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

Cardew

babbity wrote: »

If someone works long hours and is away from the house for 12 hours or more as the norm, during a cold winter, turning the heating on an hour before they return will not save money.

I am sorry but that is wrong!


When the heating is off, the laws of physics state that as the house cools then less heat is lost.


As you apparently do not agree with the laws of physics, let us extend the discussion.


You say 'away from the house 12 hours or more' So let us take the situation where someone was away from the house for 5 years*. Would you maintain it was cheaper to leave the heating on for those 5 years?


How about 1 year? 1 month? 1 week? 1 day? at what point do you feel the laws of physics do not apply?


Similarly if your theory were to hold true, we would leave a kettle/saucepan of water simmering all day, as it would cost more to heat them from cold.


Obviously there has to be a compromise between cost and comfort, and if the CH could not warm the house in 1 hour after an absence of 12 hours, then the heating would have to be on longer - say 2 hours. But that would be still be cheaper than leaving the heating on during those 12 hours.


* To pre-empt the usual distraction, no frost damage - system drained and outside temperature never below 2C

gadgetmind

WestonDave wrote: »

Which brings me to the first comment - there is such a thing (widely available) as a programmable thermostat which combines timer and stat - which is better than either option listed.

Yes, we have three of them, one per floor. They let us set different temperatures for different times of day, and to vary this from day to day. They also learn the thermal inertia of the house so they come on sooner if necessary to get it to the required temperature at the required time.

Coupled with TRVs throughout, we pretty much never have to touch the heating. Throughout the year, the settings are the same, and it just keep us comfortable. The only exception is if someone is in during the day when they otherwise wouldn't be, and gets chilly because they are sat in (say) the study. However, a couple of taps of an override button, and problem sorted.

gadgetmind

Cardew wrote: »

When the heating is off, the laws of physics state that as the house cools then less heat is lost.

Agreed 100%. Efficiency is all down to amount of energy consumed, and energy is only consumed if it's lost from the system. All losses are through doors, windows, walls, roof, etc. so you insulate and draft proof a lot *and* keep the temperate gradient as shallow as you can.

naedanger

Cardew wrote: »

You say 'away from the house 12 hours or more' So let us take the situation where someone was away from the house for 5 years*. Would you maintain it was cheaper to leave the heating on for those 5 years?

How about 1 year? 1 month? 1 week? 1 day? at what point do you feel the laws of physics do not apply?

I'm not sure this reasoning will get you very far. Instinctively someone could (wrongly) feel the optimum is somewhere between around 12 or 15 hours say. Just as they may instinctively (correctly) feel the optimum constant speed in their car for maximum fuel economy is 60 mph (or whatever the actual optimum speed is), in other words somewhere between the extremes.

I do agree with you, namely it is cheaper to only heat the house when you need it (with the heating coming on just early enough to get the house to the desired temperature for when it is needed). But I think the problem is that it can be hard to see the answer instinctively.

Personally I think the easiest way to thinking of it is to consider that, assuming you start with the property fully at the desired temperature, from that point onwards you are only paying for the heat that escapes. So then consider what heating regime will lead to the least heat loss. (Then I think the correct answer is a bit more instinctive.)

A further complication is that the temperature that people are comfortable can change with conditions. Some are obvious e.g. if you have just come indoors or been active but other times it is not so obvious. Personally I cannot work out what the missing factors that affect perception. But sometimes I think direct heat is a bit addictive. So if you feel warmth radiating from a fire you want that to continue even when the room has come to a temperature you would normally be comfortable. Therefore I can see why in some cases trying to maintain a constant temperature may enable people to feel more comfortable at a lower temperature than rapidly heating from a low temperature. (But I am only talking about at the margins.)

r2015

When I took advice to turn off my heating during the day, timed to come on about an hour before I got home from work, my gas usage was higher than if I had the boiler ticking over on a low temperature during below freezing days, or coming on at intervals during the day.

I have found that as well.

Since I retired I have the heating on from 8 am to 11 pm and if it is very cold outside sometimes 24 hours and I do not seem to use much more gas than when the heating was on from 7 am to 10 am then from 3 pm to midnight.

From when I bought my house in 1991 to when I retired in 2005 my average yearly gas consumption was 20721 kWh per year and from 2005 to present it has been 18058 kWh per year.

A difference of 2663 kWh per year.

Let the laws of physics explain that.

kintrepid

Cardew wrote: »

1. Well firstly please make this thread a sticky as the 'Cheaper all day v timed' question comes up a dozen times each year.

The normal variation of that question is:

'Which is cheaper - on 24/7 at, say 15C or in timed bursts to 20C'

A question nobody can answer!

2. The EST have a fixation about standby consumption. The manufacturers got to grip with that problem many years ago and standby consumption for, say, TVs is tiny - fractions of a watt. You can leave a modern(up to 10 years old) TV on standby 24/7 for a year and the cost is pence. Even the worst of my TV's - an old 32" CRT has a standby consumption of 0.6 watts. so left on 24/7 it will cost 63p a year with electricity at 12p/kWh.

Obviously any saving by switching off at wall is welcome, but the emphasis on standby consumption leads many people to think it is the major cause of high electricity bills - and it aint! We get countless posts complaining about high electricity bills and nearly all are at pains to point out that they switch off TV at wall.

