I think that is an interesting point, and may be behind the "immersion heater" story. With a well insulated tank, and a policy of taking a large bath every evening, and on "Economy 7", you might a disbenefit in switching off for part of the "cheap energy" window.
Sorry but that does not make sense. If its off for 8 hours out of 24 you will save nearly one third. You may save more as it usually colder at night and will use more energy than in the day. It will take slightly more energy to raise the temp back to the norm in the morning but it shouldn't take much.
Oh dear, ours can easily drop from 22C down to 12C overnight with no heating (this is with outside temps above freezing, we're in centralish London, with rarely a frost). Single-glazed sash windows are probably to blame, along with less-than-optimal loft insulation.
Well the healthiests time of my life was at college with a wonderful free and totally overspecced central heatings system.
Well right now it's minus 8 C outside (it was minus 12 earlier today). Inside it's somewhat less than plus 20 degrees. Probably around 67 F.
When retiring in a couple of hours time will definitely turn down the thermostats in the two rooms we use most. And the TV will be off so it's wasted heat will not be present either. The temp. since it is not all windy tonight will drop slowly and by morning the house will be cooler by quite a few degrees and thus losing less heat per unit of time than right now while I am still up and about.
It won't take long, maybe half an hour with the thermostats turned back up, in the morning for the two or three most used living areas to warm back up. And if I was leaving for the day I might even leave them turned down until returning tomorrow evening! As a single retiree home most of the time it is possible to keep thermostats in the less used rooms turned lower.
A basement workshop, and an attached storeroom garage are only heated occasionally when in use. The bathroom has a row of six inefficient 40 watt bulbs; much of the time since they are on when bathroom is in use they are sufficient to heat the room so the 500 watt electric baseboard heater rarely cuts in.
It's quite obvious that one heats the areas necessary to the degree needed and anything more costs more to heat. At any temperature the heat loss from wind IS a significant factor; not so much because wood frame houses here are leaky, they are not. If anything are often too well sealed and must use air/heat exchangers; but because wind carries away heat from the structure.
So would they actually have to cut away the pipes and replace or could they get them clear somehow?
Dad mentioned something similar the other day. Apparently it's just as inconvenient when a ship hold (Shell tankers in his case) full of something that should be liquid (like pitch or tar I think he said) goes solid.
They normally pumped steam through pipes in the tanks when the sea / outside temp dropped below a certain point and this would keep the cargo liquid. However if the heating system failed (or they forgot to turn it on) and the cargo did go solid, the steam heating system wasn't able to take it back to liquid again (poor convection) and it would all have to be cut out by hand. :-(
T i m
It is NOT obvious that 'heating fast' is as boiler efficient as 'keeping warm'. It is NOT obvious that 'heating fast' will not in many cases overshoot the 'needed temperature' by quite a margin. If parts of the structure take longer to warm up.
This is a case where simplistic analysis fails, because the devils are in the details.
The other case we discussed here recently and was very hard to pin down, was 'what is the most fuel efficient way to accelerate a car to its cruising speed'.
Popular myth has it 'as gently as possible' simplistic analysis says that since the energy gained is constant, it doesn't matter whether its a short sharp burst or a prolonged gentle shove. Detailed analysis implies its about operating the car engine in its most efficient part of the power band.
Which in the case of a petrol engine is assuredly NOT low throttle setting at low RPM. Although possibly it is for a Diesel.
Nor yet full revs and wide open ;-)
>
I agree with your sound logic, in theory the heat losses (and hence energy costs) through the night will be higher if the house is heated but a recent check on meter readings suggest that the case might not be quite so straightforward.
