I have an old Potterton fireside black beauty 35/51 back boiler keeps turning off pilot light

Hello I hope someone can help with this one.
I have an old Potterton fireside black beauty 35/51 back boiler.
until this morning it was running fine. It flamed up fine and the heating/water came one fine. after it bit it turned itself off, and the pilot light went out.
I turn the starter vale to off and left it to cool down.
when I went back to it to relight the pilot light after about 20 minuets. It relight fine I turned the starter vale back to on. After about 20 seconds the was a hissing sound then a clicking sound and the pilot light goes out.
I gave it all a good clean with vacuum cleaner and air duster.
It does the same, the pilot light come one fire time very time. The flame look nice a blue and is one flame.
If I turn the heating and water on before I light the boiler it will flame up fine and run okay for a bit.
It looking like it get to where it will flame down in normal operation then after about 20 seconds it stars to hiss then the pilot light goes out.
Anyone got any idea what is up with it?
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kennygw wrote:

Highly likely that you need a new thermocouple. they are pretty universal and readily available in a kit with a number of different accessories. EG http://www.toolstation.com/shop/Heating/d230/Central+Heating/sd2708/Universal+Thermocouple/p64067 Screwfux will have one too.
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Bob Minchin wrote:

Sounds about right.
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Bob Minchin wrote:

In our boiler, the top socket crusts up (and points the pilot flame in the wrong direction) sooner than the thermocouple itself packs in. I don't know if the OP's has anything like that.
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On Saturday, 22 October 2016 15:44:04 UTC+1, kennygw wrote:

The pilot light needs to heat the thermocouple enough to keep the valve open that keeps the pilot flame on. The flame pointing bits or thermocouple may be out of position. If the TC is heated too much it gets red hot & doesn't last so well.
If the thermocouple is getting all the heat it can but it still doesn't work, time for a new thermocouple.
NT
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replying to tabbypurr, kennygw wrote: Hi i think the pilot light is heating up the thermocouple okay. As it does stay alight for about seconds. Then it will start to hiss the i get a click sound as i have turn it off for 10 ten mins get the the light to stay on. I would of guess that when the boiler is fully on it would keep the thermcouple hot. When the boiler flames down. The pilot light stags on for about 20 seconds then the hissing sound is back on the click and it goes off. Sound like some safety thing is kicking in. I have had a look at the both ends of thermocople and tighten them off then back up still the same.
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kennygw wrote:

The thermocouple may be hot, but if it's worn-out it won't be generating enough current to hold the gas valve open ...
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On Sunday, 23 October 2016 15:44:03 UTC+1, kennygw wrote:

You can't tell they're faulty by visual inspection.
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On Saturday, 22 October 2016 15:44:04 UTC+1, kennygw wrote:

Thermocouple. (The bit of metal that projects into the pilot light flame). It generates a voltage which hold the safety valve open. If insufficient voltage is generated, the safety valve will close. They deteriorate as they get older. New ones are dirt cheap. You can get a universal one that fits most things.
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message

This hiss you hear sounds like there could be a slight water leak and the drip turns to steam on a hot surface.
--
Dave W



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On Sunday, 23 October 2016 17:21:08 UTC+1, Dave W wrote:

maybe that cools the TC enough to cause the problem. If so, liquid glass would bodge it.
NT
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