Should a central heating boiler be on all the time?

[Richard210363]

Given that the thermostat shows that the heating has not reached its target temperature should the boiler remain on constantly?

Mine goes on/off over a, roughly, 20/30 minute cycle.

I have a conventional system of :

• Gas boiler
• Cold tank
• Expansion tank
• Cylinder
• Not pressurized.

 

[Jackrae]

If the thermostat is demanding heat AND the boiler is not cycling on high temperature limiting due to restricted flow then it should remain 'burning'

 

[stem]

The boiler thermostat controls the temperature of the water leaving the boiler and will regulate the burner accordingly to maintain the temperature of the water leaving it.

The on / off time will depend upon the heat required by your system in relation to the output of your boiler. When started from cold, the boiler will be 'on' more than 'off' as cool water will be entering it, needing a lot of heat to bring it up to temperature. As the system warms up and hot water is returning to the boiler which requires less heating to get it back to the set temperature, then the 'on' time will decrease and the 'off' time increase.

 

[Richard210363]

So the 2 answers above contradict each other?

Or am I misinterpreting?

 

[stem]

It depends what you mean by the boiler being 'on constantly' I was referring to the burner actually being lit and consuming gas, which was was I thought you were referring to. ie, it lighting up and then going out and back on, on a cyclic basis. Although if the on / off differential is 20 / 30 minutes, that is probably not what you are referring to. So my apologies for any confusion.

I believe Jackrae is referring to the boiler having a signal from the controls telling it to run, regardless of the burner being lit or not. In which case it should be on constantly until the thermostat is satisfied.

 

[jackthom]

If the thermostat is calling for heat the boiler should ideally be firing constantly until the house is up to temperature.
Depending on the make and model of the boiler, number of radiators, whether the system is a bit clogged or the pump is not working too well, this may not be possible.

 

[stem]

If the thermostat is calling for heat the boiler should ideally be firing constantly until the house is up to temperature..

That depends on the design of the boiler. Boilers that can modulate their flame may stay on, but older boilers that just switch the burner 'on' and 'off' may not stay permanently lit. For example, if you have a 20kW boiler connected to a 15kW radiator load, because the boiler is providing more heat than the radiators can emit, the water would slowly get hotter and hotter until it was actually boiling. To prevent this, the boiler thermostat will switch off the burner when the set water temperature was achieved, even though the room thermostat was still calling for heat.

 

[jackthom]

That depends on the design of the boiler. Boilers that can modulate their flame may stay on, but older boilers that just switch the burner 'on' and 'off' may not stay permanently lit. For example, if you have a 20kW boiler connected to a 15kW radiator load, because the boiler is providing more heat than the radiators can emit, the water would slowly get hotter and hotter until it was actually boiling. To prevent this, the boiler thermostat will switch off the burner when the set water temperature was achieved, even though the room thermostat was still calling for heat.

Agreed, you didn’t quote my second sentence where I said as much. I’d be interested to know which boiler this is and what it can modulate down to.

 

[Richard210363]

Ah, then the confusion was caused by me not defining what 'on constantly' means.

By 'on constantly' I mean the burner being lit.
I take it that in heating engineer terms 'on constantly' means that the boiler being told to run by the thermostat system

The cycle time of 20/30 minutes gets shorter as the day wears on. At the moment it is about 10/15 minutes.

My boiler is 24 years old so I suspect it is non-modulating. It is a Potterton Prima 40E. I had it checked over earlier this year and it was fine then.

 

[jackthom]

Then your boiler is probably working normally as stem described above.
Some modern boilers can modulate their output power down to as low as 5kW and in most systems they would happily keep burning as long as the thermostat was calling for heat.

 

[Richard210363]

Yes, working normally that was what I was getting from the discussion.

Thanks for helping me out, I feel reassured now.

 

[Jackrae]

You need to advise what you mean by "thermostat" Is it a room 'stat or one installed on the boiler.

 

[D_Hailsham]

Given that the thermostat shows that the heating has not reached its target temperature should the boiler remain on constantly?

Mine goes on/off over a, roughly, 20/30 minute cycle.

If the room hasn't reached the target temperature then it's down to one of two things:

1. The boiler thermostat is switching the boiler on and off to maintain the correct water temperature. The length of the off period will be caused by the anti-cycling mechanism.
2. The room thermostat has TPI control built in, which automatically cycles the boiler as the actual room temperature approaches the target temperature. This prevents overshoot.

Which boiler and room stat do you have?

 

[JohnD]

it's quite common for radiators to be undersized. So although the boiler valiantly tries to heat the house, the radiators aren't capable of emitting all the heat the boiler can provide. So the circulating water will get hotter and hotter until the boiler thermostat stops it firing, and the house is still not adequately warm.

You would notice this more on a cold night.