# How to wire duct booster fan to turn on when furnance fan is on



## supers05 (May 23, 2015)

medeksza said:


> I recently installed a zoning system in my house separating my first and second floor into their own zones. It did wonders in equalizing temperature differences and will hopefully save a few bucks in heating costs too. I do still have one problem room which is always coldest from the rest. It is the furthest from the furnace, a corner room, and it has only 1 register between 2 windows instead of a register under both.
> 
> I am trying to install an inline booster fan on the duct for this room. The fan I am using is a 120VAC fan. I need it to only come on when the furnace fan is on and when zone 2 is on. So I'm trying to wire in 2 relays that take 24 VAC input coils and are able to complete 120 VAC circuits. I wired the 120 VAC parts in series so that it requires both fan+zone to be on before it provides power. When the zone is on, the zone controller sends a 24VAC signal to one of the relays just fine and the relay switches on. However, the other relay I am trying to tie to the furnace fan will not switch on. I used a volt meter and confirm I see 24 volts when the fan should be on, but the relay just won't come on. It makes a faint noise like something is happening, but it never actually switches on. To rule out a bad relay, I tried to swap the relay with the one from the zone and both work fine on the zone circuit, but neither work on the fan circuit. I also tried a different brand relay with similar results.
> 
> I wired the relay in on the (G) green fan wire coming from the furnace before it goes through the zone controller. The zone controller completes the circuit between G and R whenever it wants the furnace fan on and it works just fine, I confirmed 24 V appears when the fan is on. What am I doing wrong? Any other way to accomplish what I want to do?


Firstly, for heating using G probably won't work so well. (G is only used for AC and Heat Pumps) There's a fan output on most newer boards that deals with that part of the logic for you. It'll be 120v however. Model number of the furnace would be helpful. 

Secondly, I don't quite follow your description of your wiring. It sounds like you have the relay coil in series with another load. You also haven't mentioned which zoning board that you have. 

Cheers!


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## user_12345a (Nov 23, 2014)

if you don't have an electronic air cleaner, you can connect a relay with a 120v coil to the electronic air cleaner and use it to switch the circuit with the booster fan on and off. 

do not use the eac terminals directly for power since it's only designed for small loads. 

it will come on with either zone calling though.

you also have to make sure any wiring is code compliant.


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## supers05 (May 23, 2015)

I'm still wondering why not just bring on the duct fan with the zone call? Does the zone not have voting rights? (ie. it turns on the unit on a call by itself?) In would understand if it was a slave only zone. 

Anyways, your relay coil for the air handler would be wired from g then to common. 

Cheers!


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## beenthere (Oct 11, 2008)

Get a 120 coil relay, and have it energized off the EAC terminal on the board. 

I agree with Super. No real need to have a second relay controlled by the furnace. 90 to 120 seconds of room temp air from the other rooms should not cause an objectionable cold air feeling in the room in the winter.


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## medeksza (Dec 30, 2016)

For the zone controller I bought and installed an Aprilaire 6203 unit. I have 2 active zones and 2 thermostats connected to the zone controller. The zone controller opens and closes electric dampers on the ducts depending on which zone is calling. It makes calls to the furnace, ac, and fan on behalf of the zones. It is working perfectly in this regard.

When the system is resting, all the dampers in all zones are set to "open". So I can't have the booster fan turn on based on the damper being open alone. The booster fan should only be on if the zone 2 damper is open AND the fan is actually blowing. Here is a picture of the controller:
See attachment: [zone1.jpg]​If I attach a relay to zone 2 it works just fine. The relay detects a 24VAC voltage and is able to open/close dampers and would be able to turn on a booster fan.

On the section of the controller called HVAC I confirmed the zone controller completes a circuit between G and RC whenever it is calling for heat, AC, or fan only. So it seems it would be perfect to send a signal to my booster fan to turn on too (if zone 2 is open as well). RC and RH are connected together since the furnace controls the AC. I used a volt meter on the green G wire and confirmed I see 24 VAC only when the fan is running. However, if I connect a relay on the G wire, it doesn't work and I don't understand why.

I haven't meddled with the board on the furnace itself much yet, but here is what it looks like:
See attachment: [board.jpg]​I do see an "EAC" as mentioned above and see nothing is connected there. What is the purpose of this and how could it help me? Here is also the bundle of wires going to the fan itself. Thank you for any help you guys can provide:
See attachment: [fan.jpg]​


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## medeksza (Dec 30, 2016)

Here are higher res versions of the images above since the forum shrunk them a bit:
http://499e34e29654c5084b21-928c38c0f14efc0162712244c6d7927d.r53.cf2.rackcdn.com/inside.jpg
http://499e34e29654c5084b21-928c38c0f14efc0162712244c6d7927d.r53.cf2.rackcdn.com/zone1.jpg
http://499e34e29654c5084b21-928c38c0f14efc0162712244c6d7927d.r53.cf2.rackcdn.com/fan.jpg


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## supers05 (May 23, 2015)

Use the g and c of the "zone 2 thermostat" connection to connect the coil of the relay. You will only need one relay. 

Cheers!


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