Tom Paterson wrote:

From: Pooh Bear

Over here ( UK ) the gas pipe to the heater will be copper ( as is the water
too
).

The gas entering my home starts off as a steel pipe though - and then is
converted
to copper inside the property.

Never seen any examples over here of the 'orange discolouration'.

Your copper gas lines must be different material from water lines (USA spec):

Copper water tube is a seamless, almost pure copper material manufactured to
the requirements of ASTM B 88 €“ Standard Specification for Seamless Copper
Water Tube, of three basic wall thickness dimensions designated as types K, L,
and M. Type K is the thickest and type M is the thinnest with type L being of
intermediate thickness. All three types of tube are manufactured from copper
alloy C12200 having a chemical composition of a minimum of 99.9% Copper (Cu)
and Silver (Ag) combined and a maximum allowable range of Phosphorous (P) of
0.015 % - 0.040 %.

This stuff, when used with flare fittings for gas connections to water heaters
etc., does get orange especially where the copper was stressed to form the
flare, though they might take some years to fail. Leaks in older plain soft
copper connectors are "usual" (more than common); the flares are seen to be
orange and/or black and crumbly upon disassembly ("hydrogen embrittlement").
Coated corrugated copper lines and all-stainless steel flexi lines (still flare
ends on half union fittings) have replaced plain soft copper flare connectors,
I believe at least since the 1984 Uniform Code. --TP

Clearly very different to here.

We use the same copper pipe for water and gas. Optionally, water may be run in
plastic pipe.

Don't use 'flared' joints either.

Graham