There is a lot of cobblers talked about the use of plastic in central heating systems - plastic has been widely used in the manufacture of vehicle radiators for the past 25 years without major problems - with water at 100c +.
O rings have been used in aero-engineering for decades - next time you take off in a plane spare a thought for the fact that your life depends on those O rings in the system holding up!
A Valliant staff engineer told me that most of the blockages he sees in heat exchangers can be linked to muck being downloaded from header tanks when the system is drained.
A mild steel container such as the average household radiator is going to corrode and produce some kind of debris, however, the process can be slowed down by using inhibitors. Copper and SS are a different matter - both are alloys, and so the quality of each will dictate how well they perform in use. Copper pipe in reasonable nick has been found in Roman ruins. Good quality SS is very resistant to corrosion, and I doubt that corrosion would ever be a problem with a SS heat exchanger unless a problem was caused by a chemical within the water - most metals don't like common-a-garden salt, including aluminium alloys, and poor quality SS can be affected by salt. However, water in a CH system is the same water that goes round and round, and so the content of such water is bound to be fairly stable. Limescale for example, requires fresh water to be constantly introduced and heated to produce a scale. Magnetite (as the name suggests) is magnetic, and so will generally stay in the radiator in which it is produced. The chemicals used to clean CH systems work because they lift magnetite into suspension, making it easier to flush out.
Whereas the problems caused by sludge and scale are unlikely to be all down to myth, the companies who make flushing and cleaning chemicals have a vested interest in exaggerating the problems. If a radiator is so full of sludge that it doesn't heat up as it should, then obviously it needs cleaning out.
For a CH system to become severely sludged, or for a heat exchanger to become blocked by scale, the process is going to require freshly oxygenated water, probably a lot more than can be passed by a plastic pipe, even if it hasn't got a barrier.
How many people on here can honestly say they always check the header tank for muck and debris before draining a system?
Much easier to say that a blocked heat exchanger can be traced to plastic pipe than admit that it's become blocked due to muck being introduced into the system from the header tank, or some other external source.
Whilst copper is usually the best choice, plastic pipe is another - much hated and defiled as it is by many people in the trade.
