Underfloor heating - flow indicator shows zero despite flow

If you swap the supply and return - then they shouldn't show anything, right? Measure the temperatures of both lines and see which one is warmer.

If you swap the supply and return - then they shouldn’t show anything, right? Measure the temperatures of both lines and see which one is warmer.

Correct! The mix-up of supply and return was noticed by a Stiebel technician who was here two weeks ago and has meanwhile been fixed. So that shouldn’t really be the issue, especially since the distributor upstairs shows the flow on four of them (but not on three others). In the distributor downstairs, however, none of them show the flow.

Maybe the time during which it was connected incorrectly caused it to be "stuck"... Have you ever snapped your finger against it a little? Or tried tapping only individual circles repeatedly to apply more pressure on the circle so that the float loosens?

Close all the other circles and only open the hanging ones, flow at maximum. I once had a sticking float too, and with plenty of pressure it started working again.