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

redux

malc_b wrote: »

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.

You seem to be the only person asserting that letting the temperature drop by leaving the system off for a while, like at night, causes or should mandate it to run in shorter bursts.

I didn't recommend shorter bursts. I'm not convinced that anyone else did. Indeed if the system is colder when it restarts at 0730 or whenever, simple intuition says the first run would be a longer spell. And nobody has adjusted the boiler settings just for this start, so why are you asserting the radiators would be hotter than normal?

harrym1byt

I devised a system years ago, to dry clothes cheaply and which doesn't cause damp and mould problems. In our utility room I installed a series of lines from which to hang washing, an automatic dehumidifier and a fan. The fan moves the air around, encouraging the moisture to evaporate the dehumidifier condenses the moisture and dumps it to a drain. It easily dries a line full of washing overnight in winter and for just a matter of pence.

For use during the summer, I devised a stainless steel wire line (fed up with normal lines snapping) which was counter-weighted. The whole thing about 50 yards long, at the height of our house eaves, so it needs to be winched down to load / unload. The higher it is, the more breeze it gets, the faster it dries things. Pulleys enable the line to be moved back and forth, so washing can be loaded and unloaded from one position, by just pushing the line and its washing along.

On the subject of heating the house, no matter how well insulated it might be, it will have thermal mass - all of the brickwork, stone work, furniture and etc. inside that insulation. That thermal mass will delay the house feeling warm even if the air is up to temperature. It can also hold the house cooler in the summer.

I find when the house has been without heat in cold weather outside and the heating turned on, the heating always overshoots by a few degrees, until it settle down. The way around it, is to gradually increase the thermostat over an hour or so.

malc_b

Cardew wrote: »

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.

I didn't say it was. I was quoting it as an example of where the "common sense" answer isn't actually right. It's the same here IMO. "Common sense" tells people that turning off the boiler saves money. The reality is that it saves energy but may not save money due to the lower efficiency of the boiler at higher power.

malc_b

redux wrote: »

You seem to be the only person asserting that letting the temperature drop by leaving the system off for a while, like at night, causes or should mandate it to run in shorter bursts.

I didn't recommend shorter bursts. I'm not convinced that anyone else did. Indeed if the system is colder when it restarts at 0730 or whenever, simple intuition says the first run would be a longer spell. And nobody has adjusted the boiler settings just for this start, so why are you asserting the radiators would be hotter than normal?

By shorter bursts I'm talking about 16/8 rather than 24/0, or 4/6/6/8 (on/off/on/off) which is probably about typical for someone going out to work.

My point is that when you work out the maths what comes out is that the difference between say 16/8 and 24/0 is not that much. I hope we can all accept that energy has come for somewhere. And that if the inside of the house is warmer than the outside then the house will be leaking heat to the outside. When you have the heat off overnight where is that heat coming from? It has to be from the house structure as there is nowhere else it can come from. So as overnight you are borrowing heat from the house the next day you have to pay that back. Where else is the next night's heat to come from otherwise? You cannot continue to take heat out of the house and not pay it back. You therefore need to run the boiler harder and the radiators hotter. The latter is the most significant as hotter radiators mean no condensation mode.

Once you understand how the system works then understanding how to reduce the heating costs follows - run the boiler in condensing mode all the time, increasing heating hours rather than raising water temperature.

It's the same as planning a car journey. My optimum speed is 50mph and I have a 100 mile journey where I need to arrive exactly at 12:00. I should therefore start at 10:00. If I start at 9:30 then I have to drive around for 30 minutes at the far end before I can arrive which wastes fuel. If I start at 10:30 then I have to drive at 67 mph which means I use more fuel.

Heating (for condensing boilers) is exactly the same. You want to run the boiler in condensing mode for just long enough to provide the heat the house needs in a 24hr period. That means starting earlier rather than running hotter.

Update:

Maybe I haven't answered your question. The energy given out by radiators depends on how hot they are. 1kW radiator (1kW at 60C delta) at 20C, same as the room, gives out nothing of course. At 40C delta it is ~600W output. Hence the more power you need out of the radiators the hot they have to be and the less likelihood you will be in condensing mode.

malc_b

redux wrote: »

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.

Hopefully we can all agree that running the boiler in condensing mode is a saving. You can google that or even look on Sedbuk and compare condensing and non-condensing boilers.

redux wrote: »

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.

I totally agree with you and I've used exactly that formula in my proof. The rate is constant when inside and outside are fixed values (i.e. heating is on). And the rate falls off overnight as the house cools.

redux wrote: »

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.

I agree it saves energy. What is not certain is that it saves money. To be sure to save money then the heating system must be able to run at the same efficiency.

redux wrote: »

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.

