How each achieves its temperature performance, and therefore whether it can be fused.

PEX is crosslinked: a chemical or radiation process bonds the polymer chains permanently into a network. That gives excellent temperature and creep resistance, but it converts the material from a thermoplastic into something that cannot be melted and reformed. PEX is therefore joined with mechanical fittings - press, compression, push-fit or crimp.

PE-RT reaches comparable temperature performance through its molecular architecture rather than crosslinking. It remains a thermoplastic, so it can be butt fused, socket fused and electrofused like any other polyethylene.

That is the decision in practice. If fused joints matter - because the pipe is buried, embedded in a screed, or part of a system already using PE fusion - PE-RT has a real advantage. If the installation is conventional plumbing with mechanical fittings throughout, either material works and the choice comes down to cost and availability.

Sometimes, but it must be confirmed against the manufacturers' guidance rather than assumed.

Both are polyethylene and both are thermoplastic, so fusion is physically possible, and PE-RT to PE100 joints are made in practice - district heating and industrial systems frequently need the transition. However the two materials have different melt flow characteristics, and a joint between them takes fusion parameters that suit both, which is not simply the parameters for either one alone.

Manufacturers publish guidance on which of their grades may be fused together and with what settings. Some combinations are approved; others are not.

Where fusion between the two is not approved, or where the SDRs differ, use an electrofusion coupler rated for the combination or a mechanical transition fitting. Do not resolve the question by assumption on site - the joint will look identical either way.

Considerably more than standard PE at temperature, but always as a combination of the two rather than as separate figures.

Plastic pipe ratings for hot service are expressed as classes that define a service profile - a continuous operating temperature, periods at a higher temperature, and a malfunction temperature, all combined with a design pressure over a stated design life. A single headline temperature figure means very little on its own.

PE-RT is produced in grades, with the later types offering higher performance for a given wall thickness. The application class then determines which grade and which SDR are required.

Select by identifying the application class for the system - underfloor heating, hot and cold plumbing and district heating each have their own - and then reading the required grade and SDR from the manufacturer's table. Designing to a working temperature alone is the common error and it under-specifies the pipe.

Whenever the circuit contains ferrous components and the water is recirculated - which covers most heating systems.

Polyethylene is slightly permeable to oxygen. In a sealed heating circuit, oxygen diffusing through the pipe wall is continuously replenished, and it corrodes steel and cast iron components - boiler heat exchangers, steel panel radiators, pump bodies and manifolds. The result is sludge, blockage and premature failure of the plant rather than of the pipe.

An oxygen barrier layer, usually EVOH, is co-extruded into the pipe wall to stop that diffusion. Barrier pipe is standard for underfloor heating and sealed heating circuits.

It is not needed where the water is not recirculated, such as a potable hot and cold supply, or where the entire circuit is non-ferrous. Where there is any doubt, specify barrier pipe - the cost difference is small and the failure it prevents is expensive and slow to diagnose.

By fusion where fusion is wanted, and by the same mechanical fittings used for PEX where it is not - which is one of its attractions.

Socket fusion and electrofusion are common on smaller diameters, butt fusion on larger pipe and district heating mains. Because these produce a joint as strong as the pipe with nothing mechanical in it, they are the right answer for anything buried, embedded in screed, or otherwise inaccessible.

PE-RT also accepts press, compression and push-fit fittings, which suits visible plumbing, manifold connections and anywhere the joint may need to be broken.

The usual approach on underfloor heating is a continuous coil with no joints at all within the screed and mechanical connections at the manifold - the best of both. Where a joint in the screed is unavoidable, fuse it, and record its position on the as-built drawing.