Do combis take a breather?

[Paul D Smith]

My combi boiler seems to stop running the heating, take a 10s or so "breather" and then restart - is this normal?

It's a new Glow-worm and heats the house and water great. The external thermostat is fine but when the house is quiet I can hear the boiler "shut down" (pump and flame go off), wait about 10s and then restart.

The only thing I could think of is perhaps it's stopping to allow condensate to drain out? But I'm guessing here.

I'm sure someone on here knows. Paul DS

[Toby]

the boiler will heat the water in the radiators until it is gets to the set point, set with the thermostat, usually on the front of the boiler - maybe this is a little too high, so it reaches this only briefly?

[Stephen H]

Your boiler is probably a modulating type where it monitors the return temperature of the water from the radiators.

if most of the rooms are heated up fully, the thermostatic radiator valves close, which means the boiler is now serving fewer radiators.

This means the return water temperature will be higher than for when the house is being heated up from stone cold.

Thus the boiler doesn't need to heat the returning water up so much to achieve the expected flow output water temperature. Thus the gas valve is modulated down. Obviously when the returning water tempertaure drops, the gas valve will modulate back up again.

Normally in a modulating boiler, its the gas valve that is being modulated. the pump continues running otherwise how else can the boiler monitor the returning water temperature if its not flowing?

It is only in extreme cases where all of the radiators have shut off of if the pump has failed that the boiler will then shut the gas valve completely down in order to prevent water within the boiler turning into steam.

Anything worse than this will result the boiler going into overheat lock-out. The boiler must then be reset before it will work again.

Regards,

Stephen.

[Doctor Drivel]

It will not cut out the pump if room temp is not at setpoint. It will be monitoring the return temp and modulating the burner leaving the pump running.

But not cut out the pump.

Ah! You are getting there.

[Gazz]

you say the boiler is new, so has it done this since it was fitter?? could the test jumper have been left in place by the fitters?

[John Rumm]

How often is it doing this?

That is usually just dispensed in batches from a syphon - it empties each time it gets full - no need to stop heating.

[Paul D Smith]

...snip...

My mistake for failing to say that I have no thermostatic valves anywhere in the system. To some more info. based on what responders have been saying...

- I've normally got the temp of the boiler set low (60degC) but have tried raising this; same results.

- It's may have done this from new, but to be honest it's only recently its been on in the evening and I've been able to hear it doing this.

- Shutting down the gas (or just modulating) makes perfect sense to me and it is a modulating boiler, but not stopping the pump too thus my question.

So sounds like it's not something that would normally happen, and time to get the lads back to explain it to me.

Thanks, Paul DS.

[Paul D Smith]

What would the test jumper do? On my old boiler I believe it stopped the modulation but I know the new boiler modulates as I can hear the gas "roar up" sometimes.

Thanks, Paul DS

[gremlin_95]

10 seconds...that's not right. The sequence on my Vaillant condensing combi boiler (Glowworm are made by the same company apparently) is, assuming the thermostat is constantly calling for heat:

1. Fire up at low rate for a minute, in this minute it checks the flow/return difference.
2. It will either stay at low rate or simply ramp up to the maximum value set in d.0 (heating part load, my boiler is capable of pushing out

28kw, however I don't have that many radiators so it's set to 19kw)

1. As the target flow temperature is reached, it will slowly modulate down to maintain this.
2. TRV's close and the house gets warm so it can no longer simply stay on and maintain the set flow temperature and the flow temperature creeps up to 5ºc above the set point.
3. The burner cuts out and the pump continues to run (pump over-run). Pump over-run is set to 10 minutes.
4. The burner anti-cycle control stops the boiler firing for up to 20 minutes (depending on the set flow temperature) and the boiler can only re-fire and begin the whole process again once the burner anti-cycle time has elapsed.

Hope that makes sense, what Glow-worm is it? Have you checked what the display actually says when the boiler cycles on/off?

Thanks

[Paul D Smith]

...snip...

Some comments in-line...

Seems to do this but...

How do you do this? The glow-worm seems to have a "temperature" setting.

More and more wondering whether it's really doing this with the heating. It clearly is with the hot water as I can hear the gas "roar up" as I increase the water flow.

No TRVs at all - all radiators "ye olde valve" and on.

I have an external thermostat so not sure how that affects it but I could imagine the return temp reaches a certain point.

Looks "normal" i.e. what it normally says when idle.

Think I need to sit and watch and call back the installers for early New Year.

Thanks, Paul DS.

[gremlin_95]

Have you got the manual for it? It involves pressing a few buttons in installer mode and simply lowering the maximum possible firing rate for CH. The installer manual should tell you how to do this.

It might not even have to roar up if the flow temperature rises rapidly on CH, it could be just staying on minimum output!

Unless it's a Glow-worm stat, the only thing it could do really is tell the boiler to come on and off, a Glow-worm one could tell the boiler what flow temperature to use based on the indoor/outdoor temps. If it's a Honeywell or any stat with proportional control, it will purposely cycle the boiler on and off to prevent over shoot and to maintain a set temp.

Possibly the the best option really, they will hopefully realise it isn't normal :)

[Paul D Smith]

...snip...

Thanks for the additional info. I've got some paperwork so will look re: max output. The stat is a simple "on/off" device and I can hear it "click" - when the boiler is taking a breather, the stat is still on.

Paul DS