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

malc_b

grahamc2003 wrote: »

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.

I posted this earlier. The file I have is called "10-Energy_consumption.pdf" which I found on the net some while ago (I don't remember where and google can't find it now, the best I have is "www.esru.strath.ac.uk/Courseware/Class-16293/10-Energy_consumption.pdf" which is a reference in another doc but that link no longer works). Inside it says "16293: Environmental Engineering Science 1". The interesting bit is on page 8 where it talks about the fuel factor for night shut down. 24hrs is 1.0, for a heavy building it is 0.85-0.95.

And just so we're clear I do have A level double maths. I also have a BSc (Hons) and I'm CEng (but not in buildings). In my work I use maths and integration. However, with so many unknowns and for easy of understanding IMO it is clearer to make approximations and get rough answers which give us a ball-park figures. The thought experiment I went through came up with an 8% loss which is in the same ball park as the notes.

And, I'll say this again. I have never said that you do not reduce the heat loss from the house by switching off the CH. Above I said "My point is not that there isn't an energy loss saving." You missed the not when quoting what I said. My point is that energy loss saving is NOT necessarily the same as reducing your fuel bill, unless we are talking electric heating.

Boiler efficiency in general is higher if you are light loading it, i.e. you just need to get the radiators hand hot rather than mad hot. A condensing boiler needs cool return water to work in condensing mode. Even a non condensing boiler will give up more energy from the flame to cooler water. The other problem with running mad hot radiators is that the boiler thermostat may cut in and out, short cycling the boiler which again is inefficient.

In many situation there could well be a fuel saving. Telling people that the more they turn off the CH the more they save is not the whole story. My gut feel is that you probably don't get a saving 24 vs 16 but may do 16 vs 4+4 hrs. And more so in winter than spring/autumn. And if you don't have good boiler controls.

This is NOT going against the laws of thermodynamics. Rather, instead of a simplistic approach it is considering boiler efficiency as well. It also has the benefit of explaining why some people have measured fuel savings when running 24hr.

I also like another explanation posted here on dew point and condensation in the walls. That sounds very plausible. My own experience of going away mid winter and coming back to an unheated house is that it takes days of heat before the house feels warm again. I'd put that down to a lot of thermal mass in the house but equally that could also be the fact that this thermal mass was that much larger because of the added water. The house would then have a larger thermal time constant when very cold than at normal temperatures which would fit better with my experience.

malc_b

147718 wrote: »

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

By this I assume you mean the water temperature dial on the boiler and not the hall thermostat?

On any modern system the water max temperature dial on the boiler should be set to max and left there. The only exception would be something like a coal fired system where the boiler is always lit and the thermostat controls the tick-over rate too. Then you'd turn the dial down in spring/summer/autumn to lower the tick-over.

The lower you have the dial set the more likely it will operate and cause the boiler to short cycle. That is the hall thermostat is demanding heat, the water is up to temperature but the house isn't yet warmed so the pump is still running. The boiler stat cuts the burn then 2 minutes later it burns again, briefly, then the boiler stat cuts out again, etc.. That mode of operation is best avoided as it is inefficient.

This applies to modern oil boilers and similar gas boilers. There are modulating gas boilers which might be different. I'm not familiar with those and maybe the dial modulates the burn rather than cutting in and out. It is the cutting in/out you want to avoid.

But the best way to save money would be to fit a predictive controller in place of a simple thermostat. With these you set what temperature you want and when. It works out when to turn on the heat to achieve this, the boiler time clock is set to 24hrs on. So say you want 20C at 7am and it is a warm night. It will ramp up the heating at say 6am. If it is a very cold night it would start at 5am. It also works on set cycle which is more important for oil where each start loses a drop of oil. So oil cycle would be 15 minutes. It works out you need say 50% power and so does a 7.5min burn every 15min. For gas it would use a 5min cycle so do a 2.5min every 5min if it needed 50% power.

Cardew

malc_b wrote: »

This applies to modern oil boilers and similar gas boilers. There are modulating gas boilers which might be different. I'm not familiar with those and maybe the dial modulates the burn rather than cutting in and out. It is the cutting in/out you want to avoid.

Have there been any non-modulating gas boilers in production in the last 30 years?

Cardew

malc_b wrote: »

By this I assume you mean the water temperature dial on the boiler and not the hall thermostat?

On any modern system the water max temperature dial on the boiler should be set to max and left there. The only exception would be something like a coal fired system where the boiler is always lit and the thermostat controls the tick-over rate too. Then you'd turn the dial down in spring/summer/autumn to lower the tick-over.



This applies to modern oil boilers and similar gas boilers. .

Really?

Given all modern gas/oil boilers are condensing, I think you might have some disagreement with that statement

jalexa

Cardew wrote: »

Have there been any non-modulating gas boilers in production in the last 30 years?

Please don't mock my <10 yo non-modulating Vokera which I hope some day will be a 20yo non-modulating Vokera.

