# Furnace/Bathroom Fan Interlock Help



## shinshi (Oct 7, 2009)

I would like to install a fan timer in our guest bathroom. However, due to local codes, the bathroom fan is interlocked to the furnace fan and uses a 3-way switch. As I can't get a 3-way Leviton timer switch, I am trying to figure out how I might re-wire the existing setup to have a 2-way timer switch for the bathroom fan. I have traced the wires and included a wiring diagram of the existing circuit. Does anyone have any suggestions on how to accomplish this?

I will clarify what the furnace fan interlock is and what it does. If I turn on the ventilation fan, the furnace fan AND the bathroom fan come on. This brings fresh air in as the bathroom fan vents out exhaust air. We have natural gas appliances and thus it is important not to create backdrafts which can lead to carbon monoxide gas accumulation in the home. If I turn on the bathroom fan, ONLY the bathroom fan comes on. I'd like to maintain this operation, except with a 2-way timer switch in the bathroom.


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## oilseal (Aug 24, 2009)

A timer could be placed in series with the BLACK feed wire to the BR fan


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## shinshi (Oct 7, 2009)

Correct me if I am wrong, but wouldn't a timer wired in series with the BLACK feed wire to the bathroom fan prevent the ventilation switch from working as required? I think the timer wired that way would prevent the ventilation switch from supplying power to the bathroom fan when switched on unless the timer was on too. Ultimately, I want to remove the 3-way switch completely and replace it with a timer.


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## J. V. (Jun 1, 2007)

Deleted


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## shinshi (Oct 7, 2009)

Ah yes, now I understand the confusion. I was not clear in my original post, the goal is to replace the 3 way switch with a 2 way timer. 

I can run new wires to either electrical box if necessary or even the fan motor. I was thinking I might be able to put the 3 way switch as the ventilation switch and run wires differently, but I realized I can't do that and maintain the same method of operation I have now.

If there is any question, the common terminal is on the bottom right of the 3 way switch.


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## J. V. (Jun 1, 2007)

Thats correct we will use a single pole timer in place of the 3 way switch. I will delete the above post as it will not work that way.

Remove the three way switch. Run a new cable (2 wire) from the 3 way switch box to the fan and connect only the black. Leave the existing black connected. Connect them together. 
Connect this black to the red in the switch box. Connect the power (black) to one lead of the timer and the black fan wire to the remaining timer lead.

If you have conduit you can just pull/push one extra wire from the 3 way switch box to the bath fan.
You are basically feeding the fan twice. 
You can use the same cable connector for your new cable. The one used for the fan cable.


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## shinshi (Oct 7, 2009)

I've tried to follow this logic, but I must be missing something. First, you mention running a new cable from the 3 way switch box to the fan. Which fan are you referring to, the furnace fan or bathroom fan? The bathroom fan already has one wire running from this 3 way switch box and if they are to be connected together, I can't see what that would accomplish.

The goal is to maintain the current method of operation with the bathroom switch only operating the bathroom fan and the ventilation switch operating both the bathroom fan and the furnace fan.


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## J. V. (Jun 1, 2007)

shinshi said:


> I've tried to follow this logic, but I must be missing something. First, you mention running a new cable from the 3 way switch box to the fan. Which fan are you referring to, the furnace fan or bathroom fan? The bathroom fan already has one wire running from this 3 way switch box and if they are to be connected together, I can't see what that would accomplish.
> 
> The goal is to maintain the current method of operation with the bathroom switch only operating the bathroom fan and the ventilation switch operating both the bathroom fan and the furnace fan.


 Deleted.


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## J. V. (Jun 1, 2007)

Deleted


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## J. V. (Jun 1, 2007)

This drawing will turn on both fans from either location. We need one more contact to isolate the furnace fan which we do not have. I do not have the ability to draw a complete circuit for you in Windows Paint. You can see I did not do a very good job just editing your drawing. It's possible someone else may figure it out with what you have, but I could not. If the timer is crucial, follow the drawing. If not, well thats up to you.

Note: To make this circuit work like before, you will need a relay. A relay that energizes when you energize (turn on) the timer, that will open the red wire going to the furnace fan. FYI.


