Should a central heating boiler be kept on all the time or on a timer?

[countryman]

TL;DR Summary

This is being discussed on another forum. One school of thought says keeping the central heating 'on call' rather than 'on call when the time says' saves money. This is countered by others saying it depends on how well insulate your house is.

Yet no-one has been able to provide any formulae etc to expand the topic further. A lot is said about house construction (high mass vs low mass) but that doesn't take the discussion any further.

I think the key question is if your house is badly insulated then it's losing heat all the time. If you keep the CH boiler 'on call' 24/7 then you will burn through a lot of oil. So it seems logical to run it on a timer.

But if the house is higly insulated then why would keeping the boiler 'on call' be cheaper to run ?

Apologies if this is in the wrong place.

 

[sophiecentaur]

TL;DR Summary: This is being discussed on another forum. One school of thought says keeping the central heating 'on call' rather than 'on call when the time says' saves money. This is countered by others saying it depends on how well insulate your house is.

But if the house is highly insulated then why would keeping the boiler 'on call' be cheaper to run ?

It's a matter of the details but the general principle is that the house loses heat proportionally to the temperature difference between inside and outside. (This also works the other way round for Air Con btw.)
So the least total heat loss over the day is when the average temperature difference the boiler is running (heating) as little time as possible. Turn the boiler off at night because the heat lost (temperature difference) before the boiler comes on again in the morning will be less than if the house is kept warm whilst you are tucked up in bed.
'Just keeping the house warm' when you don't need it to be warm is costing you.
Good insulation will mean less total heat loss. This still applies If the boiler takes a long time to warm up a cold house, it means the house has a high thermal mass or bad insulation. That high thermal mass will slow down the rate of cooling but, remember, you had to put a lot of heat in to warm it up so it still costs you.

Good insulation will mean that, whatever you do with your heating times, you will save money.
A totally free mod you can do with your heating is to run the house at one or two degrees cooler and wear more clothes. We have tried this since the fuel prices went up and you really can get used to it without feeling bad.

 

[The Bill]

OP didn't mention energy source. That can be very relevant if electric heating is allowed. Some electric utility companies allow variable metering based on time of day or market prices on the spot.

So, say you have a contract where one kilowatt-hour costs a cent or two less at night. Then it might make sense to run the heat constantly for the last hour or two of the cheaper billing and shut off entirely during the more expensive part of the day.

Cost and energy savings multiply each other, too. So if you improve the insulation of the house, and use a ground-source heat pump instead of a resistive heat furnace, that could result in significant savings from a thermostat timing change.

 

[Rive]

One school of thought says keeping the central heating 'on call' rather than 'on call when the time says' saves money. This is countered by others saying it depends on how well insulate your house is.

It's too complex, and just the timing won't be enough.

With a modern system, the target temperature is timed anyway, so you have lower temperature at night and when you are not at home.
With a floor-heating system you have so much heat buffer to maintain, that fast adjustment is difficult and consumes much energy. (Not to mention the inconveniences of 'the floor is lava' scenario ).
Also, the excess capacity required by fast heat-up periods might affect the efficiency and cost.

Too many possible parameters.

It's better to tackle this from the angle of what you have and what do you want: so you'll get what (and: how) you can.

But that's case specific.

 

[sophiecentaur]

But that's case specific.

Absolutely. You need to have a general idea of what factors are important and to spot those nonsense theories that many people have. But the specifics are so important if you want good value.

 

[bigfooted]

There is no complex physics or magic going on here.

Heating costs money. If you don't want to spend money, then don't heat. That's all there is to it.

Air and other fluids have a heat capacity, which is the amount of heat that you need to increase the temperature of a given volume of fluid by one degree. For air it is 1.2 kJ/(m^3.K). In a perfectly isolated house with a volume of 1 m^3, you would need 1.2 kJ of heat to increase the temperature by 1 degree, and it will then stay at that temperature when there are no heat losses.
A heat loss can be measured with the heat transfer coefficient h, which is the heat flux q [W/m^2] per degree of temperature difference,

$$h = \frac{q}{\Delta T}$$

So that means that the amount of heat going through a square meter of wall is proportional to the temperature difference over the wall. So you can minimize the heat lost by minimizing the temperature difference between the inside and the outside of the house. In other words: do not heat.

The heat lost due to radiation is even proportional to $T_{out}^4 - T_{in}^4$ so keeping the house at 2 degrees (above outside temperature) costs more than twice the heating of the house to 1 degree above outside temperature. Want to save money? Do not heat.

 

[sophiecentaur]

Heating costs money. If you don't want to spend money, then don't heat.

Haha. You are obviously a hardy type. I am one of those frozen mortals and find it difficult to work on your principle. You have to consider new-born and old humans, newly germinated plants and freshly applied paint, in real life.
Thick clothing is a partial solution to the problem; I have no patience with people who insist on shorts and T shirt with the CH going full-bore. My modest reduction in room thermostat setting has forced me to modify my indoor clothing. (I'm not complaining)