24 that would use 24kWh/day. Heating 16/24 is a 22kWh loss (16 + 6) which the CH has to supply in 16hr. So the CH has to supply 1375 W over 16hrs rather than 1000W, at 37.5% increase. The loss saving is 8.3% which is the same order as the difference between condensing and non-condensing modes. Plus if the house stays warmer over the 8hrs, i.e. falls to 15C rather than 10C with is more realistic then the 8% falls to 4%.

BTW a radiator (1700W nominal at 80C water) which gives 1000W at 60C water would need to be at 71C to give 1375W.

Firstly taking your 1kW house example.

Using your hypothetical figures for the 8 hours the heating was off, the house 'lost' 6kWh of heat as you state. However if the house remained heated it would have lost 8kWh so it used 2kWh more.

That really is the end of the exercise. The 2kWh represents a 25% saving.

I am afraid IMO after that fact, your conclusions have no merit.

This in particular:

And running CH harder, especially condensing, usually means less efficient

If someone finds it acceptable for a house to be maintained at 16C 24/7 - then good luck to them. HOWEVER even keeping the thermostat at 16C it would still be cheaper if they switched off the heating when house wasn't occupied/people in bed.

John_Pierpoint

Ah well we will see - there are so many people wedded to the idea of a hugely oversized 1960s style boiler, burning "the fuel that obeys you" as the old gas board slogan used to be. If a house can be built to be heated to 20 degrees by a one bar nuclear electric fire, we need to start questioning the need to install any central heating at the cost of (say) £5k in the first place.

When we get down to saving say 25 pence of gas per day - it is time to think of other methods of saving money. However 80% of the homes that will be in use in 2050 have already been built, it is those people who can still save significant amounts by turning off the heating. However they are going to have to get used to increasing discomfort, unless the magic of "fracking" on a crowded island, can find lots of extra cheaper gas. That is what our chancellor "Gideon" seems to think..

Here is a turn of the century design:

http://en.wikipedia.org/wiki/BedZED

HappyMJ

John_Pierpoint wrote: »

Ah well we will see - there are so many people wedded to the idea of a hugely oversized 1960s style boiler, burning "the fuel that obeys you" as the old gas board slogan used to be. If a house can be built to be heated to 20 degrees by a one bar nuclear electric fire, we need to start questioning the need to install any central heating at the cost of (say) £5k in the first place.

When we get down to saving say 25 pence of gas per day - it is time to think of other methods of saving money. However 80% of the homes that will be in use in 2050 have already been built, it is those people who can still save significant amounts by turning off the heating. However they are going to have to get used to increasing discomfort, unless the magic of "fracking" on a crowded island, can find lots of extra cheaper gas. That is what our chancellor "Gideon" seems to think..

Saving 25 pence a day is over £90 a year. Quite a reasonable saving....in my opinion. No-one has ever said the savings are huge it's just that it is cheaper to have some hours in which when heating is not required such as when you are out or are sleeping the boiler is turned off and therefore will be cheaper.

John_Pierpoint

Ah we have someone who keeps his radiators on 365 day a year - chilli billi:D

How about a discussion of installing a bigger tank and turning on the gas to heat it one day in three - or put PV panels or solar thermal panels on your roof and heat it for nothing.

tichtich

malc_b wrote: »

But the shorter you run the heating the harder the CH has to run when it is on as you have to both balance the standing loss and replace the energy in the house structure. And running CH harder, especially condensing, usually means less efficient which is why longer CH can mean less fuel used which proves fact 2.

Boiler efficiency is a complex issue, and it's not clear to me that it favours on-all-day. Even if it does, it seems implausible that this will be enough to outweigh the saving from needing less heat with off-part-of-the-day.

grahamc2003

tichtich wrote: »

Boiler efficiency is a complex issue, and it's not clear to me that it favours on-all-day. Even if it does, it seems implausible that this will be enough to outweigh the saving from needing less heat with off-part-of-the-day.

It doesn't favour on all day imv. As some have previously suspected, there are (reliable, engineering) reports I've been reading saying that most condensing boilers simply don't return much if anything via the release of the latent heat of evaporation when at working temperature. But when initially turned on, with a low return temperature, they condense and make a useful contribution to overall efficiency. So, according to those reports which I am minded to believe, in many cases, the design of most condensing boilers don't really meet the expectations or sales blurb.

'Boiler efficiency is a comples issue' - so you've done the maths too! Yeah, many things involve very complex maths. Surprisingly, one question requiring very simple maths is the topic of this thread! (At least until John introduced heat pumps into the equation, when it then becomes extremely complex again, with abnormal time constants and highly complex control systems which may call on 9kW heating elements during warm up, which may take many hours due to the times constants. In some cases, I doubt even the manufacturer understands his own control system)

malc_b

Cardew wrote: »

Firstly taking your 1kW house example.

Using your hypothetical figures for the 8 hours the heating was off, the house 'lost' 6kWh of heat as you state. However if the house remained heated it would have lost 8kWh so it used 2kWh more.

That really is the end of the exercise. The 2kWh represents a 25% saving.

I am afraid IMO after that fact, your conclusions have no merit.

This in particular:

If someone finds it acceptable for a house to be maintained at 16C 24/7 - then good luck to them. HOWEVER even keeping the thermostat at 16C it would still be cheaper if they switched off the heating when house wasn't occupied/people in bed.

In fact 1 I stated that the house lost less energy which I showed with my 1kW house. The difference was 2kWh as you quote. Over the full day this is a saving of 2kWh from 24kWh so it is not 25% but 8%. Overnight you are saving 100% fuel since the CH is off. You have to consider 24hrs to get the correct answer.

But the interesting fact is that the CH has to run at 140% of 24/24 figure to replace the 6kWh lost during the night. Condensing boilers are less efficient if the water is too hot and the harder you need to drive the radiators the hotter the water has to be.

Also, I said in fact 2 that you might see a fuel saving. You might not but there are plenty of people who have posted on this thread saving they DO see a saving when running 24/24. And there are others who say this is against physics. I'm pointing out that there is a physical model that explains why you might see a fuel saving while actually losing slightly more heat from the house. That would explain the results reported.

malc_b

tichtich wrote: »

Boiler efficiency is a complex issue, and it's not clear to me that it favours on-all-day. Even if it does, it seems implausible that this will be enough to outweigh the saving from needing less heat with off-part-of-the-day.

Implausible does not mean impossible. It's implausible that hot water turns to ice faster than cold water but that is the case (see Mpemba effect).

As I've shown in my 1kW house the difference is 8%. That is true for 1 or 10kW. The controlling fact is how cold the house gets when the heating is off. For a tent 16/24 vs 24/24 is a saving of 1/3 since the moment you turn off the heat in a tent it is the same temperature as the outside (more or less). For a house that is so well insulated it goes from 20C to 19.9C then there is almost zero saving between 24/24 and 16/24.

The 1kW house falling to 10C gives ~8%. It is fell to 15C instead, which is more reasonable, then the saving would be halved to 4%.

As with the Mpemba effect it is no use trusting your gut feel.