Gas central heating thermostat / timer settings

Scouter

I'm interested in people's thoughts on this subject - and if ever anyone has done a true comparison, all the better. We live in a typical interwar 3 bed semi, with gas central heating. We have an old system, coupled with a modern Worcester Bosch boiler, which is only used for heating, as we have a separate gas water heater, which fires up on demand - rather like a combi, but two individual units. A relatively unusual set up, but I assume a previous owner had a water heater installed prior to the central heating being installed. At which point, I assume, the decision was taken to keep the water heater rather than install a storage tank.

Anyway, I use the programmer to bring the temp up to 20c between 4:30am for me getting up at 5am and 7:30am, by which time everyone has gone to work / college / school. It is then set for 19c until 2pm, when it is set for 20c, raising to 21c at 4pm, once most of us are in. The next change is midnight, when it is set for 18c until 4:30am, then off we go for another day.

As you can see, it is set to warm the house up when we're most likely to be in. The boiler temperature is set to max, the theory being the radiators get as hot as possible, warming the house quicker, thus allowing the system to shut off sooner.

But, is this the most efficient way, or would it work out cheaper just to leave the thermostat set permanently to, say, 21c (which seems to be the temperature at which everyone is comfortable)...or perhaps 20c most of the time, with a boost to 21c in the evenings.

There seems to be no definitive answer that I can find. The only thing I've ever found was an article in our local paper a few years back, which pretty much posed my above question.

An 'expert' suggested that for older properties like ours, the heat retention tends to be poor, even with cavity insulation and double glazing (which we have), so it is most efficient to heat the house on demand, as we do.

However, for modern, well insulated properties, he suggested the most efficient way was to set the thermostat for the desired temperature and leave it be.

Obviously, the only way to find out would be to try both setting over a couple of years and see how your usage compares, but even then, one year might be milder than the other, so I guess finding an answer will be difficult!

Any thoughts?

:cool:

Scouter

Just realised I've posted this in the wrong sub-forum, if anyone wants to move it.

:rotfl:

Cardew

Variations on the questions you pose come up several times a year on MSE.

There is a definitive answer, the longer your boiler is off - the lower the running costs.

There seems to be no reason why you would keep the house at 19C from 7:30am to 2pm, then increasing it to 20C for a further 2 hours and upping it again to 21C at 4pm. (your post indicates that the house will be empty from 7:30am until after 4pm?)

Personally I would have the heating off completely from about 30 minutes before the last person leaves the house in the morning, and to come back on about 30 minutes before the first person returns after school/work.

Also why keep the heating at 18C overnight? most people would have it off, or at least a much lower temperature.

A modern Worcester boiler - set at max water temperature as you have it - should easily bring the house up to temperature in 30 mins.

It is pertinent to point out that a condensing boiler(which I assume yours is) will be a little more efficient at lower water temperatures.

macman

Cardew wrote: »

Variations on the questions you pose come up several times a year on MSE.

There is a definitive answer, the longer your boiler is off - the lower the running costs.

There seems to be no reason why you would keep the house at 19C from 7:30am to 2pm, then increasing it to 20C for a further 2 hours and upping it again to 21C at 4pm. (your post indicates that the house will be empty from 7:30am until after 4pm?)

Personally I would have the heating off completely from about 30 minutes before the last person leaves the house in the morning, and to come back on about 30 minutes before the first person returns after school/work.

Also why keep the heating at 18C overnight? most people would have it off, or at least a much lower temperature.

A modern Worcester boiler - set at max water temperature as you have it - should easily bring the house up to temperature in 30 mins.

It is pertinent to point out that with a condensing boiler(which I assume yours is) are a little more efficient at lower water temperatures.

Make that hundreds of times a year. And always beginning in the first week of cold weather...

Scouter

Obviously, the less the boiler is on, the less fuel it uses, that goes without saying!

In reality, overnight the temp rarely drops below 18c anyway, unless it's really cold. Same with 19c during the day, so the boiler is rarely running at these times.

But surely it could also be argued that if the temp is set much lower to prevent it firing over night and during the empty period during the day, it will be running longer to bring the temp back up at the times it is needed?

I suspect there is no right or wrong answer to this!

Cardew

Scouter wrote: »

But surely it could also be argued that if the temp is set much lower to prevent it firing over night and during the empty period during the day, it will be running longer to bring the temp back up at the times it is needed?

