Hot expansion in plastic pipes - how to design for it?

[dom]

There's a couple of places in my plumbing where I have long, horizontal central heating plastic pipe runs contained by walls at either end, and run inside studwork.

Speedfit's website quotes about 1% expansion cold to hot.

The studwork isn't yet boarded over in these areas, and I can see that when hot, the pipes are pushed into slight s-bends in places along the run - due to expansion.

Two things I'm considering to provide room for expansion, either a dropped section of the run (made out of 4 elbows), or a section of flexible hose.

Flexible hose would be easier, and probably cheaper (although not all flexible hoses are rated for heating) - and at one location I would have to enclose it behind plasterboard (others I can work to be behind inspection panels that would be there anyway). I'm not too keen on enclosing a flexible hose - seems a bit iffy to me.

The dropped pipe section is obviously more fiddly, as a fair bit of vertical drop is obviously required - but is unlikely to ever cause trouble any more than any other bit of plastic plumbing.

I couldn't find any specific advice on the manufacturers website.

Anyone else come up with a solution to this?

[newshound]

I reckon for 1% expansion the "sag" on an unsupported 3 metre run is around

175 mm. But if you are inside a stud wall, presumably supported at a number of points where you go through clearance holes in uprights, as you say it will "snake" and probably go into a spiral rather than "flat" S-bends. You can't see it, so why does it matter? You probably do want the holes to have clearance rather than being a tight push fit because you don't want an expanded pipe to grab, and end up putting one of your push fit couplings into tension when it cools down. Expansion joints as you describe are the norm in long runs of steel pipework, but seem OTT to me for CH plastic. If you are using coils rather than the straight cut lengths of pipe, they will start banana shaped when they are cold.

In fact if anything, shrinkage seems to me more of a threat than expansion: if you build the system so that it is "tight" at (say) 20 C you could be putting joints in tension if the building temperature falls to a few degrees C when it is empty.

[Roger Mills]

Does it matter that they're pushed into slight S bends? Would it help to build in some intentional S bends which will simply get a bit more S-shaped when they heat up. This might be a bit more predictable than not knowing which way a straight pipe is going to bend.

[TheScullster]

wrote

If you do go down the road of including an expansion loop, it may be better to loop up rather than down. If the rest of the line is horizontal, then it would be possible to drain it completely from each end, rather than ending up with a dropped section that will never drain.

Phil

[sm_jamieson]

Also make sure the pipe cannot be abraded as it expands, such as where passing through a hole in a stud, or this could cause long term problems. Simon.

[Man at B&Q]

Conversely, you could end up with a nice airlock.

MBQ