Gas central heating on constant or timer?

mech wrote: »

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.

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.

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.

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?

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...).

markbloke wrote: »

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.

Well not necessarily. Mine just comes on a set number of times an hour with a set "bandwidth". It tries not to let the temperature vary by more than 0.5 of a degree. You will find all the modern digital roomstats do this because it's more efficient. Ie: it's more efficient to vary the temperature less with a lower peak than to vary the temperature more with a higher one. I don't see any contradiction in the theory there.

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.

No it isn't. The comparison isn't that simple. A thermostat tries to maintain a temperature. A timer is intended to let the house cool from a peak. They aim to achieve different things. If you turn the thermostat down a degree the peak temperature of the house drops by a degree. If you use a timer the peak temperature remains the same, but when the heating is on the fabric of the house is less evenly heated.

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?

A lot less than you'd think.

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.

Yes and that's ultimately why I gave up trying to determine if one method was better than the other. A lot of energy saving (and moneysaving) relies on behaviour rather than theory and as I was having so much trouble pinning it all down anyway I decided that determining cooling wasn't important and that I could probably learn more by just watching my gas meter (I built a little black box which makes a reading every 30 minutes and, when my PC is switched on, it automatically updates a CSV file via NFS).

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.

Yeah, the problem is I found out that it's not that simple. Every house has a direct connection between the air inside and the air outside (unless it's an airtight eco-house with a heat exchanging ventilation system - most aren't). Half an air change per hour would be exceptionally low by UK standards.

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.

You might find this interesting: http://www.builditsolar.com/References/energysimsrs.htm

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 think my house would get fairly close to outside temperatures after 2 days, though it never gets below 1 degree over outside even in winter... probably the fridge and freezer plus solar gain during hours of daylight. I have made some casual observations after being away from home.

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.

You may have to in order to get meaningful results. It might even be necessary to model the radiators to get the right heat distribution and the right kind of convection going inside the house when the heating is on.