Can you put a smart switch controller in a cavity wall space?

23 Jan 2012
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United Kingdom
Hi there.

I currently have a very basic bathroom extractor fan that is controlled with a manual switch - i.e. it was wired in independently of the light switch.

Basic as can be - pull the cord and the extractor goes on, pull again and it goes off.

The extractor is on the wall of a flat roof bathroom and the extractor ducting passes through a cavity wall (a permanent live feed runs from the ceiling rose, and down between the cavity to the extractor).

I have a Home Assistant server in my house and I was thinking about using it to control the fan automatically with a humidity sensor.

One of these would seem to do the trick:

though I wouldn't connect any manual switch.

My question is whether you think the device would be OK in the cavity wall - do you think it might get excessively hot or damp?
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One of the pictures in your link depict it in a wall. So unless it was in close proximity to hot pipes or there were damp issues with the wall then I can't see it being an issue.


Because it has exposed terminals it will need to be installed inside an additional enclosure as shown, and a visible accessory (in this case the light switch) to indicate the presence of the hidden wires.

Screw terminals must be 'accessible so that their connections can be available for inspection, testing and maintenance'. So, it can't just be hidden inside a plaster boarded wall.

If by cavity wall you mean the gap between the external house wall and an inside wall, i.e. the one that can be filled with insulation, then no it's not suitable.
As it's a flat roof I'd guess it wouldn't be earier to put it by the light however do you have any access in through the side of the roof void, say from a loft?
Thanks guys.

I think I might well, therefore, have to sink it into the wall as above (and use a blank face plate to indicate that it is there).

One other reservation, about otherwise putting it in the cavity, was that the signal would be that much weaker.

Good point about the roof - it might just be possible to remove the pull cord and push the device via the hole. Could that be a potential fire hazard and would this also be permissible within the current regs?
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quite apart from any regulations you certainly want this smart switch stuff to be accessible for inspection and maintinance. Otherwise you are setting yourself up for a world of pain when something goes wrong.
Further to the above, I have been looking for a fire and waterproof, enclosure to countersink and put the device in (in particular, I was looking for something with an IP66 rating as the bathroom can get so wet that water does sometimes run down the wall).

Everything I have found, so far, seems to look like it can only be surface mounted - any suggestions as to what might do the bill, please?

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