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

Cardew

macman wrote: »

My living room is currently at 19C (as set on the room stat).
So I'm officially at risk of death now!
Who do I call?
And can I get compensation?

I posted that link because I was worried about you; can't afford to lose your contributions!

retiredin2011

Your usage is already very low anyway, so hardly so critical either way

Why do you think that is low for heating only?

I also use another 2,000 kWh a year for pilot lights. Then another 6,000 to 7,000 for hot water and cooking.

Total gas consumption in a year is 17,500 to 20,000 kWh depending on the weather..

I think it is high but I have decided for my comfort that I will pay that.

I was only trying to point out that not all the gas consumption in a house goes on heating.

terry2

wantanswers wrote: »

I'm doing the same Terry (experimenting)and saving, which suggests it is not a myth....

Also have you any info on night set back?

No, and I don't think it would have much effect. What you are trying to do is keep the solid mass of the house (ie inside the insulation) at as constant a temperature as possible.

macman

retiredin2011 wrote: »

Why do you think that is low for heating only?

I also use another 2,000 kWh a year for pilot lights. Then another 6,000 to 7,000 for hot water and cooking.

Total gas consumption in a year is 17,500 to 20,000 kWh depending on the weather..

I think it is high but I have decided for my comfort that I will pay that.

I was only trying to point out that not all the gas consumption in a house goes on heating.

16,500kWh is the UK average for gas (not just heating, all usage). If yours is 17.5-20K kWh with the heating left on 24/7, then your DD is relatively low.
Indeed, not all goes on heating, but it's the biggest share, and the most seasonally variable.

macman

Cardew wrote: »

I posted that link because I was worried about you; can't afford to lose your contributions!

Not to worry Cardew, I'm hanging on.
A shame though that we can't capture all the hot air being generated by the 1st Law of Thermodynamics deniers: if we could, no one would be cold this winter.

terry2

Cardew wrote: »

You say running 'the house at 17C 24hr which still feels comfortable, and IS saving money.' - over a comparison of the house being heated to 20C twice a day.

With respect, that is a meaningless statement without defining the period of time the spells of heating at 20C lasted.

Well I was using my normal twice a day times, which I would assume are close to most normal times for twice a day heating when going to work. Namely on at 06:30 off at 08:30 then back on at 16:00 and off at 23:00.

Taking two extremes simply to illustrate the point; without doubt a constant 17C would be cheaper than two periods of 11 hours at 20C!!. On the other hand a constant 17C would be more expensive than two 30 minute periods at 20C.

I was, of course, using the values I was using to be comfortable in this house, before I started experimenting. So I wasn't using extreme values - I was trying to provide some meaningful data.

Obviously at some point between 2 x 30 mins, and 2 x 11 hours the cost would be equal; but of course that point would only apply to your house at a specific outside temperature.

Now don't be silly, I said I had lots of measurements of gas usage and the max. temperature difference between the inside and outside. So I had lots of daily data points on a plot of usage versus temperature difference. As you seem to be interested, each state (themostat & period) generated it's own set of points which formed a rough noisy curve (note that as these are real measurements, you get a few outliers - primarily due to strong winds [which end up being so off scale, whatever state you are trying to measure] so as usual with real measurements you have to ignore these outliers - ie don't make deductions from measurement when there is a strong wind). From these plots it was quite evident that the values I gave earlier were correct. if I switch from twice a day to 24hr heating and drop the thermostat by 1C then the cost doesn't change, if I drop the thermostat lower then I save money.

One further point, about comfortable temperatures. Personally I find it very difficult to accept that 17C is an acceptable temperature for sitting in a living room. That is a true 17C and not a setting on a wall thermostat. The recommended temperature is 21C

I NEVER said the house themostat was in the living room, the thermostat is in the hall, which is colder than the living room by about 3C.

Richard019

Could the "always on at a lower temperature is cheaper than two timed periods" theory have come from controlling the temperature of the water in the heating system itself rather than talking about room temperatures?

I remember at school doing an experiment comparing how long it took to heat water from 20-30 degrees, and then from 30-40 degrees. The time difference showing that with a constant input of energy from the bunsen burner it took more energy to heat the second 10 degrees than the first 10 degrees.

People saying the difference between internal and external temperature affects the energy loss rate are correct, but your boiler doesn't heat your room, it heats the water in the system, your rooms are just a layer of insulation around the system that reduce the rate at which your radiators lose temperature by acting as a buffer.

If the temperature is maintained constantly then the boiler only needs to be set to heat the water to a few degrees higher (more as your home's insulation reduces), it only has to compensate for the loss caused by the difference between the system temperature and the exterior temperature.

If it's on two timed periods a day, to get it up to the same room temperature in it's available time the water in the system needs to be a lot higher, which requires much more energy to accomplish. It'll be starting from a lower temperature (if always on loft tanks will be warmer through ambient heat being higher during the off period and working as a heat sink) and going to a higher temperature.

wantanswers

macman wrote: »

no one would be cold this winter.

Sounds like a song in there somewhere Mac! add steam tables and I suppose you got an xmas no.1!

terry2

Richard019 wrote: »

Could the "always on at a lower temperature is cheaper than two timed periods" theory have come from controlling the temperature of the water in the heating system itself rather than talking about room temperatures? ... etc

I don't think so. In my opinion it is due to the large thermal mass of the internal structures within the insulation of the building, ie all internal walls and the inside layer of the external walls. If you are on a twice heating system, then to a large extent these structures are at a low temperature (ie whatever they have declined to since the last heating). So your body finds itself in a structure which has a hot radiator, warm air and cold walls (which are effectively acting as radiators of cold - and there is a lot more cold area than hot). The net effect is that to feel comfortable you have to have a high thermostat setting. Note that during this period, a lot of the heat being produced is going into warming up these cold walls. When you go into the heating-off period, these walls cool down again.

When you are in an 24hr constant heating state all these walls have reached, and are maintained, at a steady constant temperature, which means they produce a lot less cooling (as far as your body is concerned) with the result that you can feel comfortable at a lower thermostat setting. Note that the thermal mass is so big that it can take about 48hrs for the walls to actually reach that steady state.
In this steady state all the heating being produced is going into replacing the heat lost to the outside, none of it has to be used heating up the internal structure.

The arguement is whether you believe that the heat lost to the outside by 24hr use is more or less than the heat required in the twice a day case to both lose the heat to the outside and to warm up the structure inside the insulation. Any real calculation is 'orribly complicated so it was easier to do an experiment on my house and measure it. This showed that I saved money in this house with the heating on all the time.

Now if you live in a modern building with oodles of glass and no real amount of internal mass, then this may not be relevant. In a house with no wall insulation I'm not in a position to comment, but without doing the experiment I doubt than anyone else can do more than pontificate.

wantanswers

@richard 019,

Uswitch as most others do sum it up nicely:

http://www.uswitch.com/energy-saving/guides/heating-on-all-the-time/

Should I keep the heating on all the time?

"If you have a well-insulated home, you can test whether putting on the heating 24/7 is cheaper than programming your system to come on at certain times of the day.

To get a good idea of the energy usage for each option, you can leave your heating on constantly for a week, followed by a week of programming your heating to come on twice a day.

You will need to take a meter reading at the beginning and end of each week, and from the results you will be able to see - assuming the weather and temperature outdoors have been similar across the two weeks - which approach is the most energy-efficient for you."

So really at the end of the day its....

"which approach is the most energy-efficient for you."

Thermodynamic Laws taken into consideration of course!