We have just brought a new house and I can’t get the pilot light to ignite on the boiler, it is at least a few months since the boiler has been used. Where to I start on tracking down the problem?

(It is an old UK system –pre electronic , the boiler heats up water that then heats the house)

The gas fire works, so we must have gas!


The controler is a Honeywell V4600C, I have check at the inlet pressure measure point and there is gas. Sometimes I can see a flame for a very short time when I try to light the pilot light, then the flame goes out.

  • @Jeff done, as requested, and thanks! I added it as a comment at first, since I've never seen a boiler and had no idea if it would apply – Jared Harley Nov 11 '10 at 10:55

It's quite normal for the jets to slowly reduce in size as soot and tar (depending on the gas being used) build up along the walls of the jet opening.

If the jet hasn't been lit for some time the soot can "fluff up" and effectively block the hole, at least to the point where the gas flow is no longer sufficient to establish or maintain combustion. In such a case I prefer to remove the jet and clean it properly, using an appropriately sized cleaning wire and solvent. If you don't have such tools you can try a short blast of compressed air instead.

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  • Is this safe for a normal person to do? – Walker Nov 10 '10 at 21:54
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    @Walker, it depends a bit on the heater's design. Some pilots are very easy to get to, remove and clean. Others are downright hard to even get at. I suggest you have a look at yours and make a decision based on what you see. If in doubt play safe and get someone in to service the unit. – John Gardeniers Nov 11 '10 at 0:44
  • The resisted “gas safe (was CORGI) gas person that I got out, just use a sewing needle to clear the jet and then washed it under the kitchen sink tap. So much for “appropriately sized cleaning wire and solvent”. He then put the boiler back together with use something like washing up liquid to check for leeks. (It was a very old boiler we have now had replaced) – Walker Sep 5 '11 at 10:23

Any chance the gas is shut off to the boiler (for the summer) and it just needs to be turned back on? Look for a valve leading to the boiler in that general area (usually there will be a shut off valve on the gas line close to the boiler).

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  • Thanks, I could not see any levers (or taps) on the pipe, what is a valve likely to look like. – Walker Nov 10 '10 at 16:29
  • Gas line shut off valve images google.com/… – Tester101 Nov 10 '10 at 17:03
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    I have got gass at the boiler, see edit to question – Walker Nov 10 '10 at 21:17

I've never had a boiler, so I'm not sure if this will help, but my furnace's pilot light kept shutting off after it tried to start up. I ended up having to clean the flame sensor, which is a metal bar that sticks into the middle of the pilot light. It had a white buildup on it, and it wasn't able to sense the pilot flame, so it shut down to prevent filling my attic with gas. I softly rubbed it with some steel wool, and it's as good as new.

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  • This is not my problem, as I am not getting the pilot to light, if the pilet was going out when I let go with the flame sensor override, then it could be a bad sensor. Howeever +1 as your answer will help someone else. – Walker Nov 11 '10 at 15:05
  • FYI flame sensor is called a thermocouple for the type your talking about. Good answer tho – UNECS Apr 22 '12 at 22:14

The last time I had a pilot light (my current boiler doesn't have one), I remember that re-lighting the thing was a bit of a pain. As I remember, you had to hold down something to get the gas flow to start (basically a manual over-ride of the thermal sensor), light it, and continue to hold for a bit so that the thermal sensor could heat up enough to let the gas stay on.

The thermal sensor is there to cut off the gas if the pilot light goes out. If it thought it was too cold, then it would cut off the gas, thus the need to let it heat up before letting go of whatever momentary control let the gas go on anyway.

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  • Thanks, this is not the problem I am having, however I have seen this often with people not knowing to hold down the "thing" – Walker Nov 11 '10 at 15:03

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