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

Bricks

It'll "work harder" in the sense that it will have to get quite a lot of energy into the system in a short time - but what matters is the amount of energy that goes into the system overall - and it can't be the case that it pumps *more* energy in during that short period than it would have, had it been pumping energy in all day, which would have simultaneously been leaking out.

I'd think of it like trying to keep a bath full whilst the plug is partly out. To get it full, you are probably going to have to turn the taps on full. Once it is full, though, you don't just turn the taps off, you have to keep them running at the same rate as water is flowing out the plughole.

With this analogy, surely it's easy to see that if you want to fill the bath when you get home in the evening, you're going to waste less water by turning the taps off during the day, rather than leaving them on. In the latter case water would be exiting the plug hole all day, instead of only during the time it took to fill the bath from empty.

It's not the rate of flow through the taps that matters - it's the amount of time that water is literally going down the drain.

I'm not sure if the reason people have difficulty with this is that they don't visualise energy leaking out of the building - it's as if they think all that energy pumped into it during the day somehow just stays there waiting for them to get home.

malc_b

justwilliam25 wrote: »

am I right in thinking that by having the heating on all day when nobody is in, the heat is simply being lost/wasted and is costing us a small fortune?

The answer, given you say house is poorly insulated etc. is probably costing you money. The things to understand are:

1. A house has a big thermal mass, it is a big brick hot water bottle. If you turn off the heat and go out then it is still sitting there, warm, losing heat to outside. When you come and turn on the CH you have to replace the heat in the brickwork. If when you come home the house is still warmer than outside then it has been sitting there losing heat all day. However the colder the house gets the less heat lost so keeping heating on while you are out will lose less heat than turning it off.

2. Condensing boilers run more efficient when run cooler.

3. Radiators give out less heat when cooler.

The amount of heat you can get out of CH is usually limited by the radiator area. So you turn off the heating and then come back to a stone cold house and crank it up to maximum to try and get it warm. That's hard because the walls are cold you're running the CH flat out and so inefficient. You're best option would be set the boiler timer to come on before you get home so the boiler can be run cooler and more efficient.

lstar337

Bricks wrote: »

It'll "work harder" in the sense that it will have to get quite a lot of energy into the system in a short time - but what matters is the amount of energy that goes into the system overall - and it can't be the case that it pumps *more* energy in during that short period than it would have, had it been pumping energy in all day, which would have simultaneously been leaking out.

I'd think of it like trying to keep a bath full whilst the plug is partly out. To get it full, you are probably going to have to turn the taps on full. Once it is full, though, you don't just turn the taps off, you have to keep them running at the same rate as water is flowing out the plughole.

With this analogy, surely it's easy to see that if you want to fill the bath when you get home in the evening, you're going to waste less water by turning the taps off during the day, rather than leaving them on. In the latter case water would be exiting the plug hole all day, instead of only during the time it took to fill the bath from empty.

It's not the rate of flow through the taps that matters - it's the amount of time that water is literally going down the drain.

I'm not sure if the reason people have difficulty with this is that they don't visualise energy leaking out of the building - it's as if they think all that energy pumped into it during the day somehow just stays there waiting for them to get home.

Exactly.

I fully understand what malc is trying to say, but for it to be truely money saving (or even cost the same, you are going to need a house that is rated higher than mine (rated A), with a condensing boiler, and you are going to need to be in most of the day.

The simple fact is that my house (rated A) is not typical in the UK, most waste heat like you couldn't even imagine. So heating over night, or while you are out during the day, in a house that leaks heat like a colander, is going to smash boiler efficiency values in the nuts. Vague maths based on arbitrary figures are not going to prove anything.

malc_b

lstar337 wrote: »

Exactly.

I fully understand what malc is trying to say, but for it to be truely money saving (or even cost the same, you are going to need a house that is rated higher than mine (rated A), with a condensing boiler, and you are going to need to be in most of the day.

