CH system, copper v's plastic piping

As one Luddite, I have aesthetic and other objections to the use of plastic, mainly based on the following:-
1. It sags and needs greater clipping/restraining.
2. In heating applications it expands and contracts with the varying water temperatures, this expansion is catered for by the 'o'ring joints, of which there are 3 per 'tee' fitting. There may be over 100 of there seals in a typical system, and if the mean time between failures is 25 years, there may be a first failure within 10 years, followed by regular failures every 2 years after that. If each leak means a patch of ceiling being replaced as in the case of block & beam construction then that's quite an irritation
3. The bore of plastic is smaller than copper, so it will have higher water velocities for the same heat loading, which MAY increase noise. The same can be said of the pipe inserts (eg Speedfit Superseal inserts).

The good points are it's speed of installation (should be reflected in a cheaper overall quote), the chance to use long sweeping bends (which are quieter in operation), and its best quality- it doesn't creak under floorboards when badly installed

Hope this has been seen as a balanced viewpoint, I've got no axe to grind either way.....MM

ChrisR said:

I'm sure you're adrift there, onetap. The aluminium layer makes the pipe a different animal - it stays bent where you put it, and they're not the same size as standard (ie 16, 20mm etc). The speedfit pex, and the other 15mm ones, don't have any aluminium in. The barrier layer in those is some other plastic.

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No. The aluminium is there to make the tube bendable, but it also serves as an oxygen barrier. Barrier tube usually has an external plastic coating which is the oxygen barrier.

PEX-AL-PEX is usually the same OD as any other British Standard tube. The Rehau tube is different diameters (12, 16, 20, 25, 32mm), although it's not Pex-Al-Pex, because it conforms to some German standard that I can't recall.

Onetap said:

PEX-AL-PEX is usually the same OD as any other British Standard tube.

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No! Speedfit for example, has no aluminium in it. None of the 15mm plastic pipes do.

Onetap said:

Barrier tube usually has an external plastic coating which is the oxygen barrier.

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No! The barrier layer is the middle layer in eg Speedfit pipe.

Onetap short of a bathroom?

Unipipe is DELIBERATELY differently-sized from copper to PREVENT the wrong fittings being used on it!!!

One of its advantages is that the pressure seal is formed INSIDE the tube, between the inside wall of the pipe (where there will be no scratches!) and a spigot on the fitting. The external sleeve on the fitting compresses the pipe onto a sealing washer on the spigot.

This is NOT the same technology as the O-rings used in push-fit systems. An O-ring joint DEPENDS on the internal pressure to force the ring and other joint elements together, so there's no leak. For this reason, push-fit systems often leak at LOW pressure. In particular, a 'sealed' CH system with insufficient pressure in it will let air in through push-fit joints near the top of the system, where there may easily be suction rather than pressure.

In 'crimp' systems such as Unipipe, the force holding the joint together and compressing the pipes onto the sealing washers is due to compression of the swage rings on each fitting, not the internal pressure in the pipes.

The only problem is the cost of the crimp tool!

One of your points seems particularly moot, mm:

meldrew's_mate said:

As one Luddite, I have aesthetic and other objections to the use of plastic, mainly based on the following:-
3. The bore of plastic is smaller than copper, so it will have higher water velocities for the same heat loading...

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I don't think it's a foregone conclusion that smaller bore means higher velocities when you're talking about distributing heat. For example, plastic pipe is a better insulator, so the water will arrive at radiators at a higher temperature than if copper were used.

However, with a fixed speed pump, the velocity would be higher, regardless of water temperature.

On the other hand, the use of fewer bends and couplings will serve to reduce noise, so the net benefit (or disadvantage) gets very hard to predict.

speed of installation (should be reflected in a cheaper overall quote)

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No necessarily, because the speed is at least partly offset by the extra cost of the fittings, even if fewer are used.