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Bath vent CFM’s are low, looking for ideas.

jberks | Posted in General Questions on

So after cobbling together ductwork for a bath vent fan, now that it’s all in place I measures the CFM’s and I’m getting 30CFM. 

The room is 1134 cubic feet. So based on that it looks like I’m getting 0.63 air changes per hour…. Not great… Actually pretty bad, in my thoughts anyway. 

However, it is what it is, and these are the hard facts of doing renovations, you don’t get the luxury of mechanical shafts for proper duct sizing. 

So now I’m looking for ideas on how I can run the fan longer. Currently I have a leviton occupancy sensor switch wired to the fan, however the max time it runs is 30mins. I occupancy sensors for fans because nobody ever turns on bath fans and if they do, it’s not long enough. 

Does anyone know of any longer run time occupancy sensors ? Or any other thoughts to share? 



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  1. Expert Member


    Why about replacing the existing electrical box with double one so you can add a timer? The fan will run on the occupancy sensor for 30 minutes, and also whatever regular interval you want on the timer.

  2. Expert Member
    Akos | | #2

    You can try a Air King AKDT60, has a longer delay and you can use it for driving both the light and the fan.

    There are commercial motion switches with longer programmable delay but you are looking at ~$200 for the sensor and relay module, at that point might as well buy a higher capacity bath fan.

  3. Expert Member
    BILL WICHERS | | #3

    When you say you “cobbled together” ductwork, does that many lots of goofy fittings? A bunch of zigzaggy flex duct? Either of those can DRASTICALLY reduce the actual airflow a bath fan can deliver. Obstructed vents can be another double spot, as can sticky dampers.

    If you’re pretty sure the ductwork is ok, or if it’s as good as you can get it, a beefier fan may be in order. If you just want your existing fan to run longer, you might try using the occupancy sensor to trigger an industrial time delay relay (these aren’t terribly expensive, and could be hidden in an electrical box or maybe even in the vent fan’s wiring compartment). These time delay relays are often mechanical, and you can get them with different time spans (seconds, minutes, hours), and use the adjustment knob to dial in what you want. A 1 hour time delay relay, for example, could be used to keep the fan on for an adjustable period of time from a minute or two up to an hour. Grainger carries time delay relays if you want to go this route.


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