The simple fact is that my house (rated A) is not typical in the UK, most waste heat like you couldn't even imagine. So heating over night, or while you are out during the day, in a house that leaks heat like a colander, is going to smash boiler efficiency values in the nuts. Vague maths based on arbitrary figures are not going to prove anything.

Actually I say that A rated houses are more likely to see a saving.

You seem to understand that heat leaks out of your house, and that the amount heat leaking out depends on how hot the house (and how cold it is outside). So if I say it is 0C outside and 20C inside and the heat loss is 4kW what would be the heat loss if I turn the inside up to 30C then you'll say 6kW. The difference is now 30C rather than 20C.

Now let's go the other way. 0C outside, 15C inside, = 3kW, 10C inside =2kW, 5C inside = 1kW, 0C inside = 0kW.

So let's compare a large A rated house with a small F rated house, both lose 4kW when it is 20C inside and 0C outside. At bedtime the CH goes off for the night. The A rated house stays warm so by morning inside temperature is 15C, which is a 3kW loss as shown above. 1 second after the CH goes off the temperature is still 20C so loss is 4kW. Roughly then the average loss is 3.5kW (4kW -> 3kW) for the whole night while the CH is off. Overnight the saving is 3.5/4, 12.5%.

Let's look at the F rated house. That's going to get cold overnight so let's say 10C by the morning which is a 2kW loss. Again 1 second after CH switches off the loss is still 4kW so roughly the average loss is 3kW (4kW->2kW). Overnight the saving is 3/4, 25%.

The F rated house saves more than the A rated house by running shorter.

The percentages look large but that is because they are just the night time figures. Let's assume 16hrs at 20C + 8 hr night. That is 3 x 8hr periods so A rated is (4 + 4 + 3.5)/3 = 3.83kW or 4.16% saving over 4kW loss at constant 20C. F rated is (4 + 4 + 3)/3 = 3.67kW or 8.3% saving.

You can do the same with different periods but the key point is to show that turning off heating doesn't save as much as think it would. It's comparable with the difference between condensing and non-condensing hence you are probably better off, money wise, opting for longer running in condensing mode than shorter running in non condensing mode.

The other point to show is that you can get a feel for how your house sits in this scheme just be measuring how low the inside temperature falls overnight on a cold winter night.

lstar337

malc_b wrote: »

Actually I say that A rated houses are more likely to see a saving.

That's what I said. My house would benefit more than most in the UK, but even I make a saving using timed heating periods and not heating while I am asleep or out of the house.

malc_b wrote: »

The F rated house saves more than the A rated house by running shorter.

Yes, and as we have previously said, most houses in the UK are on the lower end of the efficiency bands. Therefore it makes more sense to offer good advice to the masses, rather than the 2 people in the UK with a Passivhaus.

In the majority of cases, it is better to have your heating off when you are out and overnight when it isn't needed.

Stoke

I just don't have it on at all

And it's freeeeeeeezing!!

Bricks

malc_b wrote: »

The percentages look large but that is because they are just the night time figures. Let's assume 16hrs at 20C + 8 hr night. That is 3 x 8hr periods so A rated is (4 + 4 + 3.5)/3 = 3.83kW or 4.16% saving over 4kW loss at constant 20C. F rated is (4 + 4 + 3)/3 = 3.67kW or 8.3% saving.

You can do the same with different periods but the key point is to show that turning off heating doesn't save as much as think it would. It's comparable with the difference between condensing and non-condensing hence you are probably better off, money wise, opting for longer running in condensing mode than shorter running in non condensing mode.

For the F-rated house you've calculated a 25% energy saving overnight with the heating off.
You then calculate this as an 8% saving taken over a 24hr cycle.
Then you say that this 8% is similar to the difference between condensing/non-condensing mode on a boiler. Therefore the two effects kind of cancel out.
But that only makes sense if the boiler is running in its 8% less efficient mode for the whole of the time it's on during the 24hr cycle, doesn't it? In reality, if it is pushed into its less efficient mode, it's likely only to be for a small portion of the time it's running, a short period first thing in the morning while it's getting the heat back up to a stable temperature.