I suspect there is no right or wrong answer to this!

There is absolutely a right answer to that point; and it is contained in the laws of thermodynamics. The greater the differential between the internal house temperature and outside temperature, the greater the heat loss.

As said earlier, this discussion comes up countless times every year(I appreciate you have just joined MSE)

Let us take two(well worn!) situations to illustrate the point:

If you were to go away from the house for, say 2 years, would you leave the boiler running as 'it will be running longer to bring the temp back up at the times it is needed'

I hope you answer is you wouldn't leave it running;)

So how about a year? a month? a week? a day? At what point(period) do you consider the laws of thermodynamics don't apply.

Secondly if your theory that 'it(the boiler) will be running longer to bring the temp back up at the times it is needed' were to be true; why not apply that to your kettle.

Do you keep that kettle close to boiling all day because it will use less energy than letting it get cold and boiling it up again when required? It really is exactly the same principle.

Scouter

Interesting points, and well made.

Ive had a little tweak today actually. The trick is to tweak it to save us a little, without my wife and daughter noticing!

Now I don't wish to sound sexist, but is it my imagination, or do women feel the cold more than men? My wife and daughter complain of feeling cold, yet at the same time I could sit there quite comfortably in my underpants! [I don't though. No one deserves that! ]

Cardew

Scouter wrote: »

Interesting points, and well made.

Ive had a little tweak today actually. The trick is to tweak it to save us a little, without my wife and daughter noticing!

As you indicate your house is very well insulated, in that the temperatures don't drop much during the day and overnight, then the savings will not be high. However the principle still applies.

closed

The expert is wrong, whatever the insulation standards, if you heat when it doesn't need to be heated, which is when everyone is out or asleep, that heat is going to end up outside and provide no benefit.

malc_b

The definitive answer is actually maybe. I would agree that from a simple physics if you turn off the heat, then inside temperature drops, inside to outside difference is less so energy loss is less. And less energy loss is less cost.

BUT, there are 2 assumptions in the above.

1. That the U value is constant. If the temperature drops so low that the wall gets wet then it's U value will decrease. I'm not convinced a modern house ever sees that but it could be an issue in an old solid wall house.

2. That the boiler efficiency is constant. This is certainly not the case. Condensing boilers, heat pumps, both lose significant efficiency when you run them with high radiator temperatures. The less you have the heating on the higher radiator temperature you need because you need a higher output from them.

I've explained this in another thread but to summarise it think about what happens when you turn off the heating and go to work. Say the house is at 20C, outside is 0C, you come home 8hrs later and the house has fallen to 10C. So, roughly, during those 8hrs inside is at 15C average, heat loss is 15/20 = 75% of what it would be if you left the heating set at 20C. Compared to the whole day at 20C for 24hrs this 25% energy used instead of 33%. When you come home and turn the heating on you now have to not only balance the loss to the outside at 20C but also replace this 25% (of 20C/24hr) you sucked out of the house structure. So if this happens over 6hrs you need 2 x the radiator output.

So, if you have a condensing boiler then you should first increase the running hours before turn the water temperature up, for maximum economy.

The key point here is that unless the house inside temperature is same as outside there will be a heat loss from the house, irrespective of whether the heating is on or off.

Cardew

malc_b wrote: »

Condensing boilers, heat pumps, both lose significant efficiency when you run them with high radiator temperatures.

We have had this discussion before I believe.

Have you any data that shows the loss of efficiency on condensing boilers(not heat pumps) by running them at higher temperatures?

I have contacted a couple of manufacturers and they have no data(they will release) but indicated the loss of efficiency was marginal; and in any case it is just not possible to keep a boiler in condensing mode.

Take a situation where every radiator in the house(and hot water for tank) is on and demanding heat. In that scenario it is quite easy to envisage a very cool return water temperature.

However the situation might then change to just a couple of radiators and that will affect the return water temperature.

From the little research I have carried out, I would be inclined to disagree with your statement that having the boiler at the hot water setting causes significant efficiency loss; albeit I would welcome some data to prove me wrong.

One might question if the loss of efficiency is so marked, why have a hot water setting? Also I have looked at a couple of user handbooks for condensing boilers and not seen a warning that high water temperatures significantly lower the efficiency.