Currently there is a ventilation switch on the wall, which has 14/2 leading to a relay which will turn on my furnace and newly installed HRV. It also turns on the bathroom fan in the nearby bathroom.

However the bathroom fan switch ONLY turns on the bathroom fan on and off. And again the furnace fan switch will turn on the 'furnace' (again via relays etc) and the bathroom fan

I will attach a photo I drew and some explanation;

Wiring Diagram

  • Power comes into bathroom junction box
  • splits via marrett to lower terminal on 2 way fan switch and to black wire on 14/3 wire to furnace fan switch
  • black 14/3 wire (energized) goes to BOTH BLACK terminals on 4 way switch
  • Red wire from 14/3 goes on upper brass terminal of 4 way switch to the load wire of the bathroom fan switch (this is key)
  • Black wire on 14/2 wire to furnace goes from lower brass terminal on 4 way switch

So the interesting thing here which I have never seen before is my 4th point above, where if you look at my diagram, the red wire on the 14/3 is connected to the load wire just above the 2 way bathroom fan switch. meaning current can go down red wire from furnace switch to the bath fan and bypass the 2 way switch. However when the 2 way switch is operated (bathroom fan turned on) and current is sent down the red wire to the 4 way, the 4 way switch ignors this and does not trigger the furnace fan.

this picture shows the bathroom fan wiring

bathroom fan wiring

these show the furnace switch, note the 14/3 from bathroom is on the left and the 14/2 to furnace is on the right..

right side of switch

left side of switch

I would like to replace this switch with a timer switch, which i purchased a three way timer switch; Lutron MA-T51MN-WH

And i would like it to behave exactly how the previous switch worked, that is the bathroom switch only controls bathroom fan but the timer switch will turn on both

I tried installing the new three way timer and in most cases behaves as a three way, where if the bathroom fan is turned on/off while the furnace fan switch is on, the red wire becomes un-energized and furnace fan turns off. As well the furnace fan switch I believe did not turn on the bathroom fan, though I suspect this is because the bathroom is not a 3 way.

I know this is a long shot and it sucks because there is no 4 way timer switches on the market. But I am hoping if I swap the bathroom fan switch out with another 3 way (or 4 way???), there is the possibility of recreating what I currently have... Basically my question is;

how can I switch my ventilation switch to a timer switch which at best could be a 3way

edit: I realize I am an idiot for assuming it was a 4 way switch. its a dual pole switch.

  • Can you describe the internal wiring arrangement of the "4-way"? Why do you call it a 4-way? Does it have a model number or other data? Commented Nov 1, 2020 at 5:49
  • @Harper-ReinstateMonica No! I just realized I was assuming 4way, because when i went to home depot, the 4 way switches look the same. But when I saw this last night; pinterest.ca/pin/539446861591594519/?autologin=true I realized it couldnt be a 4 way since it would allow the bath fan to engergize the furnace while furnace switch is off.
    – batman
    Commented Nov 1, 2020 at 16:37
  • Yup, that's what I figured: as you had drawn it, all terminals of the 4-way would be hot at all times. (note that the physical position of terminals is different on every switch, so the instagram animation has terminals in different places than yours. Screw colors are consistent, though.) Those 2-pole switches are not cheap. Commented Nov 1, 2020 at 19:04

2 Answers 2


You want a DPST timer switch (or a relay) since your existing switch is a double pole switch, not a 4-way switch

From the wiring arrangement shown, the only way things could work is if your existing fan/HRV combo switch is a double pole switch, not a 4-way switch. As a result of this, we need two switching poles at the living room location, and most timers that support that are made in a larger "time switch" form factor that's designed to be surface mounted in an unfinished space, such as a mechanical room or garage.

There are a couple options here, but you'll likely have to replace the box with a bigger box, either way

Fortunately, you aren't out of options, as there are a few wallbox timers available that do provide double-pole switching, and you can also use a relay to provide the second switching pole due to the fact neutral's available at the living room box. Most of these, though, will require replacing the existing single-gang box with a two-gang box in order to make sufficient room for the parts required.

