# Second thermostat running fan issue



## beanbreeder (Oct 4, 2014)

Ok, I've searched all over the net and can't find the same problem with a resolution so here it goes.

I have an existing LP furnace. Recently installed an Outdoor wood boiler.  The wood boiler pumps water through a heat exchanger in my existing duct work. Everything is all good to go except now I'm trying to hook up a second thermostat to run the existing furnace fan.  I am doing that as it will blow across my new heat exchanger to heat the house and not fire my LP furnace.  I plan to keep the LP as only a backup, hence the two thermostats. 

So, doing research on the net told me to hook up the R and G wires from my existing thermostat to my new one. I did that and then tried to start the fan from the new thermostat.  The fan has not come on yet. I can turn it on from the old one, but neither the fan or auto setting with the heat cranked will kick on the fan. I am at a loss here.  Any thoughts/solutions?

Thanks


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## alaskawild (Oct 4, 2014)

beanbreeder said:


> Ok, I've searched all over the net and can't find the same problem with a resolution so here it goes.
> 
> I have an existing LP furnace. Recently installed an Outdoor wood boiler.  The wood boiler pumps water through a heat exchanger in my existing duct work. Everything is all good to go except now I'm trying to hook up a second thermostat to run the existing furnace fan.  I am doing that as it will blow across my new heat exchanger to heat the house and not fire my LP furnace.  I plan to keep the LP as only a backup, hence the two thermostats.
> 
> ...


 Can you give me the model of your LP furnance?


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## beanbreeder (Oct 4, 2014)

alaskawild said:


> Can you give me the model of your LP furnance?


It is a York Diamond 90.


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## davem3261 (Oct 4, 2014)

What terminