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Nabu the Wise 08-01-2009 04:14 AM

Motion Sensor and exhaust fan
Hello everyone, and thanks for listening to my question.

In a corner closet in my home, I had an exhaust fan installed (just a bathroom fan) connected to a three way switch, with one switch inside the closet, and the other outside. The handyman who installed it wasn't sure what sort of motion sensor should be installed (my ultimate goal) so he simply installed switches with the idea of replacing them later. The idea was to use this closet for the cat litter location for our cats (my wife loves her cats, but can't stand the cat litter smell that comes along with them).

I later installed the motion sensor inside the closet so it would detect a cat going into the closet, and kick on the fan for the duration of the timer. Overall, it worked rather well, except I didn't get a motion sensor that was intended to run anything other than lighting and I believe this caused the fan (and possibly the motion sensor) to fail after a year.

Right now, I'm merely speculating that this is what actually happened, but the motion sensor (a Heath / Zenith SL-6107) has a switch that basically bypasses the motion sensor and makes it a standard switch, switched to on, and it's still not turning on the fan. The switch specs state that it can run a 1/8 hp fan, but all of the instructions talk about lighting, so I question how much the system was actually intended to run a fan. (When the switch ran the fan, it was obvious that it wasn't running at full power. The tone of the fan changed when the motion sensor was running the fan and when the switch was set to run as a regular switch).

Since I likely need to replace the fan and switch, I'm looking for advice on what might make a good combination for this setup. I'm not really sure about the specs of the fan, but it seemed to be almost too strong. (Not that that was much of a problem, but in my mind the fan doesn't need to suck too much AC or Heat out along with the air, it just needs to create enough negative pressure that the smell goes out with the air) Also, the motion sensor worked fairly well, but I wasn't convinced that it really did that good a job running the fan, and the timer was a little short in my opinion (it ran for about 20 minutes). Ideally, I want a fan that was either variable speed, or just fairly weak, and a motion sensor that had a 45 minute timer.

Has anyone here set up something similar (or just a standard bathroom fan setup)? What parts worked well for you?

Thanks everyone,


locknut 08-01-2009 06:02 AM

Bill: If a motion sensor switch is not suited for control of a fan motor, you can isolate the switch from the motor by means of a relay. The motion sensor switch would activate the relay coil only. The relay contacts would do the work of switching over the power to the fan. It may be that the sensor device was affected by the relatively high inductance of the fan motor. A relay has a value of inductance, but much less than the motor. The relay would serve as a motor starter and substantially reduce the starting load through the sensor switch.

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