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

andy81_2

Cardew wrote: »

Welcome to the forum, and whilst aware we have to be nice to new posters, your post simply defies the laws of physics and your conclusions have no scientific merit.

You contend that by having the heating on for 16.25hours a day you used 50% of the gas that you consumed when the heating was on for 9.25hours a day.

So an extra 7hours heating a day halved the daily consumption!

It begs the question, using your conclusions, why don't you have the heating on 24 hours a day and make even bigger savings i.e. if an extra 7 hours heating halves the bill, another 7.75hours a day must reduce consumption even more!

I don't necessarily agree that Architect First's post defies the laws of physics. You are of the view that the cost of heating a home is a simple on/off calculation, so the more hours the boiler is running, the more fuel is used therefore the higher the cost. However the reality is more complex.

You ignore the impact of condensation build up within the walls and structure of the building which will reduce the thermal insulation ('U') value of that surface. So if the temperature of the house drops significantly overnight (as the heating has been off or the thermostat low) then when the heating comes on in the morning, most of the heat will go to drying out this condensation and heating the walls, ceiling, etc, rather than heating the air. So it will take longer to heat up your house to a given temperature than it would if this condensation wasn't there and there is latent heat in the building. Thermostats only measure air temperature, not moisture levels within walls, etc.

If a house is kept warm throughout the winter, it should prevent the build up of moisture within the walls, etc, and the walls, floors, etc will retain heat for quite some time (at least several days if the heating was switched off completely), thus providing radiant heat and reducing the work for the boiler.

There's probably a cut off point in temperature below which you would save money, but if you want the house to remain reasonably comfortable during the winter and/or you spend more than a few hours at home each day, then it may make sense to go for the 'always on' approach. If however your house temp is constantly around this 'dew point' temperature, then the heating coming on twice per day is going to be quite inefficient, mostly going into drying out moisture build up within the house, and therefore taking a long time to reach your thermostat temp.

Some boilers are also more efficient at lower temperatures (the temperature setting on the boiler), especially condensing boilers which can recover more heat from the CH return when the boiler temp is lower.

Another point you miss (and related to efficiency) is that boilers use more or less fuel whilst operating depending on the heat of the water that is coming into the boiler. Water comes in -> boiler heats water to required temp (boiler temp setting) -> water goes out to radiators. Therefore it follows that if the water coming into the boiler is warmer, the boiler works less hard to heat that water to the required temp, therefore uses less fuel (all else being equal). So it's not a simple 'boiler on/boiler off' calculation. If the water in the CH system is just being 'topped up' by the boiler rather than being heated from cold, then this fact combined with the condensation issue, the efficiency of the boiler mentioned, and the latent heat of the building, could make it more economical to use the 'always on' approach.

It also depends on each person's circumstances though, such as the amount of time they spend at home, the type of property they live in, the amount of cold you can tolerate (!), etc, but in my view the 'always on' approach would make sense financially for many people.

In my case I have started using the 'always on' method and have noticed a drop in our fuel usage. We live in a barn conversion with 2ft-thick walls, high ceilings, etc, and we're using LPG, so I'm very aware of the need to save on heating bills! We also work from home so need the house relatively warm during the day, as well as morning and evening. Previously I'd keep the house fairly cold during the winter (much to my wife's annoyance) and we were still using a fair amount of gas, and coupled with that were getting lots of (expensive) problems with mould, condensation, damaged plasterwork, etc. Since trying the 'always on' method I haven't seen our usage go up (if anything it's gone down), the mould problem has gone and the house is a lot more comfortable to live in!

Cardew

wrote:

Could I ask Cardew's favorite question, which is really good in these circumstances - do your maths again using a month with the heating off rather than 8 hours. Do you still think it's cheaper to keep it on all the time rather than turning it off for a month then on again?

Quote = malc_b;58069835

The answer to that is obviously it would be cheaper to turn the heating because over a month the house would stabilise to the same temperature as the outside. Over a single night or day that doesn't happen.

So we agree that the law of thermodynamics applies for a house left unoccupied for a month; in that it 'obviously' would be cheaper to turn the heating off.

How about a week?

A day?

4 hours?

According to your theory, there must be a point in time where the laws of thermodynamics cease to apply and the laws of malc_b take over.

The analogy to a kettle also applies. For your theory to have any substance you would keep a kettle simmering at boiling point as it would be cheaper than allowing it to cool down and then re-heating the water.

This is from the Energy Saving Trust faq

Question

Is it more economical to leave my heating on 24hrs in the
winter?

Answer

No. It is a common misconception that it is
cheaper to leave your hot water
and heating on all the time. Boilers use more power initially to heat water from
cold, however the cost of this is greatly exceeded by the cost of keeping the
boiler running all of the time.

The best solution is to programme
your heating system so that it comes on when you need it most (possibly early
morning and in the evening), and goes off when you don't need it (when you are
out of the house or asleep). There are a range of controls that can be used and
your heating engineer will be able to provide you with the most appropriate
solution.

Depending on your circumstances it may be necessary to keep the
heating on all day during winter but it will cost more than if you turn the
heating off when you don't need it.

IMPORTANT

1. we are not discussing comfort - obviously you should have your heating set for your comfort.

