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

victor2

It's that time of year again....:)
Just out of curiosity, I did a simple test the other day.
I let the house cool down from the central heating being on with the thermostat set at 19 degrees C. After 6 hours the thermostat said it was 16. The outside temperature was indicated as 4 on my cheapo digital thermometer.
I switched the heating on and monitored the gas usage (an old inefficient boiler by modern standards). It took 3 hours and 39.9kWh of gas to get the thermostat back up to 19. By which time the outside temp had dropped to 1.
From that point on, the boiler was using 9.2kWh of gas per hour.
So, if I'd left the boiler on for the 9 hours, I would have used just over 82kWh of gas, probably a bit less as the outside temperature was mostly a bit higher than when it finished. About twice as much gas over 9 hours compared to switching the boiler off for 6 hours then heating the "cold" house up.

Not a huge expense to leave the heating on, and certainly more comfortable, but an extra cost nevertheless. Which as already said, if it's on, it's costing money, if it's off, it isn't.

maxmiler

I have a technical query. Last year I had a completely new gas CH system fitted . It has a Combi boiler (condensing because it is new). I have tried to run it with a water temperature of 55 which should allow the condensing feature to improve efficiency. However, the house doesn't get warm enough until I put the water temp up to at least 65, so it does not run in condensing mode.
So what is more efficient?
1. water temp at 65 which means the boiler is on (and pumping) for longer in each cycle.
2. water temp at 75 which shortens the cycles (and saves on electricity as the pump runs for less time.

Thanks!

oldskoo1

Here are some figures

I understand the need to make sure my tests are conducted with the same weather conditions, average ambient temperatures and same starting temperature. With these things considered.

Scenarios tested
1) 17.5c on 24hr
Cost = 10 units costing £5.36 p/d

2) 17.5c timed (6am - 11pm) - then drops down to 12c but never kicks in
Cost = 13% less than scenario 1

3) 16.5c timed (6am - 8am) - drops to 16c until 5pm then 16.5c 5pm till 11pm and then drops to 12c overnight
Cost = 25% less than scenario 1 (24hr)

I was lucky enough to have a run of overcast days where the outside temperature and wind was almost identical. There is still a margin of error but there is a trend.

I've repeated these tests will similar results before.

For example, about a week later i had a day that was the same temperature but it was sunny all day and we were running scenario 3 but didn't use the gas hob that day and used 40% less gas than the 24hr on scenario. So you can see how much of an affect other factors can have when testing these things.

The best thing would be to turn down the thermostat, this seemed to save the most. You have to be comfortable nobody is disputing this but 16c - 16.5c in my hall is 21c in the sitting room and 19c in the bedrooms controlled by the TRV's.

There are times like right at the end of the cooling period before the thermostat kicks back in where you can feel it getting cooler but it gets very nice and warm after this.

sheffield_lad

Oldskool1, thanks for that it's interesting to note. I think the reason for such differences in opinion is all to do with the room stat. No one really expects a boiler running 24hrs per day to be efficient, but what me and other posters have discovered is that leaving it on and using the room stat to regulate a temperature through the day is still efficient and more comfortable. Also houses differ on how they operate and how efficient they are. For example my hall is as warm as my living room.

I guess if you are prepared for a little discomfort and lower temps savings can be made via a timer but leaving the heating on 24hrs and using a room stat can also be economical. (If some of you have not got a digital one btw buy one (£30-£40), its well worth it as it displays the current temp and the target temp. (even my wife almost gets it lol).

JerryW

We seem to have two separate discussions going. One is about what is warm and convenient. One is about which is cheaper and more efficient.

I am afraid that the idea of leaving the heating on because it is cheaper than using it only when needed is just plain wrong. It doesn't work.
If you like having the house warm 24/7 fine. No problem. Go for it. Just don't pretend its somehow more efficient or cheaper. It isn't. The longer its on, the more it costs. Simples.

Cardew

Mark_Beech wrote: »

Those who are saying that this defies the laws of physics are simply not thinking this through.
The logic is that leaving the heating on 24/7 but controlled by a thermostat and turned down low overnight or when away from home CAN be as cheap as turning the heating on and off with a timer twice a day BECAUSE it takes an ENORMOUS amount of energy to reheat the fabric of a house that has been allowed to completely cool down. This will be less of an issue where the house is extremely well insulated and will be even more likely if its an old house with solid walls.

