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

reeac

justwilliam25 wrote: »

So what's the definitive, laws-of-physics-abiding solution?

Already stated. Have it off when in bed.

malc_b

This is a case of comparing like with like. In the theory modelling I showed how it is possible to save fuel when heating 24/7 due to the inefficiencies of heating at high power with a condensing boiler. This explains why some people (including National Trust BTW) report fuel savings with 24/7 heating. I didn't say heat 24/7. What I said was it is better to heat longer and cooler rather than shorter and hotter. This can be seen from understand the maths.

The example above "Opentherm will gradually build up the boiler temperature to 65C - adjusting to maintain a 20C return difference." is a boiler operating with a return below 45C so well in to condensing which starts about 55C return. You are in the most efficient mode so operate as short as possible. If the weather turns cold I expect Opentherm would start the boiler earlier in the morning rather than increase the boiler to 85C which would be less efficient.

What you can do depends on how the heating system matches the house. Going back to 1970 and before heating design was based on -1C outside, calculate losses, fit radiators to balance this heat loss. The radiators would be working at 80C (85C out, 75C return), 60C above ambient of 20C. Now that house is likely to be double glazed, cavity wall filled and the roof insulated. The radiators have less demand on them so they can work cooler allowing the a boiler to be run in condensing mode. A modern house should be better still as radiator design temperature is lower today. And winters are warmer.

If you can run in condensing mode all the time then run shorter. If you start to need to run hot then running longer is likely to save more fuel than running shorter.

orrery

malc_b wrote: »

The example above "Opentherm will gradually build up the boiler temperature to 65C - adjusting to maintain a 20C return difference." is a boiler operating with a return below 45C so well in to condensing which starts about 55C return. You are in the most efficient mode so operate as short as possible.

I'm afraid that is simply too esoteric for most people with most systems. A boiler with a single contact demand simply won't behave like that. In most systems, the return temperature will continue to rise beyond that which is efficient and the boiler will modulate to keep the flow temperature from rising.

The most important factor in maintaining condensing mode is having a good controller which will start to run in proportional mode (turning the boiler off for ever extending periods) as the room temperature approaches the target.

lstar337

Cardew wrote: »

lstar337 wrote: »

If you left your house unoccupied for a year, would you leave the heating on? After all, if you let the house go cold for a year the boiler would have to work quite hard to bring it back up to temperature when you returned. Probably cheaper to leave the heating on I think. :rotfl:

So if we agree the 'one year heating off' analogy, at what point do the laws of physics not apply? one month? 1 week? one day, one hour?

orrery, malc_b?

Blackbeard_of_Perranporth

Cardew wrote: »

The myth - and it is a myth - comes from the false logic that it takes more energy to heat a house from cold than keep the heating 'topped up'!

This forgets that overnight your house can fall in temperature to around 5C and then slammed up to 20C which it never reaches, back to 10C during the day and then never reaches 20C in the evening. The condensate remains after the morning shower, doing more damage to the walls, tiles in the bathroom!


My thermostatic time-switch control the house temperature 24/7 and my furnace is hardly on. Current utilities are £75 a month, fixed until December 2018!

A neighbour who lives in a similar property only blasts her central heating to save money currently pays more than me!

Swipe

Blackbeard_of_Perranporth wrote: »

This forgets that overnight your house can fall in temperature to around 5C

5C? What do you live in, a barn? The coldest I've seen is my bedroom at 6C and I have no heating in there at all, even during the day.

orrery

lstar337 wrote: »

orrery, malc_b?

Sorry, don't point that at me. I'm simply pointing out that leaving a boiler on a setting that is low enough to ensure condensing mode is unlikely to do what you want, and that intelligent control is likely to be a better strategy with the boiler set to a higher level.

lstar337

orrery wrote: »

Sorry, don't point that at me. I'm simply pointing out that leaving a boiler on a setting that is low enough to ensure condensing mode is unlikely to do what you want, and that intelligent control is likely to be a better strategy with the boiler set to a higher level.

Intelligent control that costs far more in outlay than any energy savings can ever recover. I don't see them as a valid money saving option. They are cool toys though.

malc_b

The rules of physics are not being violated you just don't or won't understand my point. I've set out the maths and nowhere is physics violated. Energy loss from a house heated to a constant temperature 24/7 is MORE than a house heated say 16hr per 24. I show that in the maths.

