"RichardL" wrote in message
...


Yeah sorry. Over the shower in an en suite with 1 rad. It's an auto on
one set for humidity and works brill when shower is used. Never comes
on for anything else. Just started this winter, comes on when the
central heating has been on for half hour.

I would guess that you have a leak in the shower sealant..
water is then being evaporated by the heat from the CH and raising the
humidity enough to operate the switch.

Do you have a hygrometer somewhere that you can put in the room to see if
the humidity does go up when the heating is on?