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

grahamc2003 wrote: »

It's best to consider heat loss rather than the actual heating - heat loss is a function (and nowhere near a linear function) of temperature difference - by turning the heater off while you're out, the house temperature (and therefore the heat loss) drops compared to keeping at a higher temperature all the time. The heating required is that which replaces the heat lost.

Heat loss is, in all the calculations I have seen, a linear function of the temperature difference. I would agree that newton's law of cooling is not linear but heat loss through a house wall etc. is linear. The outside is taken to be at ambient so just conduction applies.

I agree with on the heat loss drops if you turn off the heating. But that is not the same as using less fuel. Since the difference between always on and part time on is not that much then to a first approximation, 24hr = 100%, 12hr = 200%, 8hr = 300%, 6hr = 400%. So turn your heating on just for a couple of hours in the morning and evening and you need a system to hit ~300% of it's design figure (assuming a new house with CH so sized for 100% of the demand).

malc_b wrote: »

Heat loss is, in all the calculations I have seen, a linear function of the temperature difference. I would agree that newton's law of cooling is not linear but heat loss through a house wall etc. is linear. The outside is taken to be at ambient so just conduction applies.

I agree with on the heat loss drops if you turn off the heating. But that is not the same as using less fuel. Since the difference between always on and part time on is not that much then to a first approximation, 24hr = 100%, 12hr = 200%, 8hr = 300%, 6hr = 400%. So turn your heating on just for a couple of hours in the morning and evening and you need a system to hit ~300% of it's design figure (assuming a new house with CH so sized for 100% of the demand).

OK have it your way. You're correct, every physicist, professional engineer, and all those who formulated the laws of physics are wrong. At some stage, after all reasonable debate has been exhausted, the appeal to authority is the only avenue left.

And no, water hot water doesn't free faster than cold water, all things being equal. But makes things unequal, and you can give the appearance, to the non-scientific at least, that in specific circumstances, hot water appears to freeze quicker, but again just to labour the point, all other things won't be equal in that circumstance.

I have dredged up the results of a long running study.

As well as reporting that the efficiencies are not as high as the test results in the laboratory. [Probably something to do with the kit being used by "irrational" users.]
I found no serious discussion of boilers condensing or not condensing, though surprise surprise the actual efficiency of high efficiency boilers is less than expected (just like the equivalent study into the efficiency of heat pumps). [My son's boiler flushes itself, like cistern in a gents lavatory, so the condensate must be coming from somewhere.]

The fact that the vast majority of boilers were oversized, did not come as surprise, but apparently this inevitable under-use, does not reduce their efficiency !!!!.
However in the summer time, when the boiler is only providing hot water, it is looking like the hand & face washing, shaving, showering, food preparation and cooking types of uses of hot water; would be cheaper if electricity was used to heat water at the point of use.

http://www.energysavingtrust.org.uk/Publications2/Housing-professionals/Heating-systems/In-situ-monitoring-of-efficiencies-of-condensing-boilers-and-use-of-secondary-heating-trial-final-report

.......but has the Telegraph done any research to prove this is actually happening?

SPAB has proved that if penetrating water is a problem, with an old solid wall home, then the cold internally insulated wall might now remain "sodden" inside the wall throughout the winter.
It is a huge jump from that to say a cavity wall will fall below the dewpoint in the outside leaf and that dampness will soak backwards into the cavity fill.

If the bricks start spalling in the frost, there is a problem ! However trying to check what is happening inside the bricks of a wall is difficult, possibly a surveyor could probe the actual insulation.

Shoddy cavity wall filling, with resulting "voids" is also a problem, let us hope cheap infra red cameras will become available for hire, so we can all check our homes on a cold frosty night.

tichtich wrote: »

What's your point? You seem to agree that this is 8% more heat needed if you leave heating on all day. Your argument (as I understand it) is that the 8% extra heat wasted by leaving heating on all day might be offset by greater boiler efficiency. But even your own figures don't support this as a realistic possibility.

