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

wantanswers

Cardew wrote: »

There is surely no ambiguity in the EST's statement!



The EST very clearly mean 'leaving the boiler switched on 24/7' as opposed to timed.



If we are down to semantics, don't you mean 'Firing all the time' ?

If I was to say to someone leave it 'running all the time' I would guess whatever I was referring to they would..... For example if i said leave the furnace conveyor running all the time they would.

re the EST (BTW who are EST) answer I might have said

however the cost of this is greatly exceeded by the cost of keeping the boiler programmer/controller set to 'On' all of the time...thats assuming they have a programmer of course and On means what it says 'permanently on' so whatever state the thermostat is calling for the boiler will act accordingly.

Both you and I know what a difference a word can make!

Cardew

malc_b wrote: »

As wantanswers has pointed out houses are different. Mine is 5 bed. Also, don't confuse air temperature with house structure temperature. The air in the house changes 1-2 times per hour so naturally that warms up pretty quick. The walls take longer. The walls suck heat out of the air during the day and give it back at night. They are what maintains the temperature during the night when the heating is off. Otherwise inside temperature would be the same as outside at least after 30-60 min when all the air has been changed.

And the figure of 1-2 changes per hour is the recommend value YMMV.

A completely airtight body(house) will still lose heat if the external temperature is lower; or gain heat if the external temperature is higher.

A modern fridge or freezer is extremely well insulated and, effectively, is airtight. Switch off power and the internal temperature will rise and eventually reach that of the room in which it is situated.

Take a kettle of boiling water in a room at 20C.

Turn off the power and the rate of heat loss initialy is greatest when there is a difference of 80C(100C-20C). That rate of heat loss gradually slows until the water and kettle are at 20C when there is no heat loss.

EXACTLY the same principle applies to a house.

Take a house with the internal temperature at 20C and outside temperature at 0C. Turn off the heating and the rate of heat loss is greatest when there is a difference of 20C. As the internal temperature of the house falls, so the rate of heat loss slows.

People seem to be able to appreciate that the laws of thermodynamics apply to a kettle, in that it is wasteful to keep a kettle simmering at boiling point 24/7 because of the high rate of heat loss, and so allow it to cool.

However apparently several people on this thread, do not appreciate that the same law of thermodynamics applies to keeping a house at a constant temperature 24/7 rather than let it cool.

Cardew

wantanswers wrote: »

re the EST (BTW who are EST) answer I might have said

EST = Energy Saving Trust

They are a Government funded organisation to advise on all matters on energy conservation etc.

http://www.energysavingtrust.org.uk/

It would appear that many people on this thread have decided that the EST advice on heating 24/7 against timed, is incorrect. So these people should tell the Government that an organisation they fund is giving incorrect advice and costing the public(who follow the EST's advice) money by increasing their fuel bills.

The EST are a UK organisation, perhaps the proponents of 24/7 heating being cheaper, can produce any statements from any country in the world that supports their theories.

wantanswers

Cardew wrote: »

EST = Energy Saving Trust

They are a Government funded organisation to advise on all matters on energy conservation etc.

http://www.energysavingtrust.org.uk/

Cheers for that link, i've not seen it before.

grahamc2003

malc_b wrote: »

Quote:
Originally Posted by malc_b
My 2p.


]

Here are my latest comments to Malc's post. Probably getting as bit hard to read, but my latest comments are in blue




The only thing you need to know for the myth is that heat loss is a function of temperature. That's all, nothing else at all. It pretty much follows in a single step that the myth is a myth. You are confusing yourself bringing in 'fabric of the house' and other irrelevant factors - just as most others are confusing themselves by bringing in comfort/burst pipes and other varied and wonderful irrelevant factors.

Temperature difference but otherwise I agree. So you tell me what happens at night when the heating switches off? Does the house loss to the outside drop to zero? The temperature inside is still higher than outside so there is still more or less the same heat loss. Not until the inside is the same temperature as the outside is the heat loss zero. So where does the heat energy come from? The structure aka fabric of the house.

