Gas central heating on constant or timer?
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A house would cool to background temperatures in a million years. It won't do that overnight.
Actually it wouldn't. It would cool exponentially toward background and it would do the same overnight. It's just a matter of how long it goes on for. The idea of using a million years as an example is that it magnifies the problem and makes it easier to analyse, like using a microscope. The saving might well be small overnight (depending on insulation etc.) but it's there and it can't be argued that leaving heating on is a cost advantage.Reading this signature is a waste of time0 -
I don't understand that sentence. Walls will cool with the heating off. Slowly, but they will cool. Subjective comfort levels are sensitive to air just a couple of degrees cooler than ideal. The walls will warm up as slowly as they cooled. The air will warm up quickly due to convective mixing, but the walls will create streams of cooler air as it passes over them.
I agree that there might be quite a lot of unknown detail regarding perceived comfortable temperatures versus average air temperature.
However, regarding heat flow, a very simplified mode would treat a house simply as a box of air with walls of a certain thickness. This box of air would float in a sea of air at the outside temperature.
There would be a temperature gradient through the walls, and the heat flow through the walls would be driven by the temperature difference between the inside and outside air. The instantaneous heat flow out of the building would then be a function of the air temperature difference between the inside and outside.
With a bit of thought I could develop a numerical model of this on a computer and see what happens if I make the walls heat stores, as you describe them.
In that case, the temperature drop of the air would be much slower.
Anyway, I'll do an analysis for the simplified model and see what it says.Happy chappy0 -
A house would cool to background temperatures in a million years. It won't do that overnight. Take it to the other extreme. If I turn the heating off for 1 second do I save any energy? If I turn the heating off and on again every second for a million years does it use less energy than having the heating on for a million years? How about every minute? Every 10 minutes? Every hour? Every 10 hours? Where's the point where savings start to be measurable over a normal heating season? No one that I can find has measured it.
So what we're really looking at is how much of a thermal capacitor the walls and floors are.
When I really go to town with my model I'll build in a draught air flow.Happy chappy0 -
It seems to be readily accepted that turning the thermostat down by a degree will save real money, but using a timer won't. A thermostat is simply turning the heating off for periods of time. It's on until the house is too hot, then off until it's too cool. It then switches back on, heating up the house (walls, floor, dog, everything) until it's too hot again. Turning the thermostat down just increases the off periods. Turning the heating off saves money. If you don't think a timer works then don't worry about turning the thermostat down either because it's just a timer in disguise.
You'll notice that turning the heating off for a million years saves money, and turning the heating off for very short periods several times a day (a thermostat) still saves money. What is special about turning it off for eight hours?Reading this signature is a waste of time0 -
A few things to throw in to see if they affect your ideals.
Simple mechanical room stats have quite a bit of hysteresis (is that the word) in their on/off switching so you will maybe have that helping your "savings".
Interesting that boilers now have modulating gas valves to vary the flame to match the temperature more to the flow and heat release in the system. Trying to keep a steady state rather than surging up and down.0 -
In the case of a simple room stat, hysteresis simply affects the timing of switching. The heating is still either full on or completely off several times a day. If you think turning your thermostat down saves money then you accept that turning the heating off several times a day saves money.
A modulating gas valve would probably be an excellent move (though it's one more thing to go wrong...).Reading this signature is a waste of time0 -
Imagine you heat a house to a nice temperature and then leave the heating on for a million years. That would cost quite a bit.
Imagine heating the same house to the same temperature, turning the heating off at the end of the day and then warming it back up a million years later. It would cost less. There's no escaping it.
Ah but have you factored in the effect of Global Warming in your million years analogy;)
You are of course absolutely correct in all your statements on this subject, but the 'urban myth' of 'it takes more gas to heat from cold than keep a constant temperature' persists.
The annual thread on this subject will doubtless soon be started - with the usual "I know its true because my friend is a plumber and he told me" statements.0 -
It seems to be readily accepted that turning the thermostat down by a degree will save real money, but using a timer won't. A thermostat is simply turning the heating off for periods of time. It's on until the house is too hot, then off until it's too cool.It then switches back on, heating up the house (walls, floor, dog, everything) until it's too hot again. Turning the thermostat down just increases the off periods. Turning the heating off saves money. If you don't think a timer works then don't worry about turning the thermostat down either because it's just a timer in disguise.You'll notice that turning the heating off for a million years saves money, and turning the heating off for very short periods several times a day (a thermostat) still saves money. What is special about turning it off for eight hours?0
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For the layman amonst us !
