Purging air from Gas pipe

[Chris Cowley]

I am planning to move the location of the gas hob & oven in our kitchen by about 12" at some point after Christmas. This will involve extending an existing gas pipework.

I'm completely comfortable with most aspects of doing this work safely and competently, including checking my work for leaks using a manometer attached to the gas meter when I'm done, and so on.

However, the one area of concern I have is related to purging the pipework of air. The pipework leading to the oven/hob will be fairly trivial to purge, but there is an existing combi-boiler teed off the pipework just before the part that I'll be extending. I don't know a great deal about combi boilers, and have no idea how to go about purging air from the 2m or so of pipework leading from the tee to the boiler itself - or even if this is necessary (I assume it is, as I can't imagine that having an air/gas mixture in the pipework when the boiler is turned back on is a great idea).

What say you, wise people of uk.d-i-y?

[Chris Cowley]

Hmmm... he various snippets of information that I've been able to find so far seem to indicate that this isn't something that is paid too much mind (when commissioning a new domestic boiler, at least). Is this correct?

[John Stumbles]

The boiler installation manual will almost certainly instruct you to purge the supply pipe before trying to light the boiler. If you don't it'll be forever before the pilot light demand pulls gas through even 2m of 22mm pipe, and some boilers will lock out after a few abortive attempts to light. A practical way to purge the pipe is to loosen the union of the pipe to the boiler and open the valve, checking for gas coming through (by smell, purlease, not with a lighter :-). Obviously make sure the area is well ventilated and there are no sources of ignition around while you're doing it. And test for tightness after.

[Ed Sirett]

By the book and when under scrutiny the correct method is to purge the required amount of gas (invariably 10 litres or 0.355 ft3) as described above after performing all the required prior checks and ensuring ventilation etc. In practice for gas hobs I just turn on all four burners and press the ignition button until all four have lit (Note that sometimes the burners will light then go out and then come on again after say 10 seconds). For boilers with two stage ignition (e.g. Pott. Profile) there is no way other than the correct method that will work. For most modern combi boilers which ignite at low rate (typically 8-10kW) simply turning the boiler on will suffice and just maybe pressing the reset button once if the gas takes a long time to come through and it locks out.

[Chris Cowley]

Thanks chaps, much appreciated. It's easy when you know how! It is a modern condensing combi, but I will slacken off the union nut to the boiler as described, as the main point of me doing this myself (besides paying hundreds in call-out fees for what is essentially a fairly simple job) is to make sure, as much as is possible, that everything is done completely correctly and according to best practice.

Cheers,

[Nigel Molesworth]

I don't think it would matter, the jet would act as a fire-block.

It isn't much different from allowing air into the pipe when you change a LPG cylinder, can't be avoided.