Heating on all day in house vs electric heater in single room switched on when needed

smogit2

A couple of things to think about when calculating costs:
1) Outside temperature will effect all these calculations, C/H on a cold day uses more gas than on a warm day due to the thermostat in the house and on the temperature of the water in the C/H system. Same goes for electric heating.
2) You house's construction will also have a bearing on the most efficient option. When we lived in a solid walled house, if the inside walls got cold, it would take a lot of heat to re-warm the house, probably more than running the heating all the time on a low level. In a similar vein don't count un-lagged pipes as wasted energy unless you have very thick carpets or if they are outside the house.

smogit2

SunReader wrote: »

A rated condensing boiler, in the real world outside lab conditions might be nearer 85% efficiency or less

If you have the boiler in a room you want to heat, this increases the efficiency a little, only the heat vented outside is wasted then.

Cardew

smogit2 wrote: »

2) You house's construction will also have a bearing on the most efficient option. When we lived in a solid walled house, if the inside walls got cold, it would take a lot of heat to re-warm the house, probably more than running the heating all the time on a low level. .

Yet another house then where the laws of thermodynamics don't apply;)

I can understand your reasoning - but I am afraid it is completely wrong.

Let us take an example:

You leave your house for, say, 10 years.

Would you keep the heating on at a low level all the time for the 10 years, because it would take more heat to warm up the house when you returned? I suggest the answer is not in dispute?

How about 1 year?

A month?

A week?

A day?

A hour?

At what point does the law of thermodynamics cease to apply to your solid walled property? - or any property or body!

Keeping the house at a low level of heat makes sense in that it will enable the house to be brought back to temperature quickly - but it isn't cheaper.

smogit2

Hi Cardew,

I wouldn't argue with the laws of thermodynamics. Maybe efficiency is the wrong word here. I guess what I am saying is that heating a single room with an electric heater (which costs more to run, so is money inefficient) is not always as efficient as heating the whole house on a cheaper fuel. In other words, that heat is not wasted in the rest of the house because you would have had to use it later anyway...

macman

But you are not comparing like with like, so your premise is invalid. I could equally suggest that heating the whole bedroom overnight is 'inefficient' when an electric blanket would be much cheaper and do the job. But once again, it's not the same thing.

Cardew

smogit2 wrote: »

Hi Cardew,

I wouldn't argue with the laws of thermodynamics. Maybe efficiency is the wrong word here. I guess what I am saying is that heating a single room with an electric heater (which costs more to run, so is money inefficient) is not always as efficient as heating the whole house on a cheaper fuel. In other words, that heat is not wasted in the rest of the house because you would have had to use it later anyway...

Hi,

I was merely commenting on the issue you raised about it being cheaper to keep heating on low and giving a solid walled house as a justification.

This issue is raised every year several times, and the same(false) logic as you gave is trotted out.

It is just one of those urban myths that need knocking on the head.

Also the 'is it cheaper to heat one room by electric rather than whole house by gas CH?' comes up time and again.

There is no 'one size fits all' answer.

If you switch off all radiators in rooms not in use, then generally gas CH will be cheaper.

However for the reasons given above, it is not always convenient to do so, so it becomes a compromise between cost and convenience.

I was in my study late last night after heating had gone off, and put on a fan heater for a few minutes. Not cheaper - but more sensible than wandering around the house switching off radiators.

smogit2

Yes, it depends on how you use your house, sure if you live in one room, or sleep in a bed just heat that ;-)

My tip is simple look at the whole system efficiency not just a single room or part of the use. My premise is that I want to pay the least (cost efficiency) to heat my house, to the use I put it to. How the heating options are utilised to reach this is not only dependant on energy conversion efficiency.

Another example with different assumptions. If I want to sit in a cold room for 20 minutes heating the air around me to an acceptable temperature, which is more efficient? Turning on a 1kw electric convection heater, turned on 1 hour before I enter to heat the room, or 1kw fan heater turned on and pointing at me whilst I am in the room for 20 minutes (with the rest of the room cold)? Both heaters have the same energy conversion efficiency, both options make me feel just as warm, one option costs and uses more energy than the second. Hence the time to heat the space around me is important. The same is true of all options for heating the use you put it too maters for overall system efficiency.

The OP indicated he worked from home half the week, and presumably lived in the house in the evenings, and also had a baby (who would want around a 18oC room at night), so thinking about how the whole house is heated is important.

SunReader

Thermodynamics also dictates that the rate of heat loss outside from a building would be directly proportional to the temperature differential, delta T: consider three different scenarios:

a) toasty 23.0 degrees inside, -15.0 outside = dT is 38 degrees

b) tempered 14.0 degrees inside, 8.0 outside =dT is 6 degrees

c) comfortable 20 degrees inside, average outside temp 2 degrees dT=18 degrees



(a) is pretty much worst case scenario in uk, even a c rated house will be losing a lot of heat in these conditions, an old house probably hemorrhaging heat and glowing like a lighthouse if viewed on an infra red heat camera.

(b) would be much lower heat losses ,equivalent to OP's example, house rads off daytime and heating just one room in an otherwise unheated house. 'losses' from underfloor pipework would just temper the air by just a few degress and heat losses would be only a third of (c) where the house got much warmer inside, but colder outside at night.


So leaving the whole house heated all day and night when not required : (c) applies, high average delta t= high heat loss.

On the OP's example, maybe (b) applies: all house rads turned off daytime and just one room in an otherwise unheated house, but with a moderate gain around the house near the pipe runs from heat 'losses' from the pipes.

So the increased heat losses from leaving a house warm all day far outweigh those of just heating evenings and at night . But the rate of heat losses is only a fraction when the air is only warmed only slightly. Thick stone walls have a low U value, they have quite a high thermal mass (acting l guess like a storage heater if on an inside wall) so on small temperature gradients the outside heat losses would be further minimised.

SunReader

condensing boilers work better when the CH return is cooler, hence i think best efficiencies would be had with low water temps and big radiators. I'd be interested to see some firm real world data on A rated condensing boiler efficiencies on a permanent duty cycle.