8% if the house falls to 10C inside for 0C outside, which I think is pessimistic and 15C is more likely giving about 4% difference. But you miss the point that the heat taken from the house structure has to be replaced and that is on top of the standing loss. Hence the heating demand is 140%. So for 24/24 you can run with lower water temperature which might make the difference between condensing and not.

If you check out sedbuk figures then the difference between a condensing boiler and non-condensing is about 8% and wikipedia reckons 10-12%.

I'm not saying this happens all the time I'm merely pointing out that saying all the people who report they see fuel savings may not be the deluded fools that some people seem to be implying. There is a physical model which supports their practical results.

tichtich wrote: »

You wrote: "The loss saving is 8.3% which is the same order as the difference between condensing and non-condensing modes." This is irrelevant unless you're suggesting that on-all-day gets the full benefit of condensing and the alternative (off part of the day) gets no benefit from condensing at all. On these figures, on-all-day just breaks even with the alternative if you make the most extreme possible (and absurd) assumption about the relative benefits from condensing. On anything less than that extreme assumption, on-all-day is worse.

The difference is that 24/24 is 100% load and 16/24 is 140% load because it needs to replace the heat lost from the structure that kept the house warmer than the outside temperature all through the night. This allows a lower water temperature so the boiler could run in condensing mode through out more of the season.

tichtich wrote: »

And it gets worse. You've chosen to calculate this 8% figure by averaging over 24 hours. But the two regimes you're comparing only differ from the time the heating goes off in the morning until the time the house is fully warm again. Outside that period (at night) you have both regimes maintaining a constant temperature. It would make more sense to compare over just that period when the regimes differ.

I'm assuming the house has a long time constant. I said that over 8hrs the temperature fell from 20C to 10C so about a 16hrs to 0C (outside temperature) if we use a straight line for simplicity (it will be an exponential curve of course). And it will take similar time to heat the structure back up. But whatever the logical action to compare over a full cycle which is a day.

tichtich wrote: »

But let's say we go with a comparison over a full 24 hours. Then, to compare like with like you must also average the difference in boiler efficiency over 24 hours. But at least half that time the boiler efficiency is the same under both regimes, since they're doing the same thing. So any actual difference in boiler efficiency during the day time will be watered down by at least half when you average over 24 hours. To put it another way, even if we made the extreme assumption that on-all-day had an 8% efficiency gain from condensing and the alternative regime had none, this could only be the case during the day time, when the regimes differ. Averaged over the full 24 hours, the advantage would be watered down to 4% or less. So even on the absurd extreme assumption, this efficiency saving (4%) is not enough to offset the 8% extra heat wasted.

And it gets worse. If Graham is right (and my wider reading on the web suggests he is) then on-all-day gets less benefit from condensing than the alternative does, not more. If that's the case, on-all-day is doubly-damned. It wastes heat and it produces heat less efficiently.

It depends on how you are running your condensing boiler. If you have the water temperature set high then the only time the boiler condenses is a the start of burn when the CH water is cold. If you run the CH for only a short time per day then you will need to run the boiler hot because you need maximum output from the radiators. On the other hand if you run for 24hr you can run with a cool water temperature that will always be condensing.

If you want to put some numbers then condensing mode is usually taken as a return water temperature of 55C. So output water at 65C, 10C drop across rads, return 55C. Average rad temperature is 60C, so 40C rise above 20C ambient. Rad factor is 60% of the rated output (at 80C water, 60C above 20C ambient).

Hence, we have two scenarios, boiler is set 65C output and always condensing. House meets set temperatures with rads at 60% rated output for 100% standing losses. OR, run boiler 16/24, we now need 140% from rad so need water at 73C. Condensing mode is now just partial at being of burn.

This is a mechanism which explains why some people report savings for 24hr running. I'm not saying that everyone should run 24/24 or that everyone will see savings. I'm merely pointing out that the physics is not as simple as some people seem to think it is.