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

markin

victor2 said:

markin said:

Reed_Richards said:

I hope nobody would dispute that, @Qyburn.  But what has kept this thread running and running is the suggestion that because of the way some heat sources operate, the extra energy used to bring the house back up to temperature is more than the energy saved in allowing it to cool.  Gas boilers and heat pumps tend to work less efficiently when working harder so this is indeed a theoretical possibility, and there is a paucity of actual evidence one way or the other.

The is a very clear US DOE study on air/air heatpumps in new identical homes that shows that people that set constant 24/7 temps saved almost 50%, from memory Vs the people that used on/off.

.........

This thread has been bumped almost every year from 2012.

But that then raises the question (and not for the first time!), when is having it on 24/7 more costly. If you're going away for the weekend, do you leave your heatpump on to save money?

This thread has indeed been like a phoenix rising from the ashes annually when it starts to get cold! Didn't think it was going to appear this year, but it hasn't let us down.

From the many posts from people that had looked at gas usage when they return from days, or a week away it would seem at least in brick, high mass houses then over around 5 days the savings out number the extra gas usage to re-heat that mass.

In a timber stud house or with internal insulation the only mass is the plaster board and flooring so it heats up that and the air space very fast.

JSHarris

markin said:

In a timber stud house or with internal insulation the only mass is the plaster board and flooring so it heats up that and the air space very fast.

Not always.  Our house is 100% timber, except for the ground floor (a concrete slab) and the recycled plastic roofing "slates".  It has a very long decrement delay, as the structure has a fairly high specific heat capacity, mostly the concrete ground floor, the blown cellulose insulation (300mm thick in the walls, 400mm thick in the roof) and the high specific heat of the gypsum plasterboard and skim (there's about 8 tonnes of that in the house).

Cardew

JSHarris said:

In my view it does, as the thermal time constant seem to be a critical consideration.  If your home takes a couple of days to warm up after having the heating turned off for a time, then that makes for a pretty uncomfortable time when you get home, turn the heating on and then wait around in a chilly home for the rooms to become warm (which is a direct consequence of having a long thermal time constant).

We can mitigate this to some degree by opting to use additional heating, like fan heaters that warm the air up quickly, but they would be very costly to run, and would cost a great deal more than just leaving the heating on and keeping the house warm when we're not at home.

Obviously how anyone runs their heating is ALWAYS a compromise between comfort and cost.

However that wasn't the 'exam question' i.e. the title of this thread! 

Reed_Richards

Cardew said:

However that wasn't the 'exam question' i.e. the title of this thread! 

Maybe somebody needs to mark the answers, giving no marks for not answering the question posed? 

JSHarris

How on earth can the question that is the title of this thread not be intrinsically linked to the decrement delay of the house?

To use a couple of extreme examples to illustrate this, someone living in a well-insulated, but very short decrement delay home (say a modern mobile home) could probably save money by only heating it when they are there.  Because of the short decrement delay it will heat as rapidly as it will cool.  Conversely, someone living in a home like hours, with a very long decrement delay would be unable to do this.

Edited to add this link that explains this physical property better than I could here: https://www.greenspec.co.uk/building-design/decrement-delay/

Qyburn

JSHarris said:

How on earth can the question that is the title of this thread not be intrinsically linked to the decrement delay of the house?

It doesn't, but it confuses the issue because sometimes it's not practical to allow the buildings to cool down between occupancy times, even if it would save money.

  

JSHarris

Qyburn said:

JSHarris said:

How on earth can the question that is the title of this thread not be intrinsically linked to the decrement delay of the house?

It doesn't, but it confuses the issue because sometimes it's not practical to allow the buildings to cool down between occupancy times, even if it would save money.

  

Not sure it confuses anything, it just establishes that it's an impossible question to answer generically.  There is no one right or wrong answer to the question posed, no one-size-fits-all definitive answer. 

For someone living in a short decrement delay, but well-insulated, mobile home and out at work all day it is almost certainly going to be better to only heat their home during the time they are home (perhaps starting half an hour before they get home).

For someone living in a long decrement delay home it almost certainly makes more sense to keep their home heated all the time, in fact they may not have much choice, as the heat up time may be so long that this is their only comfortable option.

victor2

So the short answer is NO.

That is, for a majority of people. Then define majority!

JSHarris

victor2 said:

So the short answer is NO.

That is, for a majority of people. Then define majority!

I think the true answer to the question is "it depends"!

Our first home was a 200 year old stone built, terraced cottage that was best kept warm all the time, as it took a long time to heat up.  Our next home was a well-insulated timber frame house in Scotland, with a stupidly over-sized oil fired boiler and massive radiators.  That was fine with the heating off all day whilst we were out at work, as it heated up very quickly when the boiler fired up.  Our current house is of all-timber construction but has such a long decrement delay that it's impractical to do anything other than keep it warm all the time.

molerat

I think the true answer to the question is "it depends"!

After many years of trying all options I found that the cost difference was minimal so it is now a case of not really caring, the comfort factor is more important than the cost.