Firstly, thanks for replying in the manner I suggested - I thanked your post for that, please don't confuse the thanks as agreeing with you, but i'll try to point out the errors in your understanding - (sorry to be blunt).

Last thing at night, if the room is 20C and outside it's 0C, then when the heating is turned off, the rate of heat loss will start to drop from 1kW, and will continue until the room reaches 0C when heat loss will stop. We assumed a linear relationship in an earlier post. The heat lost comes from the inside of the house. (Incidentally, as an aside, the thermal mass determines the temperature loss for that heat loss - the same heat loss from a high thermal mass will result in a lower temperature loss than the same heat loss from a low thermal mass). I trust we still agree? And both understand the difference between heat and temperature, energy (i.e. heat, measured in KWh) and rate of heating (or power) measured in kW, and that rate of heat loss is a function of the temperature difference (far from a liner function). So far so good.


Could I ask Cardew's favorite question, which is really good in these circumstances - do your maths again using a month with the heating off rather than 8 hours. Do you still think it's cheaper to keep it on all the time rather than turning it off for a month then on again?

The answer to that is obviously it would be cheaper to turn the heating because over a month the house would stabilise to the same temperature as the outside. Over a single night or day that doesn't happen.


This question and your answer should really indicate that your reasoning is wrong. It seems such a knockout question to me I really can't see how you and others seem to so easily dispense with it. Could you (after answering Cardew's further question a couple of posts back) give some explanation as to why it's cheaper to turn it off for a month but not 8 hours? Where is the limit and what are the circumstances you feel change the situation between a month and 8hrs?





For those who like physics we can do a thought experiment. Consider a one room house, inside 20C, outside 0C, losses 1kW. Heat it 24hrs at a constant 20C and it takes 24kWh.

Agreed


So what happens if we turn off the heating 11pm to 7am. What are the house losses at 11:01pm? 1kW since the inside temperature is still 20C.

No, losses will be 1kW-delta, where delta is a very cmall amount. This due to the temperature being 20C-delta.

That's why I said 11.01pm so near enough 20C (19.999 if you want).

Me, you and physics are together on this!



If we assume the house reaches 10C (brrr!) by 7am and take as linear (which is isn't) then average temp overnight is 15C so losses are 1000x15/20 = 750.

Crude, but OKish, we'll say average rate of heat loss is 750W for that period.

We have saved 0.25*8 = 2kWh out of 24kWh or 8.3%. During the night we have used 6kW rather than 8kWh which has come out of the fabric of the house.

OK- You've 'saved' 2kWh of energy compared to heating at 20C all night. It hasn't 'come out of the fabric of the house' - you just haven't burnt that 2kWh of heat which you would have done if you maintained 20C

But with the heating off we haven't burnt 8kWh of gas/oil/elec yet the losses have been 6kWh. Where has that 6kWh come from? It has to have come from the structure of the house.



True, we haven't burnt 8kWh of fuel (being 1kW for 8 hours we would have burnt if left on. We also haven't burnt 100kWh of fuel or 1kWh of fuel. It is irrelevant what we haven't burnt.

The heat lost from the house under our assumptions is 750W (average rate of heat loss) for 8 hours, so yes 6kWh. We still agree


In the morning we have to put that 6kW back as well as providing the 750W-1kW to balancing the losses.

You mean put back in 6kWh (not kW). Yep.

To balance the on going heat losses, you have to supply heat at an average rate of 750W over 8 hours, starting at 500W with the room temp at 10C rising to 1kW after 8 hours when the temperature in our example would be 20C for an average rate of heating of 1kW. (the exact opposite of the cooling cycle in the night). The excess of heating at 1kW over the heat loss (average 750W would supply the 6kWh the house had lost as it cooled (the 'fabric' loss).

Do all the maths and you see that theres a saving in this example of 4kWh, (2 lots of 2kWh, one during cooling, one during heating) being the extra heat loss the house would suffered had the heating been at 20C for those 8 hours at night.

