Gas pipe fitting Q's

[PM]

A couple of questions regarding extending gas pipework:

1) It is illegal to run gas pipe in a cavity wall because of the possibility of gas build-up, does this mean it is preferable where I have the choice to run gas pipe along a wall rather than under floorboards?

2) How do I clear the existing pipe of gas before soldering the new pipe on? The pipe comes down from the meter, under the floor (presumably horizontally but I've got no way of telling) and then up in a cupboard which is where I will be making the join. So I would imagine most gas will escape once I open up the old pipe - but with things like this it is best to make sure! Should I open up the test nipple at the meter to aid dispersion? I don't have any gas analysis devices. N.B. I am fully aware of the need to purge air from the new installation.

Thanks

Pete.

[Doctor Drivel]

Under boards is fine as it is ventilated on the ground floor and enough space under on suspended floors too.

Turn off the gas and undo the meter outlet union. Between the meter horn and the union insert a lid from say a flux tin. This prevents any flashbacks down the pipe from blowing the meter. There is no need to get rid of the gas in a small copper gas pipe. In larger systems the gas has to be pumped from the pipe via a flame trap hose to outside.

In reality a flame trap hose is needed on an installation when purging too. Few are ever used on domestic systems.

[Ed Sirett]

As always if you need to ask you probably shouldn't be doing it...

1) Through cavity is fine but there are extra rules. See BS 6891 2) No need to purge gas from the pipes. You will need to seal the meter outlet if working within 2m of the meter. The gas can only burn if comes into contact with air. Even when unsoldering fittings the most you even get is a small flame which goes out in a few seconds.

[Doctor Drivel]

I have seen a flash dow 6 metres of pipe and a boom as the guts of the meter blew.

[PM]

First time for everything! I've read up a lot (including your FAQs) but there are some areas such as this where I'm not sure. My mind has been made up to DIY mainly because of the incompetence and obvious dishonesty of the "tradesmen" I have had round for gas work in the past, and for quoting for a new boiler.

Through cavity yes, along length of cavity no because of gas build-up. So is it better to avoid under floorboards if possible or is it fine as Drivel says? I have the choice of running the pipe in the corner of the ceiling (unsightly) or under floorboards.

Thanks!

[Ed Sirett]

What was the pipe diameter? Pure gas can't go 'boom' as it's concentration is way above the UEL.

[Ed Sirett]

A link to BS 6891 can be found under 'gas fitting standards' below. If the under floor space is ventilated then there is no problem, this is common practice. It is quite awkward to get the pipes under the floor in these circumstances. Compression joints should not be used.

[PM]

installation.

When I cut the old pipe ready to solder the new pipe on, won't the gas mix with air at that point? The old pipe is vertical up a wall and I will be cutting it about a foot from the floor. Won't the gas will rise out of the existing pipe so could be in the 5-15% concentration either above the pipe, or within the pipe?