2. Don't confuse matters by stating that you keep the heating on 24/7 but turn down the heating thermostat for periods. Turning down the thermostat effectively achieves the same effect as turning off the heating.

e.g. I have my timer set to switch off CH at 10pm and back on at 7am.

If my house is set for 20C during the day/evening and I left heating on 24/7 but turned the stat down to, say, 12C at 10pm my heating will not have come on by 7am as the house(the room where the thermostat is located) will not have dropped to 12C overnight.

wantanswers

Question

Is it more economical to leave my heating on 24hrs in the
winter?

Answer

No. It is a common misconception that it is cheaper to leave your hot water and heating on all the time. Boilers use more power initially to heat water from cold, however the cost of this is greatly exceeded by the cost of keeping the boiler running all of the time.

That statement appears to be misleading.... NO person will keep their boiler running all the time because to do that you would probably have to set the Thermostat to max and keep the doors open 24/7

Crock

I used to get a regular update emil from the EST until I tried to do their "how energy efficient is your home ? " questionaire. There was nowhere on the form to input underfloor heating and no sign of any 'green' technology, specificaly Heat Pumps of any description. We run a GSHP (80m borehole) with a ~ CoE of 1:4. When I queried the omission on their 'efficiency' test form and asked whether they were really interested in saving energy and the planet or were just a government organisation trying to salve it's conscience I recieved no reply and my regular 'imformation' email stopped as well !!!
We run the bungalow, 2 bed semi, on a constant 19C for the core of the home with bedroom and lounge adjusted slightly lower/higher as we were advised this was the best way to run a heat pump. All electric home with current running costs of £46 per month. Recently installed solar panels should hopefully see this drop further. We were lucky to be able to rebuild a 70s bungalow from the outside in with modern green tech and insulation. A bit of cooking or 2 bodies in a room with the door shut will bring the temp up another degree or two.

Cardew

wantanswers wrote: »

Question

Is it more economical to leave my heating on 24hrs in the
winter?

Answer

No. It is a common misconception that it is cheaper to leave your hot water and heating on all the time. Boilers use more power initially to heat water from cold, however the cost of this is greatly exceeded by the cost of keeping the boiler running all of the time.

That statement appears to be misleading.... NO person will keep their boiler running all the time because to do that you would probably have to set the Thermostat to max and keep the doors open 24/7

That presumably is meant to be a joke?

andy81_2

wantanswers wrote: »

Question

That statement appears to be misleading.... NO person will keep their boiler running all the time because to do that you would probably have to set the Thermostat to max and keep the doors open 24/7

Absolutely. All systems need at least one thermostat so the 'always on' approach should really be called 'constant temperatue', as opposed to the more variable temperature you get by only having the heating on morning and evening. 'Heating on' doesn't mean 'boiler on', it just means the heating is available if the thermostat calls for it.

As for the question by Cardew above, the laws of thermodynamics are immutable as far as I know, but I would say if the house is to be unoccupied for more than a few days, then it would be worth setting the thermostat to a much lower temperature than normal as the cost of heating the house from cold would be less than keeping an unoccupied home at a constant high temperature. You would need to do the sums to work out the exact time but I would have guessed a few days or so (obviously if you were leaving for a month or a year you shouldn't leave your heating on!).

wantanswers

Cardew wrote: »

That presumably is meant to be a joke?

It is not but I think the part answer to the question (keeping the boiler running all of the time) is!

HappyMJ

malc_b wrote: »

As wantanswers has pointed out houses are different. Mine is 5 bed. Also, don't confuse air temperature with house structure temperature. The air in the house changes 1-2 times per hour so naturally that warms up pretty quick. The walls take longer. The walls suck heat out of the air during the day and give it back at night. They are what maintains the temperature during the night when the heating is off. Otherwise inside temperature would be the same as outside at least after 30-60 min when all the air has been changed.

And the figure of 1-2 changes per hour is the recommend value YMMV.

My house doesn't leak that much air. It's 70sq.m ceiling height 2.4m...So 168 cubic metres. Dividing that out at 1.5 changes per hour means 70 litres of air is changing every second. That's a very leaky house if it were. Even my extractor fans only extract 85 cubic metres of air per hour when they are turned on so that just makes 1 change per hour....but I don't leave them on. All my ventilators at the moment are shut to keep the heat in and there are very few drafts so the air change rate is very low.

macman

Heat is not lost merely by air circulating in and out of the house though. A airtight house will still lose heat if the ambient temp outside is lower.

Cardew

wantanswers wrote: »

It is not but I think the part answer to the question (keeping the boiler running all of the time) is!

There is surely no ambiguity in the EST's statement!

No. It is a common misconception that it is cheaper to leave your hot water and heating on
all the time. Boilers use more power initially to heat water from cold,
however the cost of this is greatly exceeded by the cost of keeping the
boiler running all of the time.

The EST very clearly mean 'leaving the boiler switched on 24/7' as opposed to timed.

That statement appears to be misleading.... NO person will keep their boiler
running all the time because to do that you would probably have to set the
Thermostat to max and keep the doors open 24/7

If we are down to semantics, don't you mean 'Firing all the time' ?