I really cannot believe that someone would post something so incorrect on this thread; and with such conviction!

The Laws of Physics exist - period!

Just to expand on the post I made on the first page.

Firstly we can agree on one point - it takes a lot of energy to heat a cold house and indeed it takes time to bring it to a comfortable temperature.

So let us take the scenario where you left your house empty for 2 years - presumably you wouldn't have it heated, and would accept that the ENORMOUS amount of energy to bring it back up to temperature would be less than keeping the house heated for 2 years.

How about 6 months?

1 month? 1week? 24 hours? 6 hours? 1 hour?

There must be a point at which YOU consider the laws of Thermodynamics don't apply! What period of time would that be?

Again, for your theory to hold true, you wouldn't let the water in a kettle go cold because of the ENORMOUS amount of energy required to bring it back to the boil. It would be cheaper to keep the water temperature topped up all the time.

P.S.
Please don't raise the issue of what is more comfortable, or preventing damp. This is about cost!

ab48uk

In our house, there isn't a room thermostat - just thermostatically controlled valves on each radiator. To me this seems sensible, otherwise the two are fighting each other. Often room thermostats are located in a little used hall, rather than the living room!

If we are away from home most radiators are turned down to the frost stat setting - not off completely. This should avoid frozen pipes, providing the heating system is still set to come on for a couple of hours morning and evening. The hot water can stay off.

maxmiler

ab48uk wrote: »

In our house, there isn't a room thermostat - just thermostatically controlled valves on each radiator. To me this seems sensible, otherwise the two are fighting each other. Often room thermostats are located in a little used hall, rather than the living room!

If we are away from home most radiators are turned down to the frost stat setting - not off completely. This should avoid frozen pipes, providing the heating system is still set to come on for a couple of hours morning and evening. The hot water can stay off.

I have a wireless room thermostat/timer. It cost just over £30. every radiator in our house has a TRV.
The trick is to fully open the TRV in the room with the thermostat. This gives me the freedom to control the heating from whichever room I like.

sheffield_lad

ab48uk wrote: »

In our house, there isn't a room thermostat - just thermostatically controlled valves on each radiator. To me this seems sensible, otherwise the two are fighting each other. Often room thermostats are located in a little used hall, rather than the living room!

If we are away from home most radiators are turned down to the frost stat setting - not off completely. This should avoid frozen pipes, providing the heating system is still set to come on for a couple of hours morning and evening. The hot water can stay off.

I can't imagine not having a room stat I like controling the temp, I often visit homes without one who like you use TRV's also seen some cowboys fit the boiler and leave no control other than the controls on the boiler grrr.

daveyjp

maxmiler wrote: »

I have a technical query. Last year I had a completely new gas CH system fitted . It has a Combi boiler (condensing because it is new). I have tried to run it with a water temperature of 55 which should allow the condensing feature to improve efficiency. However, the house doesn't get warm enough until I put the water temp up to at least 65, so it does not run in condensing mode.
So what is more efficient?
1. water temp at 65 which means the boiler is on (and pumping) for longer in each cycle.
2. water temp at 75 which shortens the cycles (and saves on electricity as the pump runs for less time.

Thanks!

No idea, but you raise a point which is missed on this thread. A modern boiler doesn't keep burning at full power until the stat temp is reached. A boiler heats water to a set temperature, this water then goes round the system heating the rooms. The water will never get any hotter than the temp set on the boiler, so increasing the thermostat won't mean hotter water.

What it does mean is the on off cycle of maintaining the water loop temperature goes on for longer, but the burner is on at a low level. Even from clap cold our boiler never runs at full whack to heat water in the heating circuit. At most it uses half burner temp and once the water temp is reached it burns at its lowest level to maintain the loop temperature.

On this basis there's no way our boiler uses less gas by being on 24/7 than when it is turned off as the initial higher output of the boiler is only used for a short period to get the heating water temp from ambient to whatever the temp is set at, after that it just ticks over, this initial heating phase is a matter of minutes.

As mentioned by others, it's no different to a kettle. I don't keep the kettle ticking over 24/7 to save money on boiling it from cold as it costs more.