We have a "cupboard under the stairs" which was the original entrance into a cellar which has since been filled in. It's partially below ground level and is damp. We're going through a series of mitigations, one of them being fitting an extractor fan to induce some ventilation down there. With a bit of faffing I should just be able to get the ducting routed to an outside wall.

I'm looking for one that I can leave on 24/7 but will only kick in when humidity reaches a given value. Most I've seen still work on some sort of timer, which I don't want. Can anyone suggest a suitable fan to use?

Use a cheap humidity sensor to operate the fan .?? Something like this ..??

 

XH-W3005 Humidity Sensor 12V/24V/220V Digital Hygrometer Switch Controller 0~99% RH(12V) https://amzn.eu/d/2NAtzzq

Or a dMEV fan, designed to run 24/7 at very low watt draw, so running costs should be low.

 

Shop around and they can be picked up cheaply enough, greenwood, vent axia etc.