# New AC with old style furnace



## JRaef (Mar 23, 2009)

ChadThomson said:


> Hi there I'm trying to wire the controls for old furnace with a new air condtioning unit. The furnace looks like it has at fan control centre so I hooked it up how I would do any ac install and now whenever the fan fires up the heat does. So when the ac turns on and in turn the fan comes on so does the heat. I assume I need to issolate this with a relay somehow?? I have included a wiring schematic of the furnace. It was built in 1982.
> https://www.dropbox.com/s/8xptfhj4g7wad40/photo.JPG


So where did you already make your connection?


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## ChadThomson (Jun 10, 2011)

My made my connection up to the thermostat to the furnace from g and y and then out to the condenser from y and c off the furnace.


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## bkmichael65 (Mar 25, 2013)

Y goes to the condenser contactor, g to fan, w to heat relay and then the heat relay will energize the fan.


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## ChadThomson (Jun 10, 2011)

So I do need to add a relay? How would the fan come on for when it's calling for cooling?


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## bkmichael65 (Mar 25, 2013)

The thermostat will energize the g(fan connection) in cooling. In heating the w will energize the heat relay and you'll have a wire coming off that to energize the low speed setting of the fan


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## bkmichael65 (Mar 25, 2013)

After thinking about it, all you need to do is take the y off the furnace and connect the w there. I'm used to heat pumps where you have to do it a little different


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## xlink (Mar 12, 2012)

That's a problem when two controls energize G to turn the fan on. When the stat fan switch is in the auto position, Y and G are internally connected, so energizing G energizes Y. You need a relay.


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## ChadThomson (Jun 10, 2011)

Is there a wiring diagram out there that I can use?


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## backstay (Feb 3, 2011)




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## ChadThomson (Jun 10, 2011)

Those are just for your standard hookups and this furnace doesn't have those terminals. Just wondering about the relay. Also judging by the schematic it only has a single speed blower in it. Will this be good enough for ac?


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## backstay (Feb 3, 2011)

ChadThomson said:


> Those are just for your standard hookups and this furnace doesn't have those terminals. Just wondering about the relay. Also judging by the schematic it only has a single speed blower in it. Will this be good enough for ac?


AC requires full speed fan. Heat mostly doesn't run wide open.


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## xlink (Mar 12, 2012)

ChadThomson said:


> Those are just for your standard hookups and this furnace doesn't have those terminals. Just wondering about the relay. Also judging by the schematic it only has a single speed blower in it. Will this be good enough for ac?


Usually it will be okay. Sometimes we would change the motor to .5 hp and increase the fan speed. That was usually decided by the refrigeration company. When competition got tight and plumbers started selling air conditioners with pre-charged lines, they quit changing the motors.

We used to (late 70s) replace the transformer with a combination relay/transformer, but that was to get the 40va transformer. Now, just add the relay. You need something with a motor rating. Use the contacts to switch the line voltage to the motor in parallel with the existing contacts (1 and 3 in your drawing I think).


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## xlink (Mar 12, 2012)

xlink said:


> Usually it will be okay. Sometimes we would change the motor to .5 hp and increase the fan speed. That was usually decided by the refrigeration company. When competition got tight and plumbers started selling air conditioners with pre-charged lines, they quit changing the motors.
> 
> We used to (late 70s) replace the transformer with a combination relay/transformer, but that was to get the 40va transformer. Now, just add the relay. You need something with a motor rating. Use the contacts to switch the line voltage to the motor in parallel with the existing contacts (1 and 3 in your drawing I think).


Maybe that should be line to 3. Test your hight limit after to make sure it isn't bypassed.


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## ChadThomson (Jun 10, 2011)

Ok so I could use a 24v coil with 120v contacts and bring Y & C to the coil and bring the hot to the common and N.O. to 3? Am I totally off on this? Anytime the fan would run, being it on auto ac or on (stat) the igniter would light and the heat would come on. Wouldn't I have to isolate the air proving switch so the igniter doesn't light?


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## wendon (Sep 27, 2010)

Not sure what you're saying. What terminals do you have on the furnace?
G, W, Y, R etc.

Why did you connect G and Y?


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## ChadThomson (Jun 10, 2011)

I didn't connect y to g. There is a fan control center inside the furnace with R,W and G. Y goes directly from the stat to the condensing unit outside.


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## Stuff (Oct 14, 2012)

Trying to retrofit a furnace that was never designed for A/C. The G on that furnace is not the same G as something made today. Over 30 years old - time for a new furnace.

If you really want to try this, you need to understand how the furnace works. The main blower in old furnaces is not controlled by the stat but by a temperature switch. So a relay would need to provide power directly to the blower. G from the stat and C from the controller board would feed the 24v coil. Like was mentioned the xfrmr might need to change to 40va to drive the contactor in the condenser.


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## farlsincharge (Dec 31, 2010)

If the heat is firing when you are calling for fan, and everything else you have said is true (It sounds like you wired it right) I would say your fan limit switch is stuck shut.

What us a turbo blower? Inducer motor?


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## ChadThomson (Jun 10, 2011)

So the blower motor would be the main blower and would come on and off with the fan/limit switch and what about the turbo fan does this come on with the G coil only? Is the main blower only used for heat? Problem is its for my friends parents and I told them they should have been upgrading the furnace first. The G would come directly from the stat and to the relay coil and not at all to the furnace? I would be just bypassing the fan centre then. I would want to be powering up the turbo fan correct?


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## xlink (Mar 12, 2012)

I thought the right hand blower was labelled burner. Now I think it blower. I'm guessing the turbo motor is the combustion motor. 

