Central Heating - should we run it all night in Winter ?

[JohnM]

We live in a large old house that has been converted into 5 flats. It has a large, oil-fired boiler and about 60 radiators, with a lot of large cast iron pipes. The system takes about an hour from the boiler coming on before there is a significant amount of heat coming from the radiators. In the past we have run the system continually during the winter, in the belief that if we let it cool down overnight, it would use the same amount of oil to get hot in the morning as it would have used if we had left it on at night. However it's hard to measure this accurately. Does anyone have any thoughts on this ?

I was thinking that if we decide to do this again this winter, we could turn the boiler temperature down at night and up again in the morning in order to reduce the cost - however I would need to automate the control knob using a servo motor and timeswitch.

Cheers

John

 

[MdB]

In the past we have run the system continually during the winter, in the belief that if we let it cool down overnight, it would use the same amount of oil to get hot in the morning as it would have used if we had left it on at night.

Lots of people believe this - I'm not sure why. It is completely wrong (*). The heat losses are directly related to the temperature differential so reducing the temperature at night will reduce the heat loss and hence the amount of energy needed to be put into the system.

(*) As long as the heat source efficiency at full power is greater or equal to the efficiency at low power levels. I can't think of a heat source where this isn't the case.

 

[Lime]

It is not just a simple cheaper-off-than-on question is it?
Large houses take some heating and although the radiators are hot after an hour warming a cold house takes longer still.
So switching off at night may have some comfort considerations to be take into account too.

However I'd give it a try to save costs.

Running the boiler at a lower temperature works well too in most cases (but it does depend on the boiler) so I'd try that as an alternative perhaps combined with slowing the pump down.. less noise at night.

 

[yamin]

Far less technical but I agree with MdB. Notwithstanding the time to heat up initially, a big old, presumably a nit draughty house, is going to need pretty constant 'top up' of the heat by the boiler to keep the temp consistent overnight. Surely thats going to outweigh the oil used to heat the system up once or twice a day? Bear in mind I'm coming from the opposite end of the spectrum - not turned heating on AT ALL yet - using woodburner to heat half the ground floor in the evening. For daytime comfort its jumpers and keep moving, for nighttime comfort, its a thick duvet, PJs and my other half !!

 

[Aidan]

You can get electronic controllers with optimum start/stop control, i.e., it gets the house warmed up to the set temperature as measured by an internal sensor at the 'ON' time, and it uses an outside sensor to calculate the time when the boiler must start. It starts early on cold days, later on mild days, not at all on warm days.

It sounds like it wouldn't go off at all overnight in cold weather.

You can't just turn down the boiler control thermostat on a non-condensing boiler. You'll get condensation in the flues and severe corrosion. You'd need a weather compensation device and a mixing valve.

 

[Flyfisher]

You could try both options for a month and monitor how much oil you use to confirm which is actually most economical for your particular house. You'd also need to keep a record of overnight temperatures to adjust for any significant average temperature differences. Might be an interesting experiment.

 

[JohnM]

Thanks to everyone for their replies. It might be that we are wrong about leaving the system running all night, but it does seem intutive that if it takes a couple of hours to heat the system and house up from cold, then it might be cheaper to keep it ticking over. I suppose Flyfisher's suggestion of trying both ways for a month would give us some facts to go on, although the variability in the weather (which seems pretty high at the moment !) might be hard to control for.

I don't think that the temperature control that I was talking about governs the heat at which the boiler itself runs at - it's more like a thermostat which tells the boiler to fire when the temperature of the water in the system falls below the level it is set at. So if we set it lower, the boiler will fire less often and the radiators will be cooler.

Aidan's suggestion of an electronic controller is interesting - can you point me in the direction of anyone selling one ?

Cheers

John

 

[MdB]

A £50 electronic thermostat with an outside temperature sensor will switch your boiler on off as appropriate and automatically give it an extra boost if it is colder outside. Our one (Honeywell I think) has six temperature time zones so you can easily set it up to be cooler during night time.


Regarding running it overnight:

Option 1: Switch the heating off at midnight with a 20°C inside temp and a 0°C outside temp. Leave off until 6am at which time it had cooled to 10°C inside. Switch heating back on to rise the temperature back to 20°C.

Option 2: Leave heating set on 20°C all night.

Option 1 will use approx 76% of the fuel that Option 2 does (from midnight to the point where the temperature gets back to 20°C).

If you have a more poorly insulated house then the above example then the potential saving will be higher. If you have a well insulated house, then the saving will be lower.

 

[Aidan]

Honeywell make one, CM67 I believe. Do a search for "optimum start". I haven't used one so can't make a recommendation.

The dial is the boiler thermostat, it regulates the flow temperature. The boiler will fire until the set temperature is reached and then stops firing. There is a differential, about 3 degC, so the flow temperature drops by 3 degC or so before the burner is fired again. If it is a non-condensing boiler, it would be unwise to turn this down.

You have to keep the return temperature above about 55 or 60 degC on non-condensing boilers. On commercial boilers you'd have a shunt pump or a modulating 3-port diverting valve, which would direct hot flow water straight into the return in order to keep the return temperature above 60 degC; it is switched off when the return water temperature from the building is above 60 degC.

It sounds like your system would have a low return temperature for a long time, due to the heat capacity of the system. You should monitor this, or get a condensing boiler.

 

[Craig]

I havent heard of these optiumum start up devices but they look like a very good idea. Seems the CM67 is out of production now - the CM727 seems to offer the same thing though (in a wireless format).