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My design includes 2 switches to control one fan, but it is not strictly a 3-way switch. I want the fan to be on if either switch is on and off if both switches are off.

This is an attic mounted fan, and I plan to split the input side to pull air from one of two rooms. Room 1 = shower with automatic humidity switch. Room 2 = toilet with timer switch. I also want these switches to control motorized dampers to close when the room switch is off.

Is there a device like a relay that can combine the two sources and power the fan?

EDIT: I don't think I worded the part about the dampers clearly. I only want a damper to be open if the switch for that room is on. In other words, I would expect that generally only one damper would be open or they would both be closed. If the switches were just wired in parallel I think both dampers would operate in unison.

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  • This is a job for double pole switches. One pole for the damper and one for the fan. Commented Sep 8, 2020 at 21:56

3 Answers 3

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This is the fundamental circuit you need. Use two double-pole switches. One pole of each switch controls one damper, and the other poles are connected to the fan in parallel.

topological circuit diagram

If you need to control the fan and dampers with timers, humidity sensors, occupancy detectors, etc., which rarely come in double-pole versions, you will probably have to use relays in addition to the switches. The relays will, in turn, be activated by your single-pole detectors.

topo diagram with relays per ThreePhaseEel suggestion

The physical wiring will depend upon the locations of your switches and loads, how you choose to run your cables, and where power enters the circuit.

Here is an example with power entering at one switch, the fan cable branching from the other switch, and no cables between the loads.

phys diagram with relays per ThreePhaseEel suggestion

I have omitted the fault ground wires from both diagrams. They clutter the picture and make it harder to understand. Just connect all fault grounds together wherever you see them.

Many many other physical layouts are possible. I think any layout that implements the fundamental circuit and does not have any cable loops will be safe. Once you decide where your cable runs must be then you can ask for a specific diagram.

One other possibility is to use the dampers as relays. You will need a style of damper that incorporates a switch to indicate when it is open. If you can find (or safely make) such a thing, you can wire those switches in parallel to control the fan.

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  • Thank you for the detailed response. The problem I ran into going down that path is that I can't find double-pole humidity sensors and timers.
    – roy
    Commented Sep 9, 2020 at 14:48
  • You'll have to use relays, then. I've added a relay solution to my answer. Commented Sep 9, 2020 at 20:57
  • BTW: you can run the relay coil off the same load connection as the corresponding damper; this lets you use single-pole relays instead of double-pole (I'd use AP&C PAM-1's since they can be tucked into the back of the switch boxes) Commented Sep 10, 2020 at 0:21
  • <forehead slap> @ThreePhaseEel is correct. - I have modified my answer. Commented Sep 10, 2020 at 1:59
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    Thank you @FreeMan. I've changed the colors. Commented Sep 10, 2020 at 11:47
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A relay is not needed in your application. You need to connect the two switches in parallel, i.e., wire them to each other, including grounds.

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  • I thought about that, but I couldn't see a way for the switches to also independently control the motorized dampers. Is there a way to handle that?
    – roy
    Commented Sep 8, 2020 at 17:55
  • Just use a 120v (or whatever voltage your fan runs at) damper that has a spring-return to close when power is turned off. Commented Sep 8, 2020 at 18:00
  • This answer calls for more detail. I'm experienced in wiring and can't visualize it.
    – isherwood
    Commented Sep 9, 2020 at 21:12
  • @PhilippNagel - I think with the switches in parallel, the dampers will both be energized when either switch is closed; even with a spring return to close them when de-energized, they'll both be open when either switch is closed. Commented Sep 10, 2020 at 5:40
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What you want can be accomplished with DPST switches.

Pole one of each switch will be connected in parallel to the fan. With the switches in parallel the load will be on if either one or the other or both are on.

Pole two of each switch will simply be connected to the damper for that room. This way each switch controls the damper for that room.

The common types of regular household switches available at home centers and hardware stores - plain switches, 3-ways, and 4-ways - won't do what you want here. But, you can buy double pole switches that fit regular switch boxes and covers, such as the Hubbell HBL1222I.

The only caveat I can think of is, NEC article 210.7(B) requires that the dampers and fans either be on the same circuit, or two adjacent breakers with a handle tie.

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