Xpelair comes on when central heatinf comes on!

[RichardL]

Can anyone explain why my expelair has started coming on this year when the central heating comes on in a morning? thanks, Richard

[Colin Wilson]

Humidistat ?

[John Rumm]

Some clues might help...

Where is it? What type? How is it controlled? What is it powered from?

[Tim Lamb]

Probably the same reason my PIR controlled front door light comes on when you use the downstairs toilet........... and switch off the fluorescent light!

regards

>

[RichardL]

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=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D/ 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. R

[dennis]

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?

[Dave Liquorice]

auto on

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.

[RichardL]

Dave, can I just get this right? You are suggesting that the mechanical thermostat on the rad is causing my extractor fan to come on!! What is an arc? An arc of what? cheers for staying with me on this one. R

[JimK]

er... *electro*-mechanical presumably?

[RichardL]

The =A35 radiator thermostat? The one that stops or starts the flow on a given setting 1-5? Are we talking the same thing here or am I being a bit thick! R

[John Rumm]

I think Dave was referring to the electrical CH room stat that controls the boiler, rather than a thermostatic radiator valve. Bimetal CH stats can produce a fair bit of electrical noise when they operate. I have seen for example one that triggered an external PIR controlled lamp to come one each time it switched.

[Dave Liquorice]

warm

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.

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.

[RichardL]

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