On 9 Feb, 13:54, "Dave Liquorice"
wrote:
On Mon, 8 Feb 2010 04:33:36 -0800 (PST), RichardL wrote:
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.

can I just get this right?
You are suggesting that the mechanical thermostat on the rad is causing
my extractor fan to come on!!

No not the mechanical thermostatic valve (TRV) on the radiator(s) but
the room thermostat that turns the heating on/off as required by room
temperature. Not so old systems may not have a room stat just TRVs
everywhere. These systems waste energy cause all the time the heating
is programmed to be on the chances are the pump will be running and
the boiler firing (short cycling) to keep itself and bypass loop hot.

What is an arc? An arc of what?

When ever a switch opens there is a spark or arc between the
contacts. If the contacts don't snap open the arc can be sustained
through the air for several seconds until the gap gets to large. This
arc generates not only noise and light but also radio frequenies.
These can be detected by the off tune MW radio or sensitive
electronics either by through the air radiation or more likely, in
the case of the errant fan, via the wiring.

--
Cheers
Dave.- Hide quoted text -

- Show quoted text -

Wow! Nice one Dave - I love how you learn something new from the
strangest question.
Anyway - no Room Thermostat, just the boiler on a timer and TRV's in
every room. The fan comes on about 30 mins after the boiler fires up.
Do we still think it's a leak?
I've had condensation this year inside the windows every morning for
the first time ever in a winter.
regards,
R