Exotic pilot light problem

[palmyra]

Hello,

I have a Glow Worm 45 backboiler unit, which started to exhibit an interesting behaviour, which is at the moment being 'guessed' by me & an engineer so I may end up paying for N experiments to repair it.

1. Essentially, pilot light is OK, room thermostat makes boiler come on, eventually turns off the boiler -

2. right after the main burner goes out, the pilot light is turned off after a loud click.

3. I have to leave it cool a bit, pilot light re-lights perfectly, stays on... until next proper burn cycle terminated by the room thermostat, after which again pilot light gets turned off.

4. Vital clue: the pilot light does NOT get smaller, doesn't choke etc. prior to the click shutting it off for no visible reason ---> so it's not caused by any backdraft or ventilation problem, in which case I would understand safety system doing this.

5. as long as I only give the boiler a short burn, let's say a minute, it doesn't do this... pilot light stays on. I repeated this experiment via the room thermostat many times. But long proper burn cycle leads always to this shutting off of the pilot light after burner is turned off by room thermostat.

So it makes me believe it could be thermocouple? somehow when boiler heats up, it gets confused and then after main burner goes out, it believes also pilot light went out so decides to shut off everything? As my understanding is that the boiler overheat thermostat would only cut main burner, not the pilot light as well

Will be very grateful for any thoughts, it's been driving me and an engineer I consulted up the wall - and it therefore may mean several repeated repair attempts.... My working theory is to change thermocouple via a qualified engineer?

 

[Agile]

Hello,

Will be very grateful for any thoughts, it's been driving me and an engineer I consulted up the wall - and it therefore may mean several repeated repair attempts.... My working theory is to change thermocouple via a qualified engineer?

Does that mean the engineer you "consulted" is not gas registered?

I dont have time to check the design of yor boiler but that situation would be brought about if it has an overheat stat and there is no pump over run.

You could test that theory by seperately powering the pump to ensure it stays on after the boiler shuts down.

If you have the book you could see if there is an overheat stat. They are sometimes added or supplied seperately so what the book says is not necessarily the whole answer.

Tony

 

[palmyra]

He's Gas Safe registered and he has done the annual full servicing a few days ago. Then this problem popped up - so afraid it may take N attempts, so far phoned him describing the symptoms and mused over possible thermocouple.

There is an overheat thermostat, but thing is, I was expecting it to cut the main flame only (not pilot light, too) if overheats... as it should come back on again to continue heating job etc.

Whether pump keeps going after boiler stops, haven't checked - can have a peek, although not sure what would make that behave differently suddenly. Then water could heat up as it stops in boiler after burner cut out, but why after few seconds it kills pilot light, too is beyond me.

 

[Agile]

There is an overheat thermostat, but thing is, I was expecting it to cut the main flame only (not pilot light, too) if overheats... as it should come back on again to continue heating job etc.

Whether pump keeps going after boiler stops, haven't checked - can have a peek, although not sure what would make that behave differently suddenly. Then water could heat up as it stops in boiler after burner cut out, but why after few seconds it kills pilot light, too is beyond me.

It would be far better if you stopped thinking and did this simple test!

Virtually ALL o/h stats on pilot light boilers just trip out the pilot light.

Do the test with the pump and we can then advise you further.

Tony

 

[kevplumb]

There is an overheat thermostat, but thing is, I was expecting it to cut the main flame only (not pilot light, too) if overheats... as it should come back on again to continue heating job etc.

oh stat cuts pilot
no pilot, no main burner

thats how it works

bear in mind your boiler won't have one unless its on a sealed system

it will however have an oxy pilot assembly which will have the same effect

 

[palmyra]

Much appreciated and will do test, too - just at work still so no direct/immediate access to the gear at the moment. I might come back with some silly questions if I end up looking puzzled at the pipework and the pump installed on it

 

[palmyra]

Hello again, now at home and had a look at pump and electrical installation. I have to admit I'm experiencing a sense of dread trying to power pump somehow independently, its power supply is at the back and I can see wiring going into a sealed box then there's separate timer etc.

