sinking pipework into exterior wall

Hi, I want to chase the hot and cold (both 15mm) into the corner of a kitchen exterior wall.

I think I read somewhere that you cant plaster directly onto the pipes as it eats into it over time, is that correct ?

Im not too bothered about plaster cracking slightly as it will mostly be hidden by tiles and wall unit.

not sure if it needs insulation but Id rather not chase too big a channel to accommodate that.

is plastic or copper best to use ?

1. Plastic pipes are definitely not affected by plaster. Copper pipes - some say they are, some say not. But you can wrap the copper in plastic tape which should be good enough for many years.
2. The coefficient of expansion of plastic is higher than that of copper, so you would get more cracking with plastic than with copper.
3. You shouldn't have any joints or fittings, other than soldered fittings, in inaccessible places. So unless the plastic is a straight run and only requires fittings in accessible places, you are better off with soldered copper.
4. All pipes are meant to be insulated. Many aren't. I'd be tempted to use copper, soldered fittings. Wrap the copper in insulating tape with a 50% overlap, wrap something like the thin foam you get in parcels round the tape, then another layer of tape. Shouldn't end up more than about 18 mm, so a chase of 21 mm (3 mm skim afterwards) should do you.
5. If the plaster under the tiles cracks, it is likely to push the tiles off.
6. (Added immediately afterwards). If you are talking about the exterior leaf of an outside wall, or a single skinned wall, you might need to guard against frost damage.

Plaster has no effect on copper pipes, cement or concrete does though.

thanks very much for that

Interesting to read that plastic has a higher coefficient of thermal expansion than copper. I find that hard to believe (...off to check).

My usual method is to wrap copper pipe in pvc tape, then over that feed fuzzy-felt type tubular lagging. The lagging adheres to the plaster well, and the copper is left free to expand within it. A further tip is to allow a few mm of expansion space (assuming a vertical run of pipe) at the top to accommodate any pipe expansion.

MM

MeldrewsMate said:

Interesting to read that plastic has a higher coefficient of thermal expansion than copper. I find that hard to believe (...off to check).

Click to expand...

Hard to believe, but true.
Plastic expands at about 5 times the rate of copper.