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I've got a contractor doing a home remodel in California. Code requires humidity sensors on bathroom exhaust fans, and in any case automatic on/off due to humidity is a feature I'd like. Considering something like the Panasonic WhisperFit FV-0511VFC1.

It also seems convenient to me to have a timer switch on the wall for manual override—set the timer, and the fan is guaranteed to run for the given time, regardless of humidity levels. Considering a switch like the Lutron Maestro MA-T51.

I'm worried that these two control schemes are incompatible: that in a normal installation, the wall switch and the built-in humidity sensor act as an "and" control rather than the "or" control I'm after.

My contractor defaults to a humidity-sensing switch, which addresses the control problem but requires a bulky sensor taking up space on the switch wall plate.

Am I understanding correctly how a wall switch interacts with fans that have built-in sensors? Is there an alternative way to wire it so that it will do what I'm after? Or perhaps I'm thinking about it the wrong way and there's a better product/arrangement I should be considering?

Appreciate any suggestions!

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You can do this with a single wallbox gizmo these days

As it turns out, the want of having humidistatic bath fan control "by default" with a timed override function is common enough that there are controls made that provide that functionality in a single unit. In particular, the Leviton DHS05 and DHD05 support this mode of operation, with a programmable ON time, while the Broan 82A/W controls support this as well but use a potentiometer preset for the ON time instead. (Other controls in this class, such as the Enerlites DWHS and the Eaton HDFS3P1, have the fan ON time factory-set.)

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  • Yes, understood. I was hesitant to go with a switch-based sensor, because they tend to be bulky and ugly. But maybe I'd be okay with that Leviton model.
    – Dan Smith
    Jan 19, 2023 at 19:35
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This fan will work just fine, with the right type of timer.

Read the manual. The fan includes the following possible controls:

  • Power switch on hot wire. You generally don't want to use that here, and it is not required, as that will totally shut off the fan so the automatic mode won't run. But you can have that if you want it. (Actually, one reasonable use case is if you want to provide a "stays off" switch if there is any issue with the fan (e.g., problem with the vent to the outside) and (as is often the case) the fan is on the same circuit as lighting so you can't just turn it off at the breaker. Of course, for any actual repairs you would still turn it off at the breaker for safety.)
  • Automatic humidity sensor. Can be set from 30% to 80% and runs for about 20 minutes after humidity gets to 5% below the setting.
  • A manual switch connected to the two red wires.

The manual switch is exactly what you need, as it lets you run the fan whenever you want due to smells or due to expectation of high humidity (e.g., start it when you start your shower to keep the humidity from building up instead of waiting for it to start automatically). However, this switch is a contact closure only and is not to be connected to a 120V power supply. As a result, you can't use an electronic or electromechanical timer. You must use either a plain switch (no "smarts") or a purely mechanical timer switch, such as this Intermatic from Home Depot:

Intermatic mechanical timer

Intermatic is a classic timer manufacturer, but in a quick search I found several other brands. They are also available with various time limits, from 15 minutes to 12 hours. You must make sure the timer is purely mechanical. If it requires neutral then it is absolutely no good. But even if it doesn't require neutral, if it has any electronics (LED lights, fancy buttons, etc.) then it most likely requires power and won't work here.

Actually, there is one other option. You could install an electronic timer that controls a relay/contactor which in turn connects to the red wires. But that seems a bit extreme for a bathroom exhaust fan.

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    Thanks for interpreting the manual for me! I had trouble making sense of it.
    – Dan Smith
    Jan 19, 2023 at 19:26
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Yes, it is possible to wire a wall switch and a built-in humidity sensor together so that they interact as an "or" control, rather than an "and" control. This is done by wiring the fan and switch in parallel, so that either one can control the fan.

The wiring diagram for this setup is fairly simple. The fan's power supply should be connected to the wall switch, and then the switch should be connected to the fan. The humidity sensor should then be connected directly to the fan, bypassing the switch. This will allow either the wall switch or the humidity sensor to control the fan, but not both at the same time.

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    Sounds like this will allow for a third-party humidity sensor, but not a fan with a built-in humidity sensor. Is that right? (Depends on the fan's internal electronics, I suppose. I'm operating under the assumption that the fan expects live power and then will switch itself on/off based on humidity levels. Maybe I've got that wrong?)
    – Dan Smith
    Jan 18, 2023 at 19:16
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    A fan with an internal sensor may also allow an external switch or timer etc.
    – Jasen
    Jan 19, 2023 at 11:44
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    @Jasen Which this one does, as I explained in my answer. The catch is "switch can't use 120V power". Jan 19, 2023 at 19:33

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