OK - one adittional thing... I just found the installation manual on their
website and it does mention that the heating pipe must be run in a
double-spiral config, so a flow and return pipe are alway adjacent.

Ah. That makes more sense. It controls the power by varying flow speed. As
this will produce a large temperature difference, you need to have the hot
entrance pipes next to the cold exit pipe to ensure average heat transfer
over the whole area.

However, the system would lead to localised hot spots, unless the floor is
sufficiently conductive to ensure that the hot and cold parts produce an
average.

At one end of the spiral, you get a 70C and a 20C pipe. In the middle of the
room, you both pipes finally meet at around 45C. The approach gives an
average 45C anywhere in the room, with some variation, however, depending on
the exact flow rate chosen.

I'd probably still want a proper TMV and pump, though, even for a solid
floor. I like the idea of a simple thermostat setting to set the floor
temperature.

Christian.