The pilot by itself is enough to keep the thermocouple happy. It looks a decent size but wanders a bit (that is, it's not a stationary flame). When the burner fires up, then the pilot seems to disappear, or drawn into the bigger flames. Only in failure mode does the pilot go out. If the CH is just turned off when the burner is on, then the pilot is OK.
Is anything restricting air flow into the boiler? It sounds like the flames might be "lifting off". We see this in our camper van oven sometimes if we close the door too soon after lighting.
Symptoms are that it lights fine, flames look fine initially but gradually "lift off" the burner until they reach a point when the flame failure device isn't being heated and the whole thing then shuts down.
I admit that I can't quite see how this might affect your pilot light but maybe you have a gas/air mixing problem.
When did the boiler and flue last have a proper clean out?
Tim
Its last service was in 2011. The main burner flames look good to me.
For an open vented boiler five years is probably longer than you want to leave it. Plenty of time for fluff to get ingested and soot to deposit.
Have you watched it until it cuts out? It's obviously lighting fine but that's not when the problem occurs.
Tim
I have watched it again to see what happens. It's quite tricky peeping through a little hole. I know I'll be dreaming about little blue flames tonight ;) The pilot flame by itself squirts out at about 45 degrees to the right, and goes over the thermocouple. When the burner is on then the pilot flame seems to get dragged to the left and you can clearly(ish) see the thermocouple is not being heated directly by the pilot. I have booked a visit for tomorrow so he can have a look at the crud situation. I didn't really want to get inside with the hoover in case I broke or upset something. Fortunately I haven't needed the heating today (only 4C outside at the moment), and have an immersion for hot water as a backup.
Yup. With the older style pilot valves, the valve is held open electrically. It has two settings if you like - pilot on, and burner on.
When you light the pilot you manually push a button to allow gas through. The pilot lights and heats the thermocouple which in turn feeds a solenoid coil in the valve. This has many turns of fine wire and the small current from the thermocouple is enough to hold the pilot part of the valve open magnetically.
When the system calls for heat it feeds mains to the main part of the valve. This will open the gas to the main burners, and they should then light from the pilot. They should stay lit all the time that mains is on the valve. When the controls are satisfied they drop the mains top the valve and it shuts off the main burner. The pilot should stay lit though.
If there is mains on the valve, then that means the controls are still calling for heat. Any additional internal stats in the boiler are normally placed in series with the call for heat to the valve. So if there is mains on the valve, it suggests all the other interlocks are closed.
Yup that's what you would expect.
The only bit I am not sure off (since I have never tried it) is what happens if you have mains on the valve, but then the current from the thermocouple is withdrawn. That would close the pilot part of the valve, but I don't know what it would do to the main burner. I am guessing it would have no effect.
Probably - but its possible its related to a blocked pilot jet I suppose. (my guess would be dead valve though if the lack of pilot current also allows the main valve to close while it has 240V on it)
OOI, what part number is the valve. ISTR I have a spare in the workshop I pulled out of a working boiler that was scrapped - you never know your luck!
I'd remove the pilot jet and give it a good clean. Won't do any harm.
The part number on the valve is V404A 1071 3. I think there is a modern Honeywell replacement for about £60-100. As this will be replaced by a qualified gas man, I guess he'd rather fit a new part. But thanks anyway.
The datasheet for the valve says that the thermocouple safety device shuts off gas to both pilot and main burners. The mains connection is not in series with the safety device, so I'm guessing in failure mode then 240V would still be present, but the valve would be off.
Maybe a good clean will get it running again. Fingers crossed.
Update: Gas man has visited and cleaned out the burner and pilot jet. So much crud came out. The pilot was cleaned as much as possible and its supply pipe tightened and sealed to remove the leak that was there. The pilot flame is now much stronger and still touches the thermocouple when the main burner starts. The old boiler has been running for an hour, so all seems well.
Thanks to all for your comments.
Glad you're up and running again. Don't neglect the servicing for so long again though. Open vented boilers can fill your house with carbon monoxide if not operating properly.
Tim
My older Potterton than that was room sealed.
My boiler is room sealed too, as long as the combustion chamber cover seal works as it should.
I thought open-vented just meant that there was a header tank.
Apologies, I was mixing my terms. I was thinking of "room sealed, balanced/fan flued" versus an "open" boiler that relies on convection for the flue. I was assuming (incorrectly) that yours wasn't room sealed.
Tim
Its combining two bits of information...
The burner can be "Open flue" or "Balanced Flue". Balanced flue usually implying "Room sealed"
The primary water system can be "Vented" or "Sealed".
So open-vented suggests an open flue burner and a vented primary water system. (There will be very few if any open-sealed boilers!)
Various types described here:
Apologies not needed. I will heed your comment about more frequent servicing nonetheless.
I found it an easy enough DIY job to remove and clean the pilot jet on my old Kingfisher.
That's why I have a CO monitor alarm close to by boiler
I have too. Yonks old and probably doesn't work (although the test button does) since IIRC it said replace every 3 years or summat.
But even the boiler it was first installed beside was room sealed. This one a condenser.
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