"PM" wrote in message
...
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?

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

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.

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.