How long does it take to heat your house?

[bugbear]

The only way I can see the "heat store + constant temp" solution being cheaper (as claimed) is IF
the efficiency of the boiler varies with power output (i,e, producing 200 watts is less than
half the price of producing 400 watts).

It is certainly easier to make a fixed power IC engine efficient, compared
to a variable one, which is why high power diesels have very narrow RPM power
bands and very complex gear boxes (32 speed is not unheard of)

BugBear

 

[paulrockliffe]

That's another factor, though a lot of heat sources are fixed output and just cut in and out.

My combi can be turned up and down with a knob, but it doesn't adjust itself to suit the demand. The Eco setting is actually just a click down from flat-out, so the on-off frequency can't be high enough to cause a net-efficiency problem.

It would, and does, turn on and off quite frequently, it's that and the accurate thermostat that gives the underfloor heating feel as the temp never drops more than 0.1 or goes over 0.1 above the set temperature. I'm using a smart thermostat which learns the heating and cooling characteristics for different weather conditions of the house though, so it turns the heating on and off effectively on an offset and stays ahead of the thermostat.

Without that a thermostat must over-heat and over-cool as the temperature changes as a result of the heat in the radiator system, which doesn't have a direct relationship with on and off. Actually, that might be a factor, if the system has a lot of hysteresis in the design, ie not enough radiators, but plenty of water volume the thermostat could be set to never let the room get cold, but the temperature would rise and fall by a few degrees above this point. The always on is then cheaper, but the average temperature is lower, so it's not apples being compared with apples. If you're always in of course.

 

[bugbear]

That's another factor, though a lot of heat sources are fixed output and just cut in and out.

Chop my argument off at the knees, why don't you?

Good point.

Which leave me with no idea how "constant temp is cheaper" can ever be correct.

BugBear

 

[John Brown]

"Which leave me with no idea how "constant temp is cheaper" can ever be correct."

It isn't. It's a myth.
The only thing I've ever read that I hadn't considered was some guff about condensation and wet brickwork conducting better than dry.
That's probably a myth as well...

 

[paulrockliffe]

I'd pin my hat on either confirmation bias at work somewhere or constant temperature actually being lower average temperature.

The moisture thing could affect things in theory, I'd expect a wet wall to transfer heat much quicker, just the way I can touch my woodburner handle briefly, but with a wet hand I get burned.

But I don't think in either case there would be any appreciable difference in moisture levels in the walls. Just consider that you'd need to have a significant rise in the external temperature of the brickwork and how much heat leak that must mean and it's obviously rubbish.

As a comparator, when it rains my gable wall gets wet, you can see where the chimneys are on that wall when they're on as they will dry the wall, but they're running 2-300 degrees and through the structure!

 

[DennisCA]

I dunno about you guys but constant temp is basically the only thing you can do if you have an accumulating, slow heated energy store. Our floor slab is properly insulated below and on the sides, it floats free from the foundation in fact and it does work very well to retain heat. Any change done on the temperature controls in our house takes days before we know the final outcome. With that in mind, if you reduce the temperature and cool the slab down, it will also absorb more of the heat when reheating. It would be impractical and wasteful to keep emptying and topping off that reservoir if not needed, you should build without any reservoir in that case so you don't have to fight it when reheating. And if the floor slab is exposed anywhere to the outside or just badly insulated then what you got is a heat sink that cools your whole house down and sucks away valuable and expensive energy.

In our case with the reservoir filled, the insulation of the house and the ventilation system (ours recycle waste heat from outgoing air) determines the rate of the cool down of the house and the amount of energy expended. Wich in our case is like 9000kWh per year, but that is not heating alone but all electricity usage for our property, don't know how much of it goes to heating really... I do heat my garage to 5C on direct electricity which I fitted with a monitor and it eats 1500-2000kwh per season... Though once I've expanded my diy solar collectors that should reduce. The garage slab is similarly insulated and in summer it works as an aircon, can be a sweltering 27C outside but 19C in the garage, until evening when it might be down to 16C outside and 23C in the garage, it's always just the perfect temp in relation to the outside in the warmer months and requires zero heating or cooling, I only run heating from nov-april.

Also a huge part of building to accumulate the heat of the house and re-release it when needed is due to the climate, like if the power goes out the house won't go cold and the pipes will freeze over in -30C weather on a cold winters day. That's when we would start firing up both our masonry heater and sauna stove really to stave it off. So IMO how well your house is insulated, if it has an energy reservoir and the climate determines if it's better to have constant heating on or not. So british houses are probably not worth having constant heating in and with the mild climate.

 

[paulrockliffe]

Wet underfloor heating that relies on a thermal store is very fashionable over here at the moment. You're right, that system only makes sense if you have the heating on all the time, though a learning thermostat like the one I have can work to some extent if the thermal store isn't so large that it takes days to cool down; it's possible to have it on for a time and then off for a period that isn't overnight and have the temperature fall occur over night. but the cost benefit of doing that is really limited to the size of the thermal store and the cooling rate over 12 hours. Realistically the cost saving is going to be small. In the UK it makes sense in a large house that's well insulated that is occupied most of the time, otherwise it's going to be cheaper to have lots of heat available on demand. I doubt there's anything cheaper than a smart thermostat, a combi boiler and mains gas. But that's not the whole point, if it was we'd all be sat in the house in thick jumpers.

For most in the UK there's no regular need to have backup if it goes cold and the power goes off as that just doesn't really happen.

 

[Benchwayze]

I have a 'back-boiler' behind my gas fire. I leave my heating on.

If the house overheats , which it can do, I adjust the downstairs thermostat, or control each rad independently using the 'stats I had fitted to all of them. This means I can turn off rads in the rooms I don't use often and just switch them on if I am going to spend time in them.

My bills are no higher than they ever were when I used the timer, and I never have to shiver.

HTH

John