Go with PEX/PE-RT/Multilayer Pipe (plastic in other words). You have a lower thermal conductivity than copper, so you get better performance. It's typically also easier to install and you won't have potential corrosion issues. It's also cheaper.

I would agree 22mm plastic can move close to 5kW so would be ample for your 3.5kW output

@bontwoody @upnorthandpersonal

Thank you for your replies.

What matters for flow and pressure drop is internal diameter. Thus 32mm MLC (26mm internal diameter) is equivalent to 28mm copper (also 26mm internal diameter).

Ultraviolet from sunlight destroys PEX, so not recommended for outdoors use. It also has quite high thermal expansion and gets soft when hot. Thus it must be installed in conduit to be able to dilate and wiggle around freely. PEX's main advantage is being very cheap. Non O2 barrier PEX will let oxygen into your pipes, this will corrode your steel radiators and eventually destroy your installation. When using PEX for heating, always use O2 barrier PEX, which has very low but not zero oxygen permeability.

Copper and MLC have zero oxygen permeability so no worries.

Copper tube is expensive but brazed fittings are inexpensive and cause very low pressure drop, so copper is great for short but complex pipe layouts with lots of fittings, like plant room, manifolds, etc. Press fittings are quite expensive and require a specialized press tool, also very low pressure drop.

MLC has an internal aluminium layer which solves PEX's main issues. It is inexpensive, but fittings are expensive, take a lot of space and have high pressure drop due to the reduction in internal diameter.

So MLC is the opposite of copper. MLC is great for long pipe runs with few fittings (ideally just one at each end). Unlike large diameter copper bars, MLC is delivered in a roll and can be bent and curved quite easily, by hand or with a pipe bending tool for larger diameters, which helps running a single uninterrupted pipe from point A to point B with no elbows, no joints, no risk of leaks. No fittings makes it much easier to insulate, and you can even buy it pre-insulated. For this use it is ideal, costs much less than copper and requires much less work.

MLC is plastic so it shouldn't be allowed to rub on abrasive stuff like masonry. 

You will need a press tool for MLC fittings.

Freezing bursts copper. PEX just dilates (but the brass fittings will burst). MLC pipe also tends to somewhat survive freezing, but not the fittings.

If antifreeze valves are fitted on the heat pump then all exterior piping should be sloped towards the heat pump to make sure in case of emergency it can all drain via the antifreeze valves. Otherwise the antifreeze valves will save the heat pump, but water remaining in a low point in the pipe can burst it.

Posted by: @bobflux

↑

Ultraviolet from sunlight destroys PEX, so not recommended for outdoors use. It also has quite high thermal expansion and gets soft when hot. Thus it must be installed in conduit to be able to dilate and wiggle around freely. PEX's main advantage is being very cheap. Non O2 barrier PEX will let oxygen into your pipes, this will corrode your steel radiators and eventually destroy your installation. When using PEX for heating, always use O2 barrier PEX, which has very low but not zero oxygen permeability.

Copper and MLC have zero oxygen permeability so no worries.

MLC has an internal aluminium layer which solves PEX's main issues. It is inexpensive, but fittings are expensive, take a lot of space and have high pressure drop due to the reduction in internal diameter.

I didn't appreciate that there was so much to a piece of plastic pipe.  Done some additional research to find out more, concluding that:

PEX-AL-PEX

PE-RT-AL-PE-RT 

Both types should be OK as it has the Aluminium layer to avoid 02 ingress issues.  The RT variant may be slightly better for higher temperatures.

Posted by: @trebor12345

↑

I didn't appreciate that there was so much to a piece of plastic pipe.  Done some additional research to find out more, concluding that:

Had to learn the hard way 🤣 now I'm sharing

The house has underfloor heating with non barrier PEX installed by the previous owner, this ended up with all the steel radiators being corroded, rust sludge everywhere, and a perforated heat pump exchanger. Underfloor heating means lots of pipes, thus lots of O2... so I've been looking into pipes for a while now lol

MLC is great but it's a good idea to pick the fittings from the same manufacturer.

"PE-RT" means non crosslinked PE rated for "high temperature"... but the gotcha is than "non crosslinked PE" isn't so good at high temperatures to begin with, so the "RT" version is a bit of a "less worse" version. Crosslinked PE (ie, PEX) withstands high temps much better to begin with, the basic PEX-Al-PEX goes to 95°C, much higher than PE-RT! Took me a while to figure this one out........