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## alaeth (Mar 20, 2012)

I actually registered to respond to this thread since it's the closest match to what I'm trying to do. 

Very similar to the first post - I even suspect my wiring is identical to his...

I have a Fantech VT20m on the main floor wired via 3/14 to my master bathroom fan via a 3-way switch. Unfortunately, before I realized the implications, I replaced the switch in the bathroom with a Leviton 30 minute timer (model: LTB30-1LZ).

Despite the warnings on the Leviton manual, it seems to work okay-ish. The problem is when I use the Fantech switch to recirculate fresh air in, the Levitron switch does not turn off after the Fantech cycle (20 minutes)... it stays on with the override LED red until I press the off button on the Levitron.


This is a brand new house so I had the electrical guys come out and they had no clue how to wire it "correctly" - I'm starting to think there is no way to get the Leviton to turn off. Has anyone tried the Fantech VT20A (accessory) module in this type of situation?


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## shinshi (Oct 7, 2009)

Not sure if this will help or not, but here is a drawing of the circuit I still have that was a few steps further along than my first drawing. However, I think I had to make some modifications to the circuit when I installed it and I also downgraded the RIB01P to a RIB2401B, because I only needed 1 trigger circuit through the relay switch after all was said and done.

Now I am not an electrician and I spent many hours figuring out how to make this work. Unfortunately I can't recall exactly what I did without opening the electrical box and tracing my wires once again. Therefore, this may at least get you on the right path, but I don't think the circuit shown here works completely.

The main thing here is I had to use a relay switch (RIB2401B) to make it all work. From what I recall, it made it possible to isolate the furnace fan so that it only came on when the vent switch was activated. Now when the bathroom switch is on, only the bathroom fan is on. When the vent switch is on, both the furnace fan AND the bathroom fan are on. 










Sorry I can't help you specifically with what you are trying to install, but when I got a notification you had posted in this thread, I thought I'd at least see what I could find and remember.

Good luck!


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## curiousB (Jan 16, 2012)

shinshi said:


> .... due to local codes, the bathroom fan is interlocked to the furnace fan...
> 
> ...what the furnace fan interlock is and what it does..... the ventilation fan, the furnace fan AND the bathroom fan come on....


I am confused with terminology here. In the first quote it refers to two loads being interlocked yet later it refers to three loads being interlocked. Which is it? 

Is: 

bathroom fan = bathroom exhaust fan?
furnace fan = furnace air handler (i.e. whole home) blower?
ventilation fan= exhaust fan inside an air to air heat exchanger/ventilator?

Also is the idea that when the bathroom fan is powered on the air to air exchanger is off (to stop excessive negative pressure in the home) or do they want to be on together to maximize air exchanges (at the risk of them fighting against each other due to pressure drop).

Not clear to me what the interlock is intended to prevent.


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## shinshi (Oct 7, 2009)

Let's see if I can clarify...

The fan switch in the bathroom turns the bathroom *exhaust* fan on, nothing else.

The ventilation switch by the furnace thermostat turns on the furnace fan and the bathroom exhaust fan. 

If the furnace turns itself on to heat the home, it only turns on the furnace fan, not the bathroom exhaust fan.

What I meant in my first post when I said "ventilation fan" is "ventilation switch". When the ventilation switch is on, the furnace fan sucks in fresh air from outside and the bathroom exhaust fan blows the stale air out. Apparently this switch was in the building code when my house was built as it is original and I have seen other houses with the exact same setup in my area.


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## n0c7 (May 15, 2008)

Where did you hide the relay? Just recently ran into this situation myself.


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## shinshi (Oct 7, 2009)

The RIB01P is the relay that I used. I mounted it beside the electrical panel.


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## n0c7 (May 15, 2008)

shinshi said:


> The RIB01P is the relay that I used. I mounted it beside the electrical panel.


I'll have to look at the diagram again. I was figuring I was going to have to some how attach it to the box the 3-way switch is in. Not an easy task with a finished wall.

In the end, are you able to use any 2-way switch you wanted?


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