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

victor2

The_Green_Hornet wrote: »

Surely there must be a peer reviewed scientific study somewhere that someone could post a link to?

There's too many variables and assumptions made which can sway the conclusions either way.
Valid "proof" would be so specific to a set environment that it becomes meaningless. It would have to consider the thermal efficiency of the house being heated, the weather, the direction the house is facing, the efficiency of the boiler, whether or not the boiler is a condensing type etc. etc.
Using average values for so many parameters would result in an "average" set of circumstances that are about as rare as rocking horse s***.
People can, and do, study their own particular environment and come to a valid conclusion for the most cost effective way to keep it comfortable. Trying to prove that applies to typical property is where it fails.

redux

The_Green_Hornet wrote: »

There seems to be a lot of specious arguments and false analogies being used on both sides which doesn't do anyone any credit on here.

As I understand it the basis of the myth is that once you have heated your house to the desired temperature at the start of the day it costs less to keep it at that temperature by running the heating on/off throughout the day than it does to switch the heating off, let the house cool down and then turn the heating back on to get it back to temperature later on in the same day.

Surely there must be a peer reviewed scientific study somewhere that someone could post a link to?

That is one assertion, and to elaborate on it someone is saying that running the boiler in its most efficient operating range is saving something.

But this isn't the whole picture, as the overall amount of heat needed is not the same anyway.

The house is losing heat all of the time, slowed down by the insulation, but the rate is directly proportional to the temperature difference between inside and outside.

That is why not heating the place while empty or at night will save money, because as the internal temperature falls the rate of loss also falls.

Heating things back up in the morning might take half an hour, and although the system is running harder, that's still less overall than running for parts of the extra 9 or 10 hours at night.

A better analogy would be running a tap to fill a tank with water, but the tank has a leak part of the way up, and the rate of loss at the leak is proportional to the pressure there, which is proportional to the depth of water above the leak.

Keeping a higher level all the time will lose more overall than turning the tap off some of the time. If the level drops to next to the leak point the loss will stop. But let's say it doesn't get there, instead someone turns the tap back on when the drop is halfway. At that point the average loss rate is three-quarters what it would have been if the level had been maintained by leaving the tap dribbling.

redux

malc_b wrote: »

As I explain at the end of the proof the ideal way to run the boiler is to have in always in condensing mode and ON FOR AS LONG AS NEEDED. In spring and autumn you might get away with shorter running times and still be condensing mode all the time. The optimum way to run a condensing boiler is to force it to be always in condensing mode (i.e. low water temperature) and then increase the hours as the season gets colder.

From what we can read in various places, condensing mode happens when the temperature of water getting back to the boiler is 57° or lower.

Indeed it seems things may become even more efficient if this is lower still.

When the system has not run for some time, what has happened to the temperature of this water?

malc_b

lstar337 wrote: »

I don't need to read it malc, if you had disproved to basic laws of physics, you would be famous and not posting here.

I therefore assume you haven't.

DOH! It's probably pointless but why don't you just read what I'm saying? Air and rolling resistance increase with speed so according to your argument then if I want to travel from Bristol to London the cheapest way to drive is at say 10mph because driving at 50mph will use far more energy. Your assumption is that Energy = Cost which ignores the efficiency of the car. We all know that driving at 10 mph would use far more petrol even though it take less physical energy.

malc_b

lstar337 wrote: »

What your saying is if we both went away for a 2 week holiday (we don't have to go together ), you with your heating on 24/7 and me with my heating off, You would use less gas than me?

I'm not convinced.

No, I'm not saying that at all. In the proof I compare two cases, 16/8 and 24/0 and show that 16/8 has less energy loss as you would expect. I show that over the 24hr period 16/8 does not save 1/3 of the energy loss of 24/0 but only ~8% assuming the house temperature falls over night to 10C, which I think is extreme even for 0C outside. I then go on to consider what this means for the boiler demand. Clearly if over 24hr the house requires about the same energy input then the boiler has to be running ~38% harder ((100-8%)*3/2). This is standard physics. Energy has to come from somewhere.