So while very grateful for the tips and makes sense to me, might not endeavour to hack the wiring there

By the way it's an open system - ground floor maisonette and probably the overflow tank in the loft above the upstairs neighbour? Would that be running on empty and leading to overheat?

So the root cause could be thermostat (which makes sense with the cooling/heating cycle symptoms in relation to what pilot light did) - or it may be OK thermostat but the pump cuts off too early?

There is one as I have adjustable thermostat control box with a capillary going into it from somewhere behind the part I can see at floor level.

What determines the pump overrun?

Anyway also phoned back the chap who in the meantime ordered a thermocouple, but something tells me if I don't convince him about the other cause(s) we may have several attempts at this over coming weeks/months

 

[kevplumb]

Anyway also phoned back the chap who in the meantime ordered a thermocouple, but something tells me if I don't convince him about the other cause(s) we may have several attempts at this over coming weeks/months

good luck trying to fit it

tho oxy pilot comes as an assembly

View media item 15559

 

[palmyra]

Well, I intend to leave it up to him, staying well away from the guts of that dinosaur

However, with the cooling/heating symptoms and tips above, my bets at least are on the overheat thermostat.

Looking at water temperature / radiator temp, it made zero difference whether I had boiler thermostat at min or max, and once the room thermostat made burner go off, the pilot light invariably went out with a mighty click, then once it all cooled for a little while, could re-ignite and stayed on.

In terms of overrun, had a look and it has no overrun at all, the pump is wired to the house thermostat, so it will stop immediately as it cuts off, at the same time with the boiler.

Therefore if it changed behaviour so drastically and starts thinking it overheats once burn stopped, maybe it simply doesn't sense temperature properly any more but it then goes off once it thinks it's totally overheated? Clearly, it doesn't make boiler go off during burn of any length, and only acts (totally turning off everything) once the pump and burner stopped...

 

[kevplumb]

There is an overheat thermostat, but thing is, I was expecting it to cut the main flame only (not pilot light, too) if overheats... as it should come back on again to continue heating job etc.

not on an open vented system

There is one as I have adjustable thermostat control box with a capillary going into it from somewhere behind the part I can see at floor level.

that is the boiler stat different animal

By the way it's an open system

What determines the pump overrun?

nothing you don't have one

Therefore if it changed behaviour so drastically and starts thinking it overheats once burn stopped, maybe it simply doesn't sense temperature properly any more but it then goes off once it thinks it's totally overheated?

it's nothing to do with overheating

 

[palmyra]

First of all thanks for the continued assistance - so in summary:
- it's not overheat thermostat in this case, as it's open vented system and don't even have one
- the boiler thermostat in its control box (not the room thermostat) doesn't seem to make any difference from Min to Max, radiators are invariably really hot
- not believed to be thermocouple
- indeed, it cuts pump together with the burner from the house thermostat, so there is no overrun in this system

So what could cause the pilot to be cut off with a mighty click, invariably after long burn (without pilot light first going out due to any ventilation etc. issues) ? as above seems to then say 'no' to both overheat thermostat and thermocouple theories.

 

[holty]

which 45 have you got. electronic ignition or old fashioned push button pilot. both type models have sensing tubes. id go with the electronic one. which would make more sense of the problem.

 

[palmyra]

Hi,

it's with pushbutton piezo thingy for igniting pilot light.

 

[glasgowgas]

serviced a 57/4 M today

get rgi to check ASD is clear from top of pipe right through to pilot tip

he would need to strip off a few bits n bobs from burner,
not just take out burner and brush clean


any pets???

 

[kirkgas]

an open vent system CAN have an overheat stat, fitted plenty to BAXI BBU's, they come as an accessory and are required if the vent doesnt rise up from the boiler