We have the heating set to run from 08:00 to 23:30. The overnight set point is 13C so it's effectively turned off overnight unless the weather gets very cold. The programmer uses optimum start so I don't know exactly what time it normally starts up but the maximum advance is 2 hours so I'll assume it starts up on average a bit less than this at 06:30 (I certainly don't intend waking up at 06:00 to find out). Average gas consumption for the last week has been about 300 ft^3 per day but extra meter readings last thing at night and at about 08:00 for a couple of nights show that about
100 ft^3 are consumed each morning in bringing the house back up to temperature leaving 200ft^3 for the remaining 15.5 hours, i.e. 200/15.5 = 12.9 ft^3 per hour to maintain a steady temperature. So if we left the heating on all the time the daily consumption would be 12.9x24 = 309.6 ft^3 - remarkably close to what we're using already.O.K. the losses through the night would be a bit higher due to the outside temperature being lower but I don't expect it would make a huge difference. I don't intend to put it to the test though because we don't like sleeping in a hot bedroom but don't want to turn the bedroom TRV down because we want the room to have warmed up by the time we have to get up in the morning.
It also appears to me that the room (well, our lounge) feels warmer when the radiator is hot (rather than warm). Letting the room cool down, then heating up relatively fiercely can *feel* warmer than keeping it at a steady state.
Very difficult to factor in things like that with any degree of accuracy.
Many years ago, Porsche were reported as having done fuel economy tests in town. Result: Best economy achieved by accelerating fairly briskly to around 30, then keeping steady speed.
As TNP wrote, nothing is obvious in the real world.
That's not the heating, that's because ethanol is an antiseptic.
Andy
I suspect it was the antibiotic value of cannabis actually.
The message from Mike Clarke contains these words:
Hardly sound logic to suggest that the heat needed to rewarm the house is minimal.
Consider a really simplified model in which the outside temperature is constant and the house requires a single unit to keep it up to temperature and loses one degree for every hour without heat. Heating on for 24 hours would require 24 units. Heating on for 16 hours would require 16 units plus whatever it takes to get it back up to temperature. In our simplistic model the temperature loss is 8 degrees which requires 8 units to reverse. 16 + 8 = 24. So no saving.
In the real world there is a small saving because the heat loss is a function of the temperature difference so during the time the heating is off the rate of heat loss will decay.
In the real world you have to take into account the heat capacity to work out how long it takes to reheat the house, not the heat loss. I.e. your simple model doesn't work.
Now consider the actual boiler output. The plus whatever it takes to get it back to temperature may push teh boiler into continuous flat out mode. How much less efficient is it then?
Or you may have to raise the outlet temperature to get it to warm up faster. More loss of effiiency.
Analsysing transient conditions using steady state analysis is a very precarious exercise.
Don't be too sure. Red hot rads heating up room against a cold wall, lose a lot of heat..then the wall itself gets locally very hot leading to more heat losses through that part of it. Until the room stabilises.
The more concentrated the heat and the higher temp the heat the more there is a chance of excess loss over simple steady state analysis. what about ultra hot feed pipes in the loft? sure they are insulated, but the hotter they are, the more they lose..
The better your insulation the more likely you are to get little benefit from time switching and actually stray into excess transient losses.
The message from "dennis@home" contains these words:
Time is not really an issue unless the heating system is inadequate. It is the quantity of energy required to reheat the house that matters and that depends on how much has been lost.
The message from The Natural Philosopher contains these words:
You have your condensing boiler set so it always condenses. If it is a non condensing boiler then the difference would not be significant.
Marginal.
But unless you have the heating firing for 100% of the time you do not have anything approaching a steady state in the first place. Having the heating off for 8 hours at a time is closer to normal operation than continuous firing, it is just that the hysterisis is larger.
But the better the insulation the less the heating system has to do to restore the status quo. Scalding radiators are not needed. It may not be entirely impossible to engineer a heating system in an extremely well insulated house to use more energy if it is switched off for a lengthy period but, other things being equal, the only way that is going to happen is for a single firing of the boiler to produce a larger temperature overshoot on the heating phase than the temperature drop on the non heating phase. ISTM that that is much more likely in normal operation than it is when the temperature drop is significant.
Of course deliberately buggering up the operation of a condensing boiler would make the task easier but even the 10% loss of efficiency may not be enough even if the set-up could be designed to give maximum efficiency in normal operation and maximum inefficiency in sustained firing.
Scalding radiatators happen when any system that is not modulated, goes into heat mode.
