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

Gromitt

I've tested this theory out several times over the last couple of weeks (as doing it just once is pointless) and I've come to the following conclusion:

When outside temp is 2 - 3 degrees C:
It costs about £2/day for a quick burst before I get up (typ 19c), day heating (13c), evening heating (21c) and overnight heating (16c)

If I stay at home all day it costs about £2.20 and the only difference is that the temperature is set to 20c during the day, 21 evening, and 16c after I go bed.

So a 20p difference, but 365 * 20p = £73. If only it was that easy, because before November the difference was more like 5p or so.

So I'd guess it would cost me £20 - £30 a year extra if I had my heating on all day at 20c.

Gromitt

blimeyharri wrote: »

It's like having the kettle constantly boiling all day just in case you want a cup of tea at some point...

Well, it depends. We have an urn at work which is basically exactly that - it keeps the water inside the 'kettle' above 90c. We used a killawatt meter and found out it was negligible difference in cost as when the urn did boil it did so in such short bursts because the water was already near boiling rather than freezing cold straight out of the tap. But yes if you only have one cup of tea per day it would be a costly exercise, but then again you are unlikely to be using an urn.

Cardew

updated version!

https://www.moneysavingexpert.com/utilities/energy-saving-myths

malc_b

Hearty74 wrote: »

Interested in thoughts/opinions on merits of replacing standard gas boiler & hot water tank with combi boiler for central heating / hot water. Hot water tank fitted with thermostat which can be regulated so that when temp of water is reached heating system bypasses coil in tank which has certainly improved efficiency / cost saving but plumber / heating engineer maintains further savings could be made by converting to combi. Not sure if his advice has merits or is he just looking for further work

Any heat you lose from the HW cylinder during the 9mths of the year when the heating is on just goes to heat the house so isn't wasted. The main benefit of a combi is you saving the airing cupboard so combi suits a small house which benefits from more floor space. Of course you then need somewhere to store your towels.

The disadvantage of a combi is the limited flow rate. Hot water is heated from cold so the flow can only be as fast as you can heat the water. Again this is not so much of problem in a small house but a house with 2 bathrooms with showers say might have a problem giving adequate supply to both showers at the same time. Big houses with 3, 4, 5 bathroom are obviously worse again. You then end up in the silly situation of having a lot of bathroom but you can't use them all at the same time.

Cost wise you can work out the saving by looking at the label on the tank, if it is a newish tank, or looking online for a similar sized and type of HW cylinder so see what figures that gives. It should tell you kW/day loss. Multiple that by how much you pay for gas, and say 100 (100 days when you aren't heating the house) and that will give you an idea of the saving.

Hominu

This is exactly why I don't leave the heating on all day, in pretty graph form:



House is heating in the morning to raise temperature to 19c. It then gradually decreases to 15c during the day and pretty much stays there, then comes on at 15:30 and raises the temp back to 19c for 16:30.

It probably wouldn't cost much difference to leave it on to be honest, but since I'm not there it seems a little pointless, plus I'd imagine starting from "cold" would be more efficient for the boiler.

malc_b

Interesting graph. It shows what I've tried to explain. The heat lost from the house depends on the difference between inside and outside. As we can see from this graph the inside temperature is still pretty constant even though we aren't heating the house for a long time. That means the heat lost is pretty constant whether you heat 24hr or 16/24 or 4+4/24. You save a little energy of course but not as much as people think.

It's a mistake to think that starting from cold is more efficient. Look at this way, if the energy you need to put into a house is pretty constant, let's say 100% is 24/24 heating and 90% is 4+4/24 heating. Hence when you heat for just 4+4 hours you need to get in 90% but in just 8hrs, i.e. the radiators need to be at 270% power compared to 24/24. To get more out of radiators you need to run them hotter. Running hotter is less efficient.

Of course it depends on how hot. If you radiators are oversized and the weather isn't too cold then maybe you stay in condensing mode so then the boiler is still running efficiently and you'll save gas. But if that doesn't work out and the boiler has to run too hot for condensing mode then it will be running less efficiently and that might cost you more gas than you save by running short hours. The key is to keep the boiler in condensing mode even if that means running longer. 24/24 is the extreme case of running longer.

System

malc_b wrote: »

It's a mistake to think that starting from cold is more efficient. Look at this way, if the energy you need to put into a house is pretty constant, let's say 100% is 24/24 heating and 90% is 4+4/24 heating. Hence when you heat for just 4+4 hours you need to get in 90% but in just 8hrs, i.e. the radiators need to be at 270% power compared to 24/24. To get more out of radiators you need to run them hotter. Running hotter is less efficient.