BTW I'd quite like a modulating boiler but not at the cost of going condensing.

wantanswers

jalexa wrote: »

Please don't mock my <10 yo non-modulating Vokera which I hope some day will be a 20yo non-modulating Vokera.

BTW I'd quite like a modulating boiler but not at the cost of going condensing.

Funnily enough i've just been reading some articles on Vokera and come across Opentherm Technology, any experience with this technology?

grahamc2003

malc_b wrote: »

And, I'll say this again. I have never said that you do not reduce the heat loss from the house by switching off the CH. Above I said "My point is not that there isn't an energy loss saving." You missed the not when quoting what I said.

In many situation there could well be a fuel saving. Telling people that the more they turn off the CH the more they save is not the whole story.


I also like another explanation posted here on dew point and condensation in the walls. That sounds very plausible. .

1st paragraph - I believe the expression is 'me bad' - yep, I missed the 'not', sorry.

2nd para - Saying 'there could be a saving' is a mssive understatement. Remeber the situation is 8 hours off - that is one hell of a time to counteract by boiler inefficiencies introduced.

It's also a mistake to simply assume the boiler will be less efficient when turned on after being off for 8 hours. That depends on many factors, and I bet there'd be more which are more efficient in that regime than those which are less. Simple assuming a poorer efficiency negates any savings is wrong.

It comes back to what I posted before - you have to assume 'all other things being equal' - even though we know they are not. You have to split reality down into components otherwise you get nowhere (as this thread illustrates). But at least we agree (and in fact will all other c.engs I know who post on here as you would expect) that, all other things being equal, it's cheaper to turn heating off rather than keeping it on - i.e. the myth is a myth. Once that is established, you can then introduce lesser factors like the variable boiler efficiency to layer on top of the understanding so far. My view is the change in boiler efficiency for most boilers under most conditions will not negate (by a wide margin) the typical savings from turning heating off for 8 hours (although both you and I could conceive of circumstances where that wouldn't hold - my contention is they wouldn't be typical).

3rd para - I think the humidity/condensation post was incorrect and formed from a misconception - I was going to reply to that, but atm I don't feel like taking on the replies I'd get!

Just to add a further point - heat loss is a function of temperature, net heat loss (i.e. heat loss less heat gain) is a function of a temperature difference.

oldskoo1

malc_b wrote: »

By this I assume you mean the water temperature dial on the boiler and not the hall thermostat?

On any modern system the water max temperature dial on the boiler should be set to max and left there. The only exception would be something like a coal fired system where the boiler is always lit and the thermostat controls the tick-over rate too. Then you'd turn the dial down in spring/summer/autumn to lower the tick-over.

The lower you have the dial set the more likely it will operate and cause the boiler to short cycle. That is the hall thermostat is demanding heat, the water is up to temperature but the house isn't yet warmed so the pump is still running. The boiler stat cuts the burn then 2 minutes later it burns again, briefly, then the boiler stat cuts out again, etc.. That mode of operation is best avoided as it is inefficient.

This applies to modern oil boilers and similar gas boilers. There are modulating gas boilers which might be different. I'm not familiar with those and maybe the dial modulates the burn rather than cutting in and out. It is the cutting in/out you want to avoid.

But the best way to save money would be to fit a predictive controller in place of a simple thermostat. With these you set what temperature you want and when. It works out when to turn on the heat to achieve this, the boiler time clock is set to 24hrs on. So say you want 20C at 7am and it is a warm night. It will ramp up the heating at say 6am. If it is a very cold night it would start at 5am. It also works on set cycle which is more important for oil where each start loses a drop of oil. So oil cycle would be 15 minutes. It works out you need say 50% power and so does a 7.5min burn every 15min. For gas it would use a 5min cycle so do a 2.5min every 5min if it needed 50% power.

I'm a bit confused about boiler stat / water temp. In your previous post you said you just wanted hand hot radiators not mad hot, but in this reply you said set the boiler stat to max (which would create mad hot water for the rads).

I find the only way to stop cycling in my system is to put the pump on 3 regardless of stat temp setting on boiler.

It would be good for you to clarify the correct boiler temp setting as what you posted seems to contradict unless I understood it wrong.

Btw my boiler is non condensing, non modulating afaik. I'm looking at what setting would potentially use less gas once wall stat is up to temp

Raksha

grahamc2003 wrote: »

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.

It may be the old Living with Technology (T102 30 points) from the Open University - last offered in 1999 - It's an introductory course for those looking to study technology or sciences.

Happy days!

macman

147718 wrote: »

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

Turning the 'stat up does not 'get the house warmer quicker'. All it does is continue to heat the house to the higher temp, thus using much more gas!
All a 'stat does is turn the boiler on or off a selected temp. It doesn't alter the output from the boiler, or heat the house up quicker on a higher setting.
A very basic (and expensive) misunderstanding here.