In article ,
"IMM" writes:

BTW, is there a dc version of electric UFH, so roof PV cells can be used
directly on the elements with inverters?

You would be nuts because PV cells are so horribly inefficient.
You would do much better to take the thermal heat from a collector
and use that, with a heat pump if necessary, and a wet UFH system
(possibly the condensor of the heat pump).
I haven't done the calculations, but there may be better sources
to pump heat from that a sunlight collector, which isn't going to
be working all that well just when you need it.

While on PV cells. Alternative, with high capital cost, is a normal wet UFH
system, fed from a large thermalstore/heat bank. Use roof PV cells to heat
dc immersion elements in the water store. The thermal store will "store
energy" for use in DHW and UFH.

Again, avoid conversion of sunlight to electricity. Most of
the Sun's energy is lost at that stage. Collect and use the
heat directly.

--
Andrew Gabriel