Of course it depends on how hot. If you radiators are oversized and the weather isn't too cold then maybe you stay in condensing mode so then the boiler is still running efficiently and you'll save gas. But if that doesn't work out and the boiler has to run too hot for condensing mode then it will be running less efficiently and that might cost you more gas than you save by running short hours. The key is to keep the boiler in condensing mode even if that means running longer. 24/24 is the extreme case of running longer.

As I have said previously, I have been monitoring gas usage throughout this Winter season. I have a modern modulating boiler controlled with an Opentherm connection that adjusts the size of the gas flame. With this type of set up up, my data shows that when the heating comes on at 7.30am, it is cheaper just to leave it on until 10pm. A key to efficiency is the need to avoid fiddling with the thermostat and TRVs. Any adjustment will invariably result in an increase in the boiler flow temperature and a loss of efficiency.

Cardew

malc_b wrote: »

It's a mistake to think that starting from cold is more efficient. Look at this way, if the energy you need to put into a house is pretty constant, let's say 100% is 24/24 heating and 90% is 4+4/24 heating. Hence when you heat for just 4+4 hours you need to get in 90% but in just 8hrs, i.e. the radiators need to be at 270% power compared to 24/24. To get more out of radiators you need to run them hotter. Running hotter is less efficient.

Of course it depends on how hot. If you radiators are oversized and the weather isn't too cold then maybe you stay in condensing mode so then the boiler is still running efficiently and you'll save gas. But if that doesn't work out and the boiler has to run too hot for condensing mode then it will be running less efficiently and that might cost you more gas than you save by running short hours. The key is to keep the boiler in condensing mode even if that means running longer. 24/24 is the extreme case of running longer.

We go round and round in circles on this subject. It was discussed here: https://forums.moneysavingexpert.com/discussion/5090074

In all the publications I have read, and enquiries I have made, I have yet to discover just how much more efficient a boiler is when running in condensing mode.

In any case the key to getting the boiler to run in condensing mode is to keep the return water temperature low. With a cold house, with all radiators on full, the boiler will initially be in condensing mode regardless of the set water temperature.

SwanJon

malc_b wrote: »

Interesting graph. It shows what I've tried to explain. The heat lost from the house depends on the difference between inside and outside. As we can see from this graph the inside temperature is still pretty constant even though we aren't heating the house for a long time. That means the heat lost is pretty constant whether you heat 24hr or 16/24 or 4+4/24. You save a little energy of course but not as much as people think..

If heat loss is pretty constant, but internal temperature remains the same without heating, where is the extra heat coming from?
In fact, the heat lost to bring it down to that constant is what needs to be replaced, and replaced continuously, but if on timer it only needs to be replaced once.

malc_b wrote: »

It's a mistake to think that starting from cold is more efficient. Look at this way, if the energy you need to put into a house is pretty constant, let's say 100% is 24/24 heating and 90% is 4+4/24 heating. Hence when you heat for just 4+4 hours you need to get in 90% but in just 8hrs, i.e. the radiators need to be at 270% power compared to 24/24. To get more out of radiators you need to run them hotter. Running hotter is less efficient.

Of course it depends on how hot. If you radiators are oversized and the weather isn't too cold then maybe you stay in condensing mode so then the boiler is still running efficiently and you'll save gas. But if that doesn't work out and the boiler has to run too hot for condensing mode then it will be running less efficiently and that might cost you more gas than you save by running short hours. The key is to keep the boiler in condensing mode even if that means running longer. 24/24 is the extreme case of running longer.

All your numbers are based on this arbitrary assumption.
Why 33% of the time need 90% of the energy put in? (or 66% for 90% in your earlier post) Not saying the condensing mode could not make a difference, but your initial premise undermines any later calculations.
Once the house is up to temperature there should be no difference between on 24 or on 4+4 (or 16 etc), so the actual comparison should be between the 16 (or 8 etc) hours of heating when not required and the two hours (one hour twice) it takes to get back up to temp (from Hominu's chart, 1530-1630). Yes that warm up hour will be more expensive than any one keep warm hour, but is to 800% more expensive?

Hominu

SwanJon wrote: »

If heat loss is pretty constant, but internal temperature remains the same without heating, where is the extra heat coming from?

The sun, and it makes a big difference.

If it's a cloudy rainy day then the house gets colder much quicker than if it's a bright sunny day, even if the temperature outside is 3 degrees C both times.

However, all my temperature data is from one thermostat in the hall, so YMMV, as with most things.