It doesn;t matter whethert tu re one degree or 100 degrees below TRV temp, thermostat temp. The rads and the boiler will be working full on.
What modulates the radiator surface temperature is in fact the time they are on 'heat' because they have thermal mass.
Thats teh whole point.
If you had e.g. a house made of polystrene with hot air heating, ultimately probably te lowest theral mas possible, then I would almost agree with your perspective.
But the more mass there is in the building, or the heating system, the more a high heat regime will tend to overheat parts of it with respect to other parts. And leave the possibility of higher heat loss from those parts during the transient phase.
It may not be
Well the figures and experiences of people in high insulated houses tend to bear out the proposition that it makes very little difference on a modern house whether its run 24x7 or timed.
The worse the insulation and the lower the thermal mass, the more the timed solution saves you. Simply because it both loses far more heat (and gets icy cold when the heating is off) and because its not got much to heat up mass wise, so it comes up to temp quickly.
Houses that keep heat in them well, don't benefit much, and as I have been trying to say, the heatloss from parts of the system that dont heat the house as such at all, can be a lot higher when their average temperature is now at 65C-70C for prolonged periods, I.e heating pipes in the loft or walls. If you have to use high boiler temps to get the house to heat up, you WILL be losing more heat during that phase.
A pipe, even in a couple of inches of foam, is nowhere near as well insulated as the rest of the house should be.
This is not a simple analysis.
Boilers modulate as well, because it is both better to run for longer periods than to run at lower power levels. The reasons are simple: high power means less efficiency as the condensers - if fitted - work less well, and also the alternative to modulation - cycling - leads to start up losses when the boiler fires.
And if you use the most efficient heater there is - a heat pump - the efficiency severely degrades when called for high output temperatures.
I m not saying that its always better to run a system this way or that way, just trying to point out that the simple analysis runs into trouble the more modern the house is.
There is a law of diminishing returns. And sometimes the transient behaviour of the system makes it a net loss.
For our non-condensing Worcester Bosch 350 the difference is the other way round:
71% efficient at 9 Kw 79% efficient at 25 Kw 80% efficient at 35 Kw
That would only be true if the TRVs snapped shut in a binary fashion. In practice these are analogue devices and close progressively as the set temperature is approached, gradually throttling the water flow. If they are all approaching the target temperature the boiler will modulate down due to the reduced flow.
The message from The Natural Philosopher contains these words:
snip
I don't have any figures to hand but I would have thought that the thermal mass of the radiator (as opposed to the water within it) would be negligible. The temperature of the radiators does take time to get up to full working temperature but not so long that the set temperature isn't reached on a normal firing cycle. Given a 10 degree drop across the boiler it is going to take several circuits of the heating loop to get the boiler output up to the boiler set temperature unless the set temperature is very low.
Surely a passive high mass acts as a damper smoothing out the overshoot. I am not sure passive is the right word but I am trying to distinguish between a house with plenty of solid masonry in its internal walls and a concrete slab with underfloor heating below which is really a radiator needing serious control gear to prevent a significant overshoot, but even there you have a large time constant in the equation.
If you go back to my very simplified model there is no difference. Don't forget that this exchange grew from Dennis' claim that turning the heating off for 8 hours would save very nearly one third of daily fuel use.
As I see it the thermal mass isn't much of an issue being a function both of the heat required to keep the house up to temperature when the heating is on constantly and of the heat required to bring the house back up to the set temperature when the heating is off for an extended period. What is true is that a poorly insulated house will leak heat much more quickly than a well insulated one and thus save more of the heat that would have leaked out had the heating remained on (IYSWIM).
Pipes per se give out very little heat compared with radiators even when they are not lagged.
I would expect only minor differences in boiler efficiency if the same system was in use in both timed and constant mode which would be dwarfed by the difference in heat losses.
But the more modern the house the more sophisticated the control system has to be. I do not believe for one moment there is a single centrally heated house in the country where running the heating 24 hours a day results in less fuel usage than having the heating off for a solid 8 hours in 24 and very few where it would be possible to even engineer such an outcome.
HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.