Gas central heating on constant or timer?

DirectDebacle wrote: »

I agree, timer timer timer. But one of the simplest and most effective ways to save your heat and thus money has barely been touched upon. You need to insulate and effectively. Current regs for new builds require 270mm insulation in your loft/roofspace. Check what yours is and if it isn't that, get it.

I've got 100mm and it honestly isn't cost-effective to add more. Even if I could put 170mm extra of insulation in my loft (which I can't) it would save me, what, 2% of my gas bill? That's £6 a year at present. If I can buy the insulation for £60 I break even in a decade. And then there's re-boarding the loft... oh dear.

£150.00 should do about 30 sq. mtrs and summer sales coming you may find it a lot cheaper. If you have cavity walls and they are not insulated, get it done. Draught proof doors and windows. If you have a fireplace but never use a fire then get the chimney capped and vented and block up the flue. A lot of warm air is getting sucked out your room and straight outside. These are relatively cheap solutions and are going to look even cheaper with spiralling energy prices. Your home will have a more even temperature with fewer cold spots and over time will pay for itself far more rapidly than double glazing. So stop keeping the birds warm and jeep yourself warm instead.

That's certainly true. Double glazing can never pay for itself in energy savings during its lifespan. Even if you believe the outrageously optimistic figures. It can only pay for itself in the increased value/saleability of the house.

mute_posting wrote: »

Very true, my point was that the bean counters and energy centre at work have already crunched the numbers from the trial buildings and determined that there is a significant enough saving to make it worthwhile to pay myself and others to make the changes for the rest of the buildings.

I don't know about commercial properties, they may well be of a different typical construction. But the domestic UK housing stock is most often brick built and relatively small. The walls make up a large thermal mass relative to the size of the building. Do calculations for stored heat energy and compare it to the heating demand and you find what you have is a giant storage heater. 90something% of the heat is in the walls, 3% is in the air inside the house and it's only the temperature of the air that we really care about.

I discovered something interesting about my house when experimenting this past winter. When the heating goes off at night there's a 20-40 minute delay while the radiators give up their heat and after that the air temperature drops by about 1 degree C fairly rapidly (another 90 minutes?). That isn't the interesting bit. The interesting bit is that after that time the cooling slows dramatically. I'm not talking about exponential decay here, I'm talking about a sudden reduction to a very low rate of cooling.

My theory is that the first degree C or so of heat loss is due to ventilation (0.5 air changes per hour is not unreasonable) and soon after that, the fabric of the house starts warming internal air. In other words, when the heating is on the walls are cooler than the air in the house (because the air is the means of carrying the heat to them in the first place). When the heating is turned off, the walls become warmer than the air in the house (because the air is being replaced with cold air from outside) and the walls lose their heat more quickly with the heating turned off than when the heating is on (at least initially). This is 90-something% of the stored heat in the house remember. No wonder the savings are so small. Lost heat gets replaced again just a few hours later. Typically twice a day (as opposed to once a day in a commercial property.

Maybe the answer to getting savings is to not turn the heating on at all in the morning. But I wouldn't want to live with that in the winter. I'd have terrible trouble persuading myself to get out of bed into the nasty cold house.

Doing this will usually have a negative effect on boiler efficiency which is why people are advised to run boilers on max for shorter periods of time.

I don't agree. People keep saying that, but I can't understand how it could possibly be true. Even for non-condensing boilers. Systems with hot water reset rely on the opposite being the case.

The efficiency of a heat exchanger depends on the temperature difference across it. If the flue gases are 170 degrees C and the water temperature is 90 degrees C it's going to be less efficient than if the water temperature was 70 degrees C. There's nowhere else for the non-transferred heat to go other than up the flue. Add to that: increased heat loss pipes under suspended floors, higher localised warming of exterior walls where they have radiators, and increased overshoot of room temperature.

I can't see anything except potential savings from a lower water temperature.

http://www.rmcotton.com/_mod_files/ce_images/compeff2.jpg

I'm going to do some temperature logging with some low mass air temperature probes when winter sets in. I'm quite interested in the house as storage heater analogy.

tomstickland wrote: »

It's not conjecture. The rules of heat flow are universally accepted.

Not in this case. You are assuming the house is a solid object and that heat flow through the walls is the only mechanism for heat loss. You are also assuming that comfort levels perceived by the occupants is an accurate measure of overall air temperature. That isn't an accurate model and I haven't read any studies that attempt to measure relative costs.

If a house stores most of its heat energy in solid surfaces (which they do tend to) and walls don't warm up quickly (which they don't) and cold surfaces produce cold draughts when the house is being heated (which they also seem to), it's quite likely that people will want a higher thermostat setting. Without knowing how much this effect affects consumption it's difficult to know what is best without guessing. That's where the conjecture comes into play.

The walls would be warmed because heat would had been used keeping their temperature up.

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.