Everything for Electronics

Tech Forum

February 2012

Two AC Switches, One Fan

I have two different bathroom vents going into the same exhaust fan. I want to sense either or both switches to access the main relay to run the fan (kind of like an OR gate which I can make from logic gates for DC).

However, my situation calls for AC switches and fans, and I cannot think of a way without converting to DC to be able to switch the AC (long way around).

Is there a simple solution using two 110 VAC switches to turn on the exhaust fan, and then off when both switches are off?

Fairfax, VA


Of course, you can use two three-way AC switches or double pole switches. It is easily available in market. If you have the knowledge of electric powers than you can also do easily yourself, otherwise call electrician. You can get here https://aashirwadinterior.com/services/industrial-electrical-contractors-vadodara-ahmedabad/

Asit Patel
Vadodara, Gujarat

The problem with standard SPST switches connected in parallel, if either is left on, the other cannot turn off the fan. If the switches are connected in series, when one switch is left off, the other cannot turn on the fan. The solution is a three way switch arrangement.

This uses two SPDT switches instead of the SPST version. Either switch can turn on or off the exaust fan. If you are not familiar with 3-way switch wiring, go to your home repair store and get a book like "Home Wiring For Beginners".

There is one problem, if someone turns on the fan in the first room, someone in the second room can turn it off if they did not notice the fan was running.

Dennis Hewett
Frontenac, KS

The answer to question # 2124 "Two AC Switches, One Fan" by Stan Strom on page 80 of the March 2012 edition of Nuts and Volts will not meet the questioners needs.

The circuit shown in the figure is a neat implementation of an inverted eXclusive OR (XOR) circuit using switches. In this circuit the fan relay will activate (thus turning the fan ON) when BOTH switches are in the ON position or BOTH switches are in the OFF position (XOR output is high if either input is high BUT not both inputs high). With the switches in opposite positions (ON-OFF states) the relay will deactivate and turn the fan OFF. [I have had students try to use switches to simulate logic gates like this. Good job Stan!]

Looking at 120 VAC bathroom fan specs, most draw 0.8 to 1.5 amps. If you have a light fixture with two (2) 60-watt bulbs, they will draw 1 amp. The 14-gage (14 AWG) wire used in most lighting circuits will handle 15 amps at 120 VAC and most lighting circuits are on 15 amp breakers. [Be sure to check that the bathroom lighting circuit is capable of handling the extra fan load just in case the installer was cutting corners]

Most vent fans are powered from a 120 VAC switch without a relay. I would locate the light fixture junction box and simply hook one 14 AWG wire from the HOT side of the lamp (its the BLACK wire if the light was wired in standard fashion, if the fan does not run when the switch id ON use the other wire at the light fixture) in each bathroom to the hot terminal of the fan. The vent fan neutral is hooked to the lighting circuit neutral. This hookup avoids using two (2) extra switches and a relay. One of the tenets of reliability says: "The more parts in a circuit, the more probable the system will fail." A tenet of economics says: "The more parts in a circuit, the more the circuit will cost."

If you want separate switches for the fans and lights, just wire the fan switches like the light switches but run the HOT wires from the fan switches to the HOT terminal of the vent fan. Two switches in parallel turned ON at the same time will not hurt anything, they will each carry half of the fan motor's current.

The original question mentioned using logic gates. The logic circuit would be simple: Hook the HOT wires from the light switch to the OR gate inputs through a device that converts 120 VAC to logic level voltage (e.g, an opto-isolator) and hook the OR gate output to the vent fan  HOT terminal through a relay (logic voltage rated coil and 120 VAC rated Normally Open contacts). This circuit would have a reduced reliability and higher cost due to the high parts count. Engineers try to minimize parts count whenever possible.

On the mechanical side, be sure to vent the bathroom fan's exhaust to the outside to avoid mold and mildew in the attic. Also check all electrical and building codes to make sure you do not anger the "regulatory gods."

Sorry for the wordiness but engineers try to cover all of the possibilities.

Tim Brown
Honea Path, SC

All that is necessary to make two switches control a single relay for your bathroom fans are two single pole double throw switches. They're very often sold as three-way switches for controlling a lamp with two switches and are readily available at Lowes or Home Depot.

Feed the hot line (the black mains wire) to the common terminal of one of the switches; the two legs of the switches connect to one another — these are called 'traveler' lines. The relay connects between the common of the second switch and the neutral line (white mains wire).

Using the circuit below, if the relay is energized, then flipping either switch will turn it off. Then with the relay off, flipping either switch will turn it on. It is shown in the on position.

Stan Strom
via email

Maybe I'm missing your intent, but it seems you should be able to just wire two 120 VAC switches in parallel.  Either OR both switches on will turn on the fan.  Both switches off will turn off the fan.

Daryl Reece
Atlanta, GA

I can see no reason not to connect two switches to one fan, that is, in parallel, as long as you check that the same circuit provides the "hot" lead to each switch. The US standard is Up = On, so it would be relatively easy to see which switch was used to turn on the fan.

If you are rewiring anyway, I would use single pole double throw switches as they are used when controlling a light with two or more switches. Then flipping any one of the switches will change the mode off to on or on to off. This requires that some of the cables have a third conductor known as a traveler.

Andrew Eliason
Mashpee, MA

If I understand this, then all you need are 2 three-way wall switches. One at each location of the two vents and then either switch will control the fan.

B Jim Russell N4ICU
via email

A simple solution is to use a three-way switch at each of the two locations (connected with a 3 wire cable) to control the Fan. Most wall switches are “single pole, single throw”. Outwardly, three-way wall switches” look the same, however they are single pole, double throw. Analogous to your situation, a common three-way switch use is to have one at the top & one at the bottom of a stair connected to control a light on the stairway.

Three-way wall switches” are inexpensive, easily found at home centers, electrical and hardware stores. The wiring is not complicated and there are many Internet sites for details, e.g., www.electrical-online.com/electrical/diagrams/

Ray M
Surf City, NJ

We have three bathrooms and one exhaust fan which is ducted to all three rooms. Our solution was to use a double pole switch in each room (commonly available in regular toggle wall switch format) where one pole does the lights in that room, and the other pole is wired in parallel with the second pole of the other two switches, and then to the fan motor, so that if any of the three switches is turned on, the fan runs, as well as the light in only that room turns on. Be sure to take safety all precautions, and follow electrical codes, as we are dealing with 110 VAC.

John C
via email

Use two three-way AC switches. These are very commonly used for lights so two switches can control the light. It will allow switching the fan off from either location no matter which location turned it on.

The problem might be getting the wire from switch #1 to switch #2 but since they both control the same fan there should be a way to get the needed wire from one to the other.

The switch instructions that should come along with them, or a good "How To" book will show how to wire them up correctly.

Philip Karras KE3FL
via email