For those following still, I'm sure the comment about taking 8 hours to bring back to 20C isn't acceptable. In practice thermostats will ensure tha average rate of heating is far in excess of 1kW - maybe 10/15kW, and this excess over the heat loss will cause a rapid temperature rise. This means the 2kWh saved during reheating in the example will in practice be much less - say 0.25kWh, making the total saving overall 2.25kWh.

So in this example, with our simplifications, theres a saving of 2.25kWh made by turning the heating off for 8 hours overnight and then quickly reheating the house to 20C the next day compared to keeing the temperature at 20c all through the night.

I think, and hope, this has identified where your reasoning was incorrect. If not, I think I'll back out now - getting too much like hard work (not technically - this is elementary stuff - but pragmatically.).

malc_b

Cardew wrote: »

So we agree that the law of thermodynamics applies for a house left unoccupied for a month; in that it 'obviously' would be cheaper to turn the heating off.

How about a week?

A day?

4 hours?

According to your theory, there must be a point in time where the laws of thermodynamics cease to apply and the laws of malc_b take over.

No. If you actually read what I wrote rather than getting on your hobby horse you'd see that I said turning the heating off over night reduces the heat loss of the house. So I am not disputing the "laws of thermodynamics". The point I made is that the saving for 8hr off in 24 is not 8/24. This is because (more thermodynamics) the house retains heat which is lost during the night. The saving I worked in an example was 8% not 1/3. That could be typical or not. The university course notes I have say it could be a low as 5%.

What I then went on to say is that at that level of saving the efficiency of the boiler comes into play. If you turn off at night but this then means the boiler runs inefficiently then that could negate the 8% saving. I'm not saving it would, just that it is not the cut and dry case that is claimed.

Cardew wrote: »

The analogy to a kettle also applies. For your theory to have any substance you would keep a kettle simmering at boiling point as it would be cheaper than allowing it to cool down and then re-heating the water.

This is only true because kettles are heated with electric with is 100% efficient. There could well be conditions where it might not be true. And in any case it is a silly analogy since you heat a house for long periods and it has a very long lag (days for my house), kettles are boiled infrequently and have a short lag.

Cardew wrote: »

If my house is set for 20C during the day/evening and I left heating on 24/7 but turned the stat down to, say, 12C at 10pm my heating will not have come on by 7am as the house(the room where the thermostat is located) will not have dropped to 12C overnight.

Precisely my point. The house is still leaking heat throughout the night. Where do you think that heat comes from? 1st law of thermodynamics, energy cannot be created or destroyed. You cannot turn the heating off each night and have the house keep its temperature above the outside without there being a source of energy. That would be perpetual motion. All you do is borrow energy from the house fabric during the night. You have to repay that debt in the morning.

In fact its a no win situation. If you live in a tent then there is no energy storage. Turn the heat off overnight and you do save 8/24. But it is so cold you have to run 24hr to be comfortable. If you have a very solid house it keeps warm but you don't save a lot of energy. Hence there may be situations where is uses less energy to run 24hr.

malc_b

grahamc2003 wrote: »

Here are my latest comments to Malc's post. Probably getting as bit hard to read, but my latest comments are in blue




The only thing you need to know for the myth is that heat loss is a function of temperature. That's all, nothing else at all. It pretty much follows in a single step that the myth is a myth. You are confusing yourself bringing in 'fabric of the house' and other irrelevant factors - just as most others are confusing themselves by bringing in comfort/burst pipes and other varied and wonderful irrelevant factors.

Temperature difference but otherwise I agree. So you tell me what happens at night when the heating switches off? Does the house loss to the outside drop to zero? The temperature inside is still higher than outside so there is still more or less the same heat loss. Not until the inside is the same temperature as the outside is the heat loss zero. So where does the heat energy come from? The structure aka fabric of the house.

Firstly, thanks for replying in the manner I suggested - I thanked your post for that, please don't confuse the thanks as agreeing with you, but i'll try to point out the errors in your understanding - (sorry to be blunt).

