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

malc_b

Cardew wrote: »

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.

No one has pointed out any flaw in the maths or anything amiss in the physics equations to my knowledge. Perhaps you would care to point out these posts or point out the errors again.

Cardew wrote: »

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.

Ah, now who is changing the laws of physics? So electronics can some how make the boiler run in condensing mode with the return water at more than 55C? Or can it make radiators give out more energy at lower temperatures? Can you point out the proof of that some where so I can read up on it.

Cardew wrote: »

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

No, I haven't. I have shown that running 16/8 vs 24/0 saves ~8% for 0C outside, 20C inside and an overnight fall to 10C. From that it follows that radiators have to work harder during the day and hence run hotter and hence may not be able to run in condensing mode. You have to make up the energy lost from the house structure during the night in the following day.

Cardew wrote: »

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:

And at one time we were told beef was safe to eat too. That wasn't right then and EST etc. are right now, or at least they are giving too simplistic an answer which I suppose is possible given the general trend to dumb everything down as much as possible. I remember the days when you returned a £9.99 item and offered the cashier a 1p so she could give a a tenner back, and she didn't say I can't use that it will put my till out, I have to give you £9.99 in notes and change.

Cardew wrote: »

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.

I have answered this multiple times, but let me try a different tack. In the proof I just look at the average over a 24hr period. In 24hrs, when the heating is off overnight then the inside temperature does not fall to match the outside temperature so the house is always losing heat. I've picked the figure of a 10C fall overnight and shown that saves ~8%. I think that is a very poor house but let's stick with that for now. So if 8hrs is 8% then taking fall as a straight line (which it wouldn't be but it makes the maths easy) then 16hrs would be 16%, and inside temperature at 0C, same as outside. After that no more heat is lost from the house and so you save 1/24 for each hour the heating is off.

If the house overnight fall is a more reasonable 15C by morning then you can roughly double all those figures and for a well insulated house probable double them again. 16%+ is of course more than you save in condensing mode so in this case having the boiler off saves more. I doubt anybody would seriously think that leaving the heating full on for 2 weeks would save money and I didn't say not did my proof suggest it or discuss it. I guess I kinda thought that was obvious from the maths.

To use the car analogy again the way to save fuel on that is not to use it at. If you need to use the car then drive a steady 50mph not 70 and not 10.

This is why I say, repeatedly, the optimum way to run a condensing boiler is to force it into condensing mode and then run as shorter hours as possible. Where I differ from the EST is that I say extend the hours rather than raise the temperature so you are then not in condensing mode. If you like, except when you are going away,

victor2

malc_b wrote: »

Ah, now who is changing the laws of physics?...

Aren't you, to "make the maths easy"?

malc_b wrote: »

So if 8hrs is 8% then taking fall as a straight line (which it wouldn't be but it makes the maths easy) then 16hrs would be 16%...,

Perhaps illustrate the heat loss with another analogy, as you seem to be quite keen on them:
I make a cup of tea with boiling water, and after a few minutes, the temperature has dropped enough for me to comfortably drink it. But it will not drop to room temperature for say an hour. But for the ease of maths let's say the temperature loss is linear, so I should wait half an hour before attempting to drink it?

malc_b

redux wrote: »

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.

You're not seeing the physical picture I feel. Let's say the boiler is putting out water at 85C. This flows to the radiators so the input side of the radiator is 85C. Typically pumps are set to a speed so that the return is 10C lower so that would make the return water 75C. The average temperature across the radiator is 80C, 60C above a 20C room. The radiator is giving out heat at its delta 60C rating. Let's say this is a big radiator of 10kW (or 10 1kW radiators). The boiler is putting 10kW into the water, the radiators are giving 10kW out. A modulating gas boiler will adjust the gas flow to maintain the set output temperature of 85C.

Let's now try to get 55C as return. 85C out, 55C return, 30C difference. Radiator average temperature is now 70C. Radiator efficiency is down to 80% so it only gives out 8kW. Boiler can only put in 8kW. And in reality 30C is a big difference. To get that the water must be moving 3 times slower than for 10C so it takes 3 longer to heat up the radiators from cold. The norm is to set a 10C drop although I believe very new boilers have variable speed pumps.

The shorter the running hours the more energy is taken from the structure to keep the inside temperature higher then outside, and the shorter the time you have to replace that energy. Hence the more efficient the radiators need to be which means the higher temperature they need to run. This is why you need longer running hours to maintain condensing mode.

Cardew

malc_b wrote: »

No one has pointed out any flaw in the maths



No, I haven't. I have shown that running 16/8 vs 24/0 saves ~8% for 0C outside, 20C inside and an overnight fall to 10C. From that it follows that radiators have to work harder during the day and hence run hotter and hence may not be able to run in condensing mode. You have to make up the energy lost from the house structure during the night in the following day.



I doubt anybody would seriously think that leaving the heating full on for 2 weeks would save money and I didn't say not did my proof suggest it or discuss it. I guess I kinda thought that was obvious from the maths.

,

We are not saying there is a flaw in your maths. However there is a flaw in your assumptions on which you base that maths.

If I advise someone on how to save a lump sum of money and state(assume) there will be a 15% interest rate on savings, for the next 26 months, I will produce a final total far removed from reality; but my maths will be faultless.

You appear to concede that the boiler may not run in condensing mode.

I have shown that running 16/8 vs 24/0 saves ~8% for 0C outside, 20C inside and an overnight fall to 10C. From that it follows that radiators have to work harder during the day and hence run hotter and hence may not be able to run in condensing mode.

However your maths assumes the boiler will not run in condensing mode, and assumes that for x hours the loss of efficiency will be y%

orrery

Certainly with my system, which is set at 64C flow temperature and has about 20C across the boiler, a long burn (to heat the house from cold) results in a gradual restriction of the flow from radiator valves as they start to close, the boiler modulates, the return temperature rises gradually and the boiler allows itself to go 5C over temperature. So the flow rises to 69C, with the return moving above 60C before the boiler shuts down as it can't modulate further.

With the house close to target the Tado controller uses proportional control, short bursts where the flow temperature only seems to get to about 60C and the return down at 40C.

So, whilst I side with the "Keeping the heat on means more heat loss therefore higher fuel costs" I really can't believe that it really make much of a difference (in my installation, with my reasonably well insulated house). I'm much closer to sitting on the fence than when I started reading this forum.

lstar337

malc_b wrote: »

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.

I was having a joke with you there. :D

lstar337

malc_b wrote: »

For those of you who are still convinced that the world is as you imagine it

Ironic statement. :rotfl:

Half of your "proof" is based on your imagined scenario.

lstar337

malc_b wrote: »

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.

My boiler won't run at high power. The output is set at 55c so the return is always less.

Not to mention that the efficiency savings from condensing to non-condensing are very small, and pale in significance to the amount of heat wasted by heating while you are away from the building.

ASavvyBuyer

malc_b wrote: »

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.

That is a rather strange scenario that re-enforces the doubts in your assumptions. Who would be daft enough to drive around for 30 minutes if they arrived early at a destination ? Most people would park up and just wait. The above statement does not do much for the credibility of your "proof".

Your assumption in the calculation is that the boiler will not run in condensing mode if it is switched back on after being off for a period of time. However, the return temperature of the water will be well below 50 degrees when it is switched back on, and with proper controls, will stay below 50 degrees. Therefore, it will be running in condensing mode whenever it is running, providing the user has not used the controls to set it at too high a temperature.

richardc1983

The cooler the return flow the better condensing - so get it as cool as you can. There isn't just condensing mode or not at all there's further efficiency the lower you run it.