You also ignore the fact that outside temperature is generally quite a bit lower at night compared to daytime.

If the outside temp is around 10C in the 2 x 8 hrs of "day" and 0C in the 8 hrs of "night" then your figures for the F house would be, with the heating on constant, divided into 8 hr periods:
(2 + 2 + 4)/3 = 2.67 average
and with the heating off overnight:
(2 + 2 + 3)/3 = 2.33 average
This means that your calculated saving of 8% would become more like 12%.
Yes I have probably exaggerated the difference between night/day temperatures but you get the point.

Finally, I think the reality is that many people asking this question are wondering whether to turn the heating off not just at night but also during the day whilst out at work. In that case, the proportion of the day where energy is being wasted is more like 16/24 than 8/24. Then the calculations would become more like
(2 + 2 + 4)/3 = 2.67 average, heating on constant
(1 + 2 + 3)/3 = 2.00 average, heating only on for 8 hrs of day
Now we are talking about a 25% saving.

My overall point being: your figures don't convince me that the energy savings of turning the heating off in a poorly insulated house are small enough that they are likely to be cancelled out by the effect of a boiler having to operate in non-condensing mode for a proportion of the time it's working.

But how about in a well insulated house? Well, I'd agree that the energy savings of having the heating off become less significant. However, because a well-insulated house is not going to cool down as much, it's also less likely that the boiler is going to be pushed into non-condensing mode for a significant portion of its time in operation.

And finally - all this assumes that in the "heating off" scenarios, the boiler is forced to raise the temperature in a short period of time. In reality, control systems allow the boiler to start raising the temperature more slowly, a little in advance of the target time for the 20C to be reached. Again this can mean that it doesn't have to be forced into inefficient non-condensing modes.

lstar337

Stoke wrote: »

I just don't have it on at all

And it's freeeeeeeezing!!

Good money saving though. :money::money::money:

reeac

Stoke wrote: »

I just don't have it on at all

And it's freeeeeeeezing!!

Sounds like you don't have a resident SWMBO.

malc_b

Bricks you're missing a key point.

Can I take it as accepted that the difference between the heat lost in a day is less if you heat intermittently than say 24hr but that this difference is not that much (under 10%). This comes about from knowing that heat lose depends on how warm it is inside and experiencing that after turning the heat off the house takes a long time to cool down.

So, from that then lets say in 24hrs you need to put into a house say 100 kWh running 24hr or 90 kWh if you run 16/24. That heat comes from the boiler via the radiators. For the 24hr that is 100/24 = 4.2kW continuously out of the radiators. For 16hr running it is 90/16 = 5.6kW. The 16hr is lower energy but is asking over 30% more from the radiators. The only way to get more power out of radiators is to turn the water temperature up. For a standard radiator the factors I have are 30C = 0.51, 40C = 0.76, 50C = 1.0, 60C=1.25, where the temperature is the radiator above ambient figure. If we are looking at 55C return (for condensing), 10C drop, 65C out then that is 60C average radiator temperature so dC of 40, 0.76 factor.

So, if our house has been installed with radiators that just balance the heat lost in middle of winter while running at 65C out and we then run 16/24 the only way we can keep the house warm is to increase the boiler temperature up 75C or more. We'd be better to run longer rather than have to put the temperature up.

Every house is different but the old calculation method was just to size the radiators for the calculated heat loss and for a radiator factor of 1 (no condensing back then). I don't know what the new method is, probably to assume a lower radiator factor but I doubt it considers running shorter hours too.

I would agree that intelligent thermostats help with this. They should learn the house heat up time, and if the boiler is limited to 65C so it is in condensing mode the stat should learn the curve for that mode. The stat will then start the heating earlier and earlier as the weather gets colder. That's exactly what I have said, run longer not hotter.