OK, lets take it as water in a bucket. It's a big bucket 20in deep (corresponding to 20C). The hole is in the bottom of the bucket (corresponding to 0C). It leaks 1 pint per hour so if we turn a tap on giving 1 pint/hr to the bucket then over 24 hrs we'd use 24 pints. Let's call that day 1, 24/0 run. Next day we keep the tap on during the day (16hr) and turn it off at night (8hr). We have used 16 pints during the day but at the night the level falls so the rate rate drops off. Let's say that by morning the bucket is half way, 10in (aka 10C). Leakage is proportional to water height so at the start of the night just after we turning off the tap when the bucket was still 20in full the leakage was 1 pint/hr. At the end of the night the leakage for 10in is 0.5 pint/hr. To keep the maths simple let's take that as a straight line so the average leakage over the night was 0.75 pint/hr. Over 8 hrs that is 6 pints. Plus the 16 pints we used during the day is a total of 22 pints on day 2, 16/8hr, rather than the 24 pints we used on day 1, 24/0. That is a saving of 2 pints in 24 pints, ~8%.

Let's say we fix the leak partially, (aka add insulation) and so that we now leak 0.5 pints/hr. Let's repeat. Day 1, 24/0 would be 12 pints. Day 2, 16 hr during the day would be 8 pints. At night the level falls less because we've partially fixed the leak. Instead of falling to 10in (i.e. by 10in) it now only falls to 15in (i.e. by 5in). Leakage starts at 0.5 pint/hr and et the end of the night is 0.375 pint/hr (0.5 x 15/20). Average is 0.4375 pint/hr, total over 8hr night is 3.5 pints. Total for day 2 is 11.5 pints (8+3.5) so saving over 12 pints of day 1 is 0.5 pints or 4%.

Also, for the first case, 1 pint/hr leakage, on day 2 we had used 6 pints over night so during the next 16hr period when the tap was on we would have to top that up. During the day we'd have to put in the 16 pints lost during the day plus the 6 pints lost from the night before. Hence the tap would need to be set to 1.375 pint/hr ((16+6)/16) in order to fill the bucket up again which is 37.5% more water flow. For the second case, 0.5 pint/hr leakage, we lost 3.5 pints over night so the next day the flow rate must be 0.71875 pint/hr ((8+3.5)/16) which is 43.75% more water flow.

QED

victor2

redux wrote: »

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...

malc_b wrote: »

OK, lets take it as water in a bucket. It's a big bucket 20in deep (corresponding to 20C). The hole is in the bottom of the bucket (corresponding to 0C)...

Slight adjustment to the analogy there in that the first one loses no water (aka heat) when the level drops below the leak, while the second one loses water until it is empty.
I'm sure that can be averaged out though.

Cardew

malc_b wrote: »

Hopefully we can all agree that running the boiler in condensing mode is a saving.

That is agreed, it is the whole point of a condensing boiler.

Your 'proof'(I use the term loosely!) has been debunked in all the earlier posts over the years in this thread.

To get to 'your' proofYou have decided that the after the CH has been switched off it will run in non-condensing mode regardless of the electronics designed to keep it in condensing mode.

You have then decided the loss of efficiency of the boiler for the period that you have decided it will be in non- condensing.

You know full well that every manufacturer, or organisation(EST) etc recommends that the heating is switched off where possible, yet you alone insist in giving poor advice; as said earlier:

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

Much as you don't like answering the question. The oft quoted example of switching the boiler off for 2 years/1 year/ 1 month/ 1 week/ 1 day/ 1 hour still needs an answer - at what point does 'your proof' override the law of physics.

redux

The boiler settings include what temperature the system is supposed to work at.

If the return temperature is lower than about 57° the boiler is more efficient as it runs in condensing mode, getting back heat that otherwise would go up the flue.

That isn't in dispute, but we have someone persistently describing only part of the picture.

I don't know why we keep getting motoring analogies, but let's stay there a moment. Two drivers can produce the same average speed in the same car but different consumption, by judicious use of acceleration.

Yes our heating system is working harder when getting the place warm again after a cooler period, but it isn't obliged to get there as fast as possible with absolute maximum power. If its control system is good, and it's been set such that the return temperature is in the desired range, it does not need to be operating outside condensing range. There seems to be some mis-thinking or mis-statement about where that range is; saying that the radiators need to be at say 60° to get decent rate of heat transfer into the room still doesn't counter the return temperature being say 55°. As I already said, nobody has changed the settings overnight.

richardc1983

The cooler your return temp the more efficient it is. So not just having it st 55c return if you can have a return flow of 30c and still keep your house comfortable your saving energy.

malc_b

victor2 wrote: »

Slight adjustment to the analogy there in that the first one loses no water (aka heat) when the level drops below the leak, while the second one loses water until it is empty.
I'm sure that can be averaged out though.

Maybe I wasn't clear enough. The point about a 20in deep bucket and a hole at the bottom is so I have 20in head of water corresponding to 20C temperature difference. You can have have a 30in bucket if you want with the a hole in the side 10in up. Or 293in deep with the hole 273in up. It makes no difference. Water flow stops when the water reaches the hole. The head of pressure in 20in at the start.

The width of the bucket you'll notice does enter in to it as we remove the need to know that by just taking start and end heights (temperatures), something we can easily observe.