Last thing at night, if the room is 20C and outside it's 0C, then when the heating is turned off, the rate of heat loss will start to drop from 1kW, and will continue until the room reaches 0C when heat loss will stop. We assumed a linear relationship in an earlier post. The heat lost comes from the inside of the house. (Incidentally, as an aside, the thermal mass determines the temperature loss for that heat loss - the same heat loss from a high thermal mass will result in a lower temperature loss than the same heat loss from a low thermal mass). I trust we still agree? And both understand the difference between heat and temperature, energy (i.e. heat, measured in KWh) and rate of heating (or power) measured in kW, and that rate of heat loss is a function of the temperature difference (far from a liner function). So far so good.


Could I ask Cardew's favorite question, which is really good in these circumstances - do your maths again using a month with the heating off rather than 8 hours. Do you still think it's cheaper to keep it on all the time rather than turning it off for a month then on again?

The answer to that is obviously it would be cheaper to turn the heating because over a month the house would stabilise to the same temperature as the outside. Over a single night or day that doesn't happen.


This question and your answer should really indicate that your reasoning is wrong. It seems such a knockout question to me I really can't see how you and others seem to so easily dispense with it. Could you (after answering Cardew's further question a couple of posts back) give some explanation as to why it's cheaper to turn it off for a month but not 8 hours? Where is the limit and what are the circumstances you feel change the situation between a month and 8hrs?





For those who like physics we can do a thought experiment. Consider a one room house, inside 20C, outside 0C, losses 1kW. Heat it 24hrs at a constant 20C and it takes 24kWh.

Agreed


So what happens if we turn off the heating 11pm to 7am. What are the house losses at 11:01pm? 1kW since the inside temperature is still 20C.

No, losses will be 1kW-delta, where delta is a very cmall amount. This due to the temperature being 20C-delta.

That's why I said 11.01pm so near enough 20C (19.999 if you want).

Me, you and physics are together on this!



If we assume the house reaches 10C (brrr!) by 7am and take as linear (which is isn't) then average temp overnight is 15C so losses are 1000x15/20 = 750.

Crude, but OKish, we'll say average rate of heat loss is 750W for that period.

We have saved 0.25*8 = 2kWh out of 24kWh or 8.3%. During the night we have used 6kW rather than 8kWh which has come out of the fabric of the house.

OK- You've 'saved' 2kWh of energy compared to heating at 20C all night. It hasn't 'come out of the fabric of the house' - you just haven't burnt that 2kWh of heat which you would have done if you maintained 20C

But with the heating off we haven't burnt 8kWh of gas/oil/elec yet the losses have been 6kWh. Where has that 6kWh come from? It has to have come from the structure of the house.



True, we haven't burnt 8kWh of fuel (being 1kW for 8 hours we would have burnt if left on. We also haven't burnt 100kWh of fuel or 1kWh of fuel. It is irrelevant what we haven't burnt.

The heat lost from the house under our assumptions is 750W (average rate of heat loss) for 8 hours, so yes 6kWh. We still agree


In the morning we have to put that 6kW back as well as providing the 750W-1kW to balancing the losses.

You mean put back in 6kWh (not kW). Yep.

To balance the on going heat losses, you have to supply heat at an average rate of 750W over 8 hours, starting at 500W with the room temp at 10C rising to 1kW after 8 hours when the temperature in our example would be 20C for an average rate of heating of 1kW. (the exact opposite of the cooling cycle in the night). The excess of heating at 1kW over the heat loss (average 750W would supply the 6kWh the house had lost as it cooled (the 'fabric' loss).

Do all the maths and you see that theres a saving in this example of 4kWh, (2 lots of 2kWh, one during cooling, one during heating) being the extra heat loss the house would suffered had the heating been at 20C for those 8 hours at night.