Going old school here

The first option I'd look at if the box replacement is practical is a Marktime 72131AB timer. This is a two-gang, spring-wound, DPST timer switch with a hold function that lets you override the timer for both poles; the black wire and jumper go to L1/L2 on this switch, with the black furnace wire and the red fan wire going to T1 and T2 (or vice versa), and no need for a neutral wire.

If you want a fancier timer...

If you want a fancy electronic timer instead of an old-school wind-up timer, that's possible too, but requires a timer that uses neutral for its power return, such as a Lutron MA-T51MN, and a 120V SPST or SPDT relay such as the AP&C PAM-1, in addition to likely needing extra box space to fit it all in for practical reasons. (The extra box gang is covered by a blank faceplate.) In this case:

  • A white pigtail from the silver screw on the timer and the white wire from the relay module get nutted in with the existing bundle of white wires in the box
  • The green ground screw on the timer is pigtailed to the existing ground bundle in the box with a green or bare wire
  • The incoming black hot from the bathroom is nutted to the blue wire on the relay and a black pigtail to the black screw on the timer
  • The outgoing black switched-hot headed for the furnace is nutted to the black wire on the relay and a pigtail to the brass screw on the timer
  • The outgoing red switched-hot headed back to the bathroom gets nutted to the orange wire on the relay
  • And the yellow wire on the relay is capped off by itself

If replacing the box isn't an option

If swapping junction boxes is a non-starter, it's still possible to do this, although you have to give up the notion of a "hold" function on your timer to do so, as the Intermatic FD460MW (aka the only DPST timer I could find that fit your existing box and faceplate) is not available with that feature. If that is acceptable, though, you can wire said timer as the existing double-pole switch was wired, with the black hot wire and jumper to the LINE terminals, and the switched-hot wires (black and red) to the LOAD terminals on the timer.

  • This is awesome @ThreePhaseEel! With the second method (for fancier switch) could I place the new box in the register below the switch (ie run wires to a junction box) and contain it all there so I dont need to add a faceplate etc to the wall?
    – batman
    Commented Nov 1, 2020 at 20:54
  • As well I had been talking to a friend earlier and what he suggested is run a from the vent switch box down to the junction box in the furnace room that connects to the transformer/relay for the furnace. I cant remember specifically how the wiring worked but if functionally acts as a second pole, your method would involve less running of wires
    – batman
    Commented Nov 1, 2020 at 20:55
  • @batman -- can't put the new box in the register as that's a "no go zone" for electrical stuff Commented Nov 1, 2020 at 23:59
  • I figured as much. But I can run the wires to the furnace room and put a box and the relay there. I guess I would need 5 wires total from your description. So a 14/2 and 14/3 for simplicity?
    – batman
    Commented Nov 2, 2020 at 1:28
  • Oh and... is there any other relay you know if that’s the same. The one you mentioned I can’t find anywhere in Canada. I’m sure it’s common just not sure how to search
    – batman
    Commented Nov 2, 2020 at 1:29

The "4 way" switch you have is actually a two pole switch, not a change-over switch like the common "4 way" switch used for what you guys call "4 way" light switch circuits.

So you'll need a dual pole timer, or need to use a relay to add pole to a single pole timer.

As to whether or not the switch is really a "4 way" switch I can't say, 2 pole switch is two "2 way" switches behind a single control. The American system of counting switch "ways" is foreign to me.

  • this is awesome, yes I realize now it couldnt be a 4 way switch. I guess the only thing I am wondering if you have any advice or direction on how to add a relay to add a pole to a single pole timer switch? @jasen
    – batman
    Commented Nov 1, 2020 at 16:43
  • use the timer to operate a two-pole relay
    – Jasen
    Commented Nov 1, 2020 at 19:14

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