Which is which? Maybe you don't have a combustion air motor, but having the air proving switch suggests that you do.

You're right about the G from the stat only going to the relay.


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## farlsincharge (Dec 31, 2010)

ChadThomson said:


> So the blower motor would be the main blower and would come on and off with the fan/limit switch and what about the turbo fan does this come on with the G coil only? Is the main blower only used for heat? Problem is its for my friends parents and I told them they should have been upgrading the furnace first. The G would come directly from the stat and to the relay coil and not at all to the furnace? I would be just bypassing the fan centre then. I would want to be powering up the turbo fan correct?


Are there two fans in the furnace?

I have never seen a factory installed fan centre before.

So contacts 1 and 3 and 2 and 6 are operated by the coil that G energizes?

I still think turbo blower is an inducer motor. Does it vent into the chimney or through pvc or abs pipe?


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## farlsincharge (Dec 31, 2010)

The more I think about it the more it makes sense. That fan centre is not for future a/c, it is a way to run a mid efficient furnace without printed circuit boards. I have never seen such a beast.

You need to disregard it and its G terminal altogether and install another fan relay.


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## bkmichael65 (Mar 25, 2013)

If you have an air proving switch, you'll have an inducer motor


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## ChadThomson (Jun 10, 2011)

Yes I agree with you I have never seen one either. It is vented through the chimney I believe. 

So to summarize: 

1. Install relay ( 24v coil with 120v contacts with motor ratings)
2. Hookup G from thermostat to coil and Common from furnace transformer
3. Common on contact side will be the line and N.O. will go to the blower fan (not the turbo fan??)
4. Can I bypass the Fan/limit switch in this case, I would have to right as the switch is only activated by heat?

So the stat calls for cool which also energizes the G terminal on the stat which energizes the new relay coil and turns the fan on WITHOUT energizing W?


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## xlink (Mar 12, 2012)

If the turbo motor is the motor that blows air into the combustion chamber, then the motor you want to start is the blower motor. That's red and yellow on the fan switch. I think you'll find the black and red on the fan and limit switch are jumpered. Leave the jumper in.


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## farlsincharge (Dec 31, 2010)

ChadThomson said:


> Yes I agree with you I have never seen one either. It is vented through the chimney I believe.
> 
> So to summarize:
> 
> ...


Correct, so in effect you are paralleling yellow and red on the fan limit switch with your NO contact.


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## wendon (Sep 27, 2010)

ChadThomson said:


> Yes I agree with you I have never seen one either. It is vented through the chimney I believe.
> 
> So to summarize:
> 
> ...


Something's not making sense. Are you saying if you jump R and G on the furnace, the heat kicks in? If you install a relay for the blower fan, how is the blower fan going to start when it gets a call for heat? You might have to use a SPDT relay so the fan can be powered by the furnace's fan switch when the relay is de-engergized. Hook 120 volt to the NO contact, Hook the fan lead to the Common terminal, and hook the original wire from the fan/limit switch to the NC terminal. If it was me, I'd power that relay off the Y terminal together with your condensing unit contactor.


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## mgraw (Jan 14, 2011)

wendon said:


> Something's not making sense. Are you saying if you jump R and G on the furnace, the heat kicks in? If you install a relay for the blower fan, how is the blower fan going to start when it gets a call for heat? You might have to use a SPDT relay so the fan can be powered by the furnace's fan switch when the relay is de-engergized. Hook 120 volt to the NO contact, Hook the fan lead to the Common terminal, and hook the original wire from the fan/limit switch to the NC terminal. If it was me, I'd power that relay off the Y terminal together with your condensing unit contactor.


 Exactly what I would and have done. Leave the system where the heat cycle will run as it is. Cut the switched leg of the motor. Put the motor side of the switched leg on common. Put the relay side of the switched leg on the contact that is closed when the relay coil is de-energized. Run 120v to the open contact when the coil is de-energized. Power the coil from Y.


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## xlink (Mar 12, 2012)

ChadThomson said:


> Yes I agree with you I have never seen one either. It is vented through the chimney I believe.
> 
> So to summarize:
> 
> ...


I think you have it. I believe the turbo fan injects injects combustion air. You aren't bypassing the fan switch, you are paralleling your contacts with it. Both, the fan switch and your relay must turn the blower on.


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## wendon (Sep 27, 2010)

xlink said:


> I think you have it. I believe the turbo fan injects injects combustion air. You aren't bypassing the fan switch, you are paralleling your contacts with it. Both, the fan switch and your relay must turn the blower on.


Canadians must have their own terminology! First time I've ever heard a draft inducer blower called a "turbo fan"!:laughing:


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## ChadThomson (Jun 10, 2011)

wendon said:


> Something's not making sense. Are you saying if you jump R and G on the furnace, the heat kicks in? If you install a relay for the blower fan, how is the blower fan going to start when it gets a call for heat? You might have to use a SPDT relay so the fan can be powered by the furnace's fan switch when the relay is de-engergized. Hook 120 volt to the NO contact, Hook the fan lead to the Common terminal, and hook the original wire from the fan/limit switch to the NC terminal. If it was me, I'd power that relay off the Y terminal together with your condensing unit contactor.


If you parallel the switch leg from the fan switch with the N.O. of the contacts It would turn on exactly the way it was originally with heat. Xlink I think you have it.


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## ChadThomson (Jun 10, 2011)

Hey guys just an update. I installed the relay as mentioned and worked like a charm. This is the greatest forum around so glad I found it. Thanks everyone!


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