We live in a 2 bed masionette , with people above , and the alley way one side and the entrance to the yard the other , the two bedrooms are in the basement and the living areas upsatirs , last winter our gas usage was ridiciolous i think
We have a crappy old combi boiler , potterton or similar and a rudimentary timer box next to it , with a old fashioned dial thermostat in the frontroom , the house is vicotrian
What is the cheapest way to heat our place ?
any tips welcome because it appears theres some pretty clever people on here !!
:beer:0 -
tomstickland wrote: »Having the heating on all of the time will produce warmer walls (I believe you said that). The reason for this is that extra heat has been put into the system by the heating system. I agree that there might be quite a lot of unknown detail regarding perceived comfortable temperatures versus average air temperature.
At the same thermostat settings I decided I save 11% of my winter gas bill putting the heating on timed versus constant. About £50 a year. But it's definitely less comfortable on timed. If it wasn't a controlled experiment I would have turned it up. I have managed to verify that the usual claim that you save (approximately) 10% by turning your heating down a degree holds reasonably true - at least at settings between 18 and 24 degrees. That's what makes me think only a real-world study testing real people in real houses with normal day-to-day behaviour can give a proper answer. Turning up the heat one degree would wipe out the savings. More than that and one wonders if keeping the heating on constant wouldn't be better. It certainly would mean less wear and tear on my boiler as then I can get away with lower water temperatures.However, regarding heat flow, a very simplified mode would treat a house simply as a box of air with walls of a certain thickness. This box of air would float in a sea of air at the outside temperature.
There would be a temperature gradient through the walls, and the heat flow through the walls would be driven by the temperature difference between the inside and outside air. The instantaneous heat flow out of the building would then be a function of the air temperature difference between the inside and outside.
What happens is that the heat in the air indoors is lost relatively rapidly through ventilation/draughts. The other 9x% of the heat is stored in the solid parts of the house and now it loses heat to the cooler air in the house (which continues to be replaced). The walls were losing heat more slowly when the air was at equilibrium with them on the inside surface. Now they lose heat from both sides. This is probably more than countered by the fact that the house as a whole no longer loses such warm air, but it does mean the heat transfer from the inside isn't exponential decay - the warm air is fairly quickly replaced with slightly cooler air which stabilises at a temperature less than that of the walls as they don't conduct heat to the air very quickly. Anyway it would mean measuring the air temperature in the house won't give a good measure of how much heat energy really remains.With a bit of thought I could develop a numerical model of this on a computer and see what happens if I make the walls heat stores, as you describe them.
In that case, the temperature drop of the air would be much slower.
Anyway, I'll do an analysis for the simplified model and see what it says.
Specifically the HEED software. http://mackintosh.aud.ucla.edu/heed/ Though I think I was using an older version than is available now, it got the gas consumption totally wrong for my house, even after I spent ages refining the parameters. It overestimated my fuel consumption by a factor of two, even when I tried to bias the results towards my actual usage by fiddling the figures. I still don't know why. It was then that I realised that if such a comprehensive piece of software couldn't get it right, nothing I could come up with was going to be any better. I'm nowhere near a good enough mathematician.I see what you're saying. The critical piece of data is some knowledge of the time constant for the system. If after 2 days the house was still within 1 degree of the starting temperature then I agree that there'd be no real difference between on all of the time and timed.
I did try to take lots of temperature measurements, but the weather wouldn't cooperate and I got fed up of letting the house get cold in the middle of December. I don't think I have kept the figures I did record. I can't find them anyway.
The only snippets I can remember: The air in my house will lose less than half of its temperature advantage over outside after 12 hours of darkness and no heating in winter (outside temperatures between 4 and 6 degrees C, indoor air temperature starting at 18-20 C depending on the room). If you plot a graph of indoor temperature after I turn off my heating it has about a 15 minute delay as the radiators lose most of their remaining heat, then a definite bend at 90 minutes or so after which the cooling slows down markedly.So what we're really looking at is how much of a thermal capacitor the walls and floors are.
When I really go to town with my model I'll build in a draught air flow.0
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