I am updating a 1993 house in Texas and now have exhaust fans going through the roof, so I am also adding a fresh exterior air intake to the HVAC return plenum for make-up air. I want to open the HVAC fresh air intake damper anytime ANY of the house's exhaust fans are ON (range hood, bathroom 1 exhaust fan, bathroom 2 exhaust fan, bathroom 3 exhaust fan), but have the damper closed when the exhaust fans are ALL OFF so that the HVAC properly circulates interior air. Already the HVAC system is barely adequate to circulate interior air without connecting to the outside (HVAC company didn't calculate the system size correctly), so I'm hoping the answer isn't just "don't use a damper; rely on barometric pressure".

There are three circuits involved (range hood, bathroom 1, and bathroom 2+3). Each fan is controlled by a wall switch. How should I wire the power for the HVAC intake damper? I don't want to end up with 3x110V to the fan when all the appliances are on, nor do I want to backfeed power into other circuits.

If this were software, I'd do something equivalent to an inclusive OR: ((IF power1=110V) OR (IF power2=110V) OR (IF power3=110V) OR (IF power4=110V)) THEN (damperpower=110V).

Please don't tell me to call an electrician or HVAC company; I've already gone through FIVE licensed "master electricians" who did NOTHING to code and one was so bad he nearly burned down my house with his mistakes; the licensed HVAC company which installed the HVAC system did everything wrong; and the city inspector is worthless -- greentags everything without checking anything -- even after I point out code violations to him (I've heard he may be accepting bribes from the contractors)!! It is one of those "if you want it done right, you have to do it yourself" situations. I minored in Physics and tutored an electrician through his electronics coursework, so I know this is in the range of do-able for me.

Thanks in advance for your advice!

  • 2
    Relay logic. Replace the OR's in your software pseudo-code with relays. The wiring might be a bit of a nightmare, though.
    – SteveSh
    Sep 25, 2021 at 1:34
  • Or, maybe one of the home automation systems. You basically want to energize a circuit when one or more switches located someplace else in the house is switched. I think they should be able to do something like that.
    – SteveSh
    Sep 25, 2021 at 1:36

4 Answers 4


It sounds like you know a little about logic. The problem you posed would best be solved using a small PLC, commonly referred to as a smart relay. I have used Zelio in the past and had fantastic results.

Smart relays are very easy to setup and program. Zelio, in particular, has a visual design app to simplify programming. They come with a number of different in options for operating voltage, inputs, outputs, and some have a built in LCD display that you can customize.

As for your inputs, I recommend you use an air flow proving switch at each exhaust fan enclosure. All it is is a lightweight flag that actuates a switch when there is air movement. Kind of like a limit switch or a whisker but for air movement. Then you'd do a simple OR logic gate for all three inputs and set the output to TRUE. I believe they have 24VAC as a voltage option and dry contact outputs so you can use your furnaces control transformer to power everything, including your dampers.

The Zelio is a little pricey but they sell on eBay all day for a steal.


If the equipment that you are dealing with is marginally sized and you are worrying about exceeding the capacity of the existing system do as @Gil stated and add a "heat recovery system"to the fresh air intake to capture as much of the cooling capacity as possible. And, instead of an always open pipe you could add a barometric damper which when properly set will open only when the space goes negative, such as when an exhaust fan is switched on, (no elaborate wiring needed).


You have many possible solutions. When I moved into my current home we were having negative air pressure problems because of the fans etc. I installed a fresh air makeup vent as my solution. I live in the northern climate and it has not caused any problems in sub zero weather. A fresh air vent It is a vent open to the outside (protected so nothing unwanted can get in). The typical location is in the room with the HVAC. I actually installed mine so it was open at the base of my hot water heater. The idea is it would burn more outside air then conditioned inside air. It has been great. I used 4" PVC, the hardest part was going through the cement wall (basement). I placed the inlet under the with a 90 pointing down and used a simple shower drain screen to keep critters etc out. Nice part about this solution no electrical connections or usage.

  • How is this answer, although works for you, a solution to the OPs problem posed? She wants to have the outside air intake damper open when any one exhaust fan is running.
    – DrSparks
    Sep 25, 2021 at 20:41

You can build an OR gate by simply wiring switches (relays) in parallel: if switch 1 is closed, or switch 2 is closed, or switch N is closed.. then power flows through any one (or more) of them to the load.

What to use for the switch? Well, it depends on what you want to sense. You could sense air flow as proposed by DrSparks. An alternative is to simply sense that AC power to the fan is switched on. One way of doing that is to use a wide-voltage "relay" device like the Functional Devices RIBU1C-N4. Wire its coil side to be powered together with the fan; home-run a pair of conductors from its NO and C terminals back to the damper location. Tie all the C terminals from each of the relays together; tie all the NO terminals together too. Think of the collection as a single switch: daisy-chain wiring from the power source to the C terminals, from the NO terminals to the damper, from the damper back to the power source.

Don't forget to verify that your chosen relay, conductors, and wiring method are appropriate for the voltage and current required by your damper's motor. That RIBU1C-N4 carries ratings for a number of mains-powered loads and also mentions 10A @ 28 Vdc. It's not entirely clear whether it'll work with 24 Vac, which would be a likely voltage for HVAC equipment, but I might be tempted to try. Or choose a better relay and wire in the same way.

  • Why propose a more expensive, more convoluted solution to a problem that, thanks to PLCs, have replaced the need for clunky relay logic? More active devices (more expense, more prone to failure), more wiring, more work.
    – DrSparks
    Sep 25, 2021 at 20:38
  • @DrSparks Don't get me wrong; I agree a PLC is elegant. Depending on circumstances, a relay could be much simpler to retrofit than a sail switch in the air stream. As for convoluted - anybody can figure out what a simple relay circuit does in a few minutes; far fewer DIYs or even residential HVAC technicians can look at a PLC and do the same. Also, those Functional Devices parts are under $20; three of them are well cheaper than even a used Zelio from eBay and they can be had immediately rather than waiting for a deal to come up.
    – Greg Hill
    Sep 27, 2021 at 4:37
  • You make a good point. The reason I proposed such a solution is was apparent to me that the OP had some knowledge of programming and/or logic circuits.
    – DrSparks
    Sep 27, 2021 at 4:48
  • Also, for the record, I'm a huge fan of Functional Devices, particularly their line of current sensors. 😄
    – DrSparks
    Sep 27, 2021 at 4:50

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