BUT, what we are concerned with is $$$$ not Joules. It is more efficient to run the heating in condensing mode. Hence it CAN save money to run the heating longer if that means staying in condensing mode.

I can't understand why that is so hard to take in. It's the same as saying 100 miles trip is cheaper if you stick to 50 mph rather than 70 mph as you will use less fuel. Clearly only a fool would say therefore it must be cheaper to travel at 30 mph or 3 mph because you said it was cheaper at 50 mph than at 70 mph and 50 mph means the car runs for longer so must use more fuel. Silly me, when the fuel gauge is near empty what I need to do is put my foot down so I can get to a filling station sooner, before I run out of fuel.

Orrery I would agree with you that better controls are the way to go but it's not as bad as you might think. All boilers have a output temperature control so out water temperature is fixed to the chosen temperature. The return temperature depends on the flow speed, the radiator area and the room temperature. Basically the boiler is adding energy which the water is carrying away. The radiators are giving out energy, how much depends on their area and the difference between them and the room. The drop in temperature, out to return, depends on the flow rate and how much much the radiators give out. Slower the flow the larger the drop. In practice the difference is pretty constant in a well set up system. The only time you would see the difference head towards zero is if all the TRVs were hard off so radiator area was reduced to near nothing. If system is working correctly all the house radiator will be giving out heat and the TRVs just tweaking the flow rates to individual rads. In other words the main stat needs to switch off before all the TRVs have gone hard off. I think if that wasn't the case the rooms would be uncomfortable and they'd get very hot until the TRV shut hard off and then gte very cold while you waited for the main stat to turn on. And the boiler would be cutting in and out as it tried to deliver heat with no rads on..

Cardew

malc_b wrote: »

The rules of physics are not being violated you just don't or won't understand my point. I've set out the maths and nowhere is physics violated. Energy loss from a house heated to a constant temperature 24/7 is MORE than a house heated say 16hr per 24. I show that in the maths.

BUT, what we are concerned with is $$$$ not Joules. It is more efficient to run the heating in condensing mode. Hence it CAN save money to run the heating longer if that means staying in condensing mode.

.

Malc,
As someone who posts valuable information, I have resisted getting into discussion with you on this subject.

The principle of having a gas boiler operating more efficiently(condensing mode) is obvious.

What is not obvious is how you quantify those savings for any situation where there are so many variables i.e. house insulation, rooms heated, inside temperature required, outside temperature. Then we have to take into account the boiler characteristics - efficiency at various modes and temperatures.

Over the years we have had many discussions on the difference in efficiency between a boiler operating in condensing mode and non-condensing mode. This was an interesting thread over 5 years ago:

https://forums.moneysavingexpert.com/discussion/3772983

In it I posted:

Excellent question about condensing/non-condensing mode.

I have raised this matter several times on this forum - including a separate thread.

The principle of getting the boiler to operate in condensing mode is one thing, but to quantify the savings is quite another. I too have tried to get information from manufacturers without success.

It is commonly accepted that the theoretical efficiencies of boilers shown in SEDBUK are much like theoretical MPG figures for cars - rarely achieved in practice.

Unless on MSE we have heating engineers who work on the design of boilers I doubt we are going to get any answers. I doubt if even the most experienced RGI is any better placed to give an answer either.

Another aspect is what happens about condensing mode if you have a different heating load. For example using your 57C example.

A. You have every(say 12) radiator on in a cold house.

B. Just one small radiator on.

The difference in the return temperature of water will be considerable. In case B with very little heat being dissipated the return temperature will not be much lower than output temperature.

I just don't know how we are supposed to operate boilers to achieve, and stay in, condensing mode.

Let me pose some scenarios for you. (I now have a condensing boiler)

1. I am out of my house 14 hours a day. I want to return to a house with just 2 rooms heated to 21C for those 10 hours. For my house and my boiler how should I set up my heating.

2. As above when outside temperature is -10C, 0C, 5C, etc etc, Inside temp to be 16C,18C, 20C etc etc

3. As above when I want 10 rooms heated.

With respect, whilst the principle(theory) you espouse is understood, your 'proof(maths)' is simply based on guesstimates. To have 'proof' you would need back to back trials over a long period.

To me the most disappointing aspect of this technical discussion on this particular thread is that it sends the wrong message to non-technical readers.

There is no question that in vast majority of cases the longer your heating is off/or turned down, the lower the fuel bills.

Edit. This discussion is only concerning those who don't use electricity for heating.