3. This is misleading IMO:



That might apply to the TRVs and thermostat in the same room. However the purpose of TRVs is also to control the warmth of other rooms. Most people would want their bedrooms set to a low temperature during the day which you can achieve with a TRV. e.g a reception room set to 21C and bedrooms to 16C

4. The recommendation to use an airer instead of a tumble dryer is questionable in modern houses that are almost hermetically sealed!! The problems of condensation and mould can be serious.

Cardew - since you mentioned TVs in your comment, can I ask you a (honestly) genuine question? I'm confused about modern TVs and standby. New TVs don't have an on/off switch, you can only do it via the remote. Does that mean they're constantly on standby? Or does the remote power button now have the 'power' of the old on/off switch? Basically, am I always leaving my TV on standby or am I turning it off properly with the remote?

Cardew

kintrepid wrote: »

Cardew - since you mentioned TVs in your comment, can I ask you a (honestly) genuine question? I'm confused about modern TVs and standby. New TVs don't have an on/off switch, you can only do it via the remote. Does that mean they're constantly on standby? Or does the remote power button now have the 'power' of the old on/off switch? Basically, am I always leaving my TV on standby or am I turning it off properly with the remote?

Modern TVs are in standby if switched off by the remote - and usually the red 'power light' on the front panel is on.


To turn them off completely they need to be switched off at the 13 amp socket.

Fitzmichael

I haven't read this thread right from the beginning, so maybe there's an answer I've missed. I'm asking via this Cardew's post because he raises another question that has puzzled me, saying: The problem with a combi is that in winter they struggle to produce a good flow for a shower even if that is the only HW demand.
I have a combi and from the shower there's always a v good flow, and far too hot for me well below max on the temp control setting, but my wife turns it right up and stays under it for a good quarter hour.
The problem is that the temp setting (at my daughter's, too) is only approximate and depends on the ambient temp (presumably of the intake). It, too, of course, can suddenly go colder/hotter if somebody runs water elsewhere. The only thermo-controlled domestic shower I have used which adjusted v quickly and reasonably precisely was electric, but it gave poorer flow the higher the temp; this was not the case in a couple of hotels I've used. Is this, too, dependent on the combi/hot tank question? Assuming the inflow is sufficient, is it not possible to have the outflows from the boiler to the feeds to the bath/shower(s) and kitchen, for that matter, controlled from the outlets? Would it be prohibitively expensive?
It's particularly puzzling as my grandparents' house, long in the family and having housed different combinations of relatives, had the cold and hot water tanks on the top (2nd) floor and 2 bathrooms and a shower-room on that and the 1st floor. On the ground floor were a toilet next to a 'wash-house' upgraded to 'utility room' and two kitchens at different extremities of the house. These were a long way from the hot tank and seemed never to have been connected to it, but each had a small gas geyser above the sink, which supplied instant hot water.
This last has made me wonder, over the last 20 years of increasing emphasis on minimising waste of fuel and water, why we still have water heated far from where it is wanted, so that yards of cold water run to waste until the warm/hot water gets into the, colder, pipes, which consume some of its heat, in a repeated cycle a dozen or more times a day. Is a more efficient system not possible (why?) and if it is, why have our, supposedly concerned, governments not altered Building Regs to impose it?
Now to my intended question about efficient regulation of house temperature. Posts seem to concentrate on adjusting the TRVs according to room usage but surely what's needed is to vary the temperature according to time, and I'd never heard of valves to do that until, in the course of this post, I spotted Weston Dave's "there is such a thing (widely available) as a programmable thermostat which combines timer and stat", but I'm not sure he's talking of something to control a radiator. Can he, or anyone, enlighten me? Thanks.

gadgetmind

naedanger wrote: »

But I think the problem is that it can be hard to see the answer instinctively.

Maybe, just maybe, but this is why we have maths. It gives us the right answers even when our monkey brains leap to the wrong conclusions.

malc_b

Fact: The laws of physics say turning off heating gives lower losses because as the house temperature falls the losses decrease.

Agreed, but this is lower losses,that is NOT the same as lower fuel bills. It is a well know FACT that condensing boilers are less efficient when running hard, i.e. high water temperature. The most efficient way to run a condensing boiler is with a low water temperature, less than 55C on the return is the usual advice.

So maximum fuel efficiency is to heat for as short a time as possible with 55C return, AND, when the house fails to reach the desired temperatures, increase the heating TIME.

To draw an analogy, it's like making a journey in a car. Bristol to London is always the same distance but which uses less fuel, making it at 50mph or 100mph?

A condensing boiler run in non condensing mode is ~10% (or more) less efficient (google it or compare figures between condensing and non condensing boilers). A well insulated house heated x/24 rather than 24/24 saves about 5% of the heat loss. If you want a simple math proof of this see here. And note the proof needs no values other than the house temperature when you go to bed and in the morning before the heat turns on. And it uses the physics "rules2.

The other point to realise is that because the difference in x/24 and 24/24 is so small the demand on the radiators is dramatically affected by shorter heating hours. So, roughly, heat 12/24 and radiators have to put out twice as much, which means they need to run hotter, exactly want you don't want with a condensing boiler.

Again to draw an analogy, heating a house is like filling a leaky bucket. If you let the level drop then the leak slows but then when you fill it up to the top you have to put in water a lot faster than if you just matched the leak.