Hopefully, everyone is ok with that so far. The first key thing is that running shorter hours doesn't save that much energy loss (and I've shown you can estimate this by checking the overnight fall). There are papers on this fact and they give a 5% difference between 16/8 and 24/0 but that depends on whether it is a dense or light building (most houses I would think are dense because of the brick walls and solid floors). The second key thing is that condensing mode saves you more than 8%. Put the 2 together and you can see it is possible to have a larger energy loss but save money because you are running more efficiently.

I do not say run 24/7 blindly. Nor do I claim that running the heating while you go away for 2 weeks will save money. It's the same as driving a car. We all know that a steady 50mph gives the best fuel economy. Go faster and we use more, go slower and we use more. Heating is the same. We want to run in condensing mode as much as possible (aka 50mph) for as short as possible. That is the conclusions you'll find at the bottom of my proof.

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Cardew

malc_b wrote: »

No, I'm not saying that at all. In the proof I compare two cases, 16/8 and 24/0 and show that 16/8 has less energy loss as you would expect.

This has all been discussed at length at several places in this thread and iirc everyone has disagreed with you at various times - with Graham2003 leading your critics.

You keep saying 'PROOF' and, as said earlier, you have provided no proof at all. All you have done is invented a scenario, and made unsupported assumptions to attempt to justify your case.

However leaving that aside, you must know that the message you are trying to convey is almost mischievous in that you are giving the impression that all the correct advice(to have off heating as much as possible) is flawed.

malc_b

redux wrote: »

From what we can read in various places, condensing mode happens when the temperature of water getting back to the boiler is 57° or lower.

Indeed it seems things may become even more efficient if this is lower still.

When the system has not run for some time, what has happened to the temperature of this water?

When the system hasn't run for a while the water will be at room temperature so yes as you start up you will be in condensing mode. But you need to look at this from the radiator function. The boiler is dumping kW of power into the water in system. The water heats up and that heat has to get out from the radiators. The boiler can only put in the same energy as the radiators give out or the excess raises the water temperature. If the room is at 20C and the radiators at 60C (i.e. 65C in, 55C back to boiler, just low enough to be in condensation mode) then the radiators output is 60% of the rated performance at 80C (60C delta).

Shorter running is pushing things all the wrong way. The shorter the run time the shorter the time you have to put the energy back into the house structure so you need higher power out of the radiator which means higher temperatures, exactly the wrong case for getting condensing mode to occur.

malc_b

Cardew wrote: »

You keep saying 'PROOF' and, as said earlier, you have provided no proof at all. All you have done is invented a scenario, and made unsupported assumptions to attempt to justify your case.

A proof is a reasoned mathematical argument. I've used a simple case to explain the point. If you would like to point which equation is incorrect then please do. Or which assumption is unsupported for that matter.

The simplification I have made is to make the maths easy to understand and so I've used a straight line rather than a curve. I've checked this and it gives about a 10% error in the night time heat loss which is not that significant over the 24hrs being considered. And I mention this in the proof.

I do wish people would try to have an open mind and look at this on its merits. If I have made a mistake then fine, point it out and I will look again.

For those of you who are still convinced that the world is as you imagine it can I suggest another idea. I want to make ice cubes as fast as possible as I've forgotten to do it and guests are arriving soon. Do I use cold water, tepid water, or hot water? What rule do I follow? Should I get the coldest water I can? That will give me the ice cubes quickest won't it?

Google it. It's called the Mpemba effect after Tanzanian Erasto Mpemba. He described in 1963 in Form 3 of Magamba Secondary School, Tanganyika. It was part of a tutorial I had as an undergraduate and it taught me not to be so quick to jump to conclusions about what is "obviously true". Since then I try to be more open minded and look to prove "facts" by maths.

bsod

instead of thought experiments with wild assumptions, read the meter, see the reality, then do the maths

Even if the 8% was correct, it's 8% of pure waste that could be 0%

Cardew

You also raised the Ice cube (Mpemba) effect earlier in this thread - and Aristotle noticed it before you! It has nothing to do with this subject.