Posted by: @robinbennett

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Thanks, we're planning on a large normal radiator. Heat Punk suggests that we need a 1800x600mm K2 radiator for a 45 degree flow temperature, but it's possible I've done it wrong! It's a 2x2.5m room with two outside solid brick walls, and I'm adding 70mm of PIR. I'm assuming that a TRV will prevent the room overheating while we insulate and upgrade the radiators in the rest of the house.

That is what I originally planned, then I fell back to 'ignore and rely on heat from adjacent room and steam when the bath is in use.@

The TRV will indeed prevent any overheating.

Posted by: @robinbennett

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I had been considering turning the old 35mm copper pipe into a heated towel rail. If I do, it sounds like I should just use an electric element and not connect it to the heating.

Not sure about this!  Personally I would either recycle the pipe or turn it into a sculpture.

@jamespa An alternative is to use a dual fuel towel rail. Regards, Toodles.

Posted by: @toodles

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@jamespa An alternative is to use a dual fuel towel rail. Regards, Toodles.

I think thats what I meant by the suggestion to fit an electric element to a towel rail?

@jamespa Sorry James, I mis-parsed your sentence there! The towel rail when heated by the heat pump will provide a low level warming / drying action to any towels but the plan we adopted is to have the electrical element switch on for a few hours in the evening but, with the valve shut to prevent the heating element having to warm circulating water throughout the system. I might add that I only shut one towel rail valve thus allowing for any expansion to take place without pressure on the rail and plumbing. Regards, Toodles.

Posted by: @robinbennett

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Could I leave some pipes in that space, ready to be used in a year or two? Or some conduit to make it easier to feed pipes through? Or will an installer be able to push pipes in from the end if I can mark the right spot? It's about a 3 meter run, parallel to the joists.

That's not a particularly long run.

There are other pipe options than copper which provide better choices for retro-fitting below floors and above ceilings.
Pushing 3m of copper pipe beneath a floor means that you must have 3m of 'space' from which to push it.

PEX/Al/PEX is more commonly used for underfloor heating (UFH),
but is lightweight and can be pushed through a void easily.

It's semi-flexible, and lengths greater than 3m are supplied as a coil.

PEX means Cross-linked polyethylene, and the Al is for Aluminium between the two PEX layers.

Plumbing and heating outlets like BES offer a range of lengths at different bores, and fittings to convert back to copper pipe or brass-threaded couplings.
32mm pipe with a 3mm wall thickness each side gives you an internal bore of 26mm.

PEX/Al/PEX can be bent by hand, and the aluminium ensures that it stays in that shape.
With ingenuity you can guide it around corners!

Remember to slide on a length of pipe insulation before you put fittings onto it.

When fixing floors and walls in place I always leave lengths of polypropylene string to act as pull-throughs for cable at a later date.

Tie each end around an old nail to prevent it accidentally being pulled into a hole...
... and label the ends with PVC-tape.

Colours help to differentiate strings emerging at the same spot.

The above set is from Amazon.
Other rivers are available.