For those following still, I'm sure the comment about taking 8 hours to bring back to 20C isn't acceptable. In practice thermostats will ensure tha average rate of heating is far in excess of 1kW - maybe 10/15kW, and this excess over the heat loss will cause a rapid temperature rise. This means the 2kWh saved during reheating in the example will in practice be much less - say 0.25kWh, making the total saving overall 2.25kWh.

So in this example, with our simplifications, theres a saving of 2.25kWh made by turning the heating off for 8 hours overnight and then quickly reheating the house to 20C the next day compared to keeing the temperature at 20c all through the night.

I think, and hope, this has identified where your reasoning was incorrect. If not, I think I'll back out now - getting too much like hard work (not technically - this is elementary stuff - but pragmatically.).

We seem to be on the same page now. I'd make the point in practice what you would do in the morning is to turn on the heat early so that it was back to 20C by 7pm (assuming in bed at 11pm, up at 7am after 8hrs sleep). So you'd save less than 2kWh, say 1.75kWh.

My point is not that there isn't an energy loss saving. Any time the average inside temperature over a day is less (i.e. turning off at night) then the total heat loss over a day is less too. However, because of the latent heat in the house the saving is quite small and in the realms of boiler efficiency variation. It's possible to have situations were the heating is running very inefficiently and struggling because it isn't run for long enough. An example of this would be a condensing system. At low power this is more efficient.

BTW I do turn off my heating overnight but then I have a PID control which will ramp it up steadily in the morning without pushing the boiler into the poor efficiency region. But most don't so I can believe that some people find there system works best if just ticking over 24/7 rather than going flat out twice a day.

Since you like physics I'll throw in a odd ball question - which freezes faster hot water or cold water for an ice cube? The answer is not what you expect since if you were to try various water temperature and measure time to freeze it is not a linear graph where the colder the water the quicker it freezes into a ice cube. Google it :-).

grahamc2003

malc_b wrote: »

We seem to be on the same page now. I'd make the point in practice what you would do in the morning is to turn on the heat early so that it was back to 20C by 7pm (assuming in bed at 11pm, up at 7am after 8hrs sleep). So you'd save less than 2kWh, say 1.75kWh.

My point is not that there isn't an energy loss saving. Any time the average inside temperature over a day is less (i.e. turning off at night) then the total heat loss over a day is less too. However, because of the latent heat in the house the saving is quite small and in the realms of boiler efficiency variation. It's possible to have situations were the heating is running very inefficiently and struggling because it isn't run for long enough. An example of this would be a condensing system. At low power this is more efficient.

BTW I do turn off my heating overnight but then I have a PID control which will ramp it up steadily in the morning without pushing the boiler into the poor efficiency region. But most don't so I can believe that some people find there system works best if just ticking over 24/7 rather than going flat out twice a day.

Since you like physics I'll throw in a odd ball question - which freezes faster hot water or cold water for an ice cube? The answer is not what you expect since if you were to try various water temperature and measure time to freeze it is not a linear graph where the colder the water the quicker it freezes into a ice cube. Google it :-).

Eh? So you agree with me that theres a saving, in the example, of about 2kWh, then the very next sentence you say 'My point is there isn't an energy loss saving'!

grahamc2003

malc_b wrote: »

The university course notes I have say it could be a low as 5%.

.

Just as a matter of interest (and I am genuinely interested), what exactly is the course, and at what University, are the notes taken from?

Heat loss such as the subject of this thread is probably around year 12 school physics, and such a question about the myth could well be posed as an introductory question to relax a candidate at interview for a numerate university course. It certainly would be assumed knowledge at the start of an undergraduate course, along with the underlying 'A' level maths such as integration (which isn't 'averaging'!) necessary to mathematically solve simple heat loss problems.

147718

There are a lot of brains contributing to this debate.
So a related question whilst we have assembled all this knowledge.
Is it more economical to have the gas central heating boiler set at a high temperature and hence getting the house warmer more quickly but maybe be losing some radiant heat from the radiators when they get cold quicker
Or a low boiler setting giving any radiant heat for longer.
I do hope you all have a warm Christmas together with low energy bills