On Sat, 6 Feb 2010 13:15:43 -0000, dennis@home wrote:

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.

Except that as the air warms up the relative humidity drops as warm
air can "hold" more moisture than cold...

I'd be more tempted to think that the switching circuit of the fan is
detecting a thermostat opening in the CH system some where. As
mechanical thermostats age they can become sluggish in opening and
generate and arc for several seconds. Tune a MW radio between
stations and listen for loud pops, splats or long buzzes and see if
one of those coincides with the fan turning on, then try and work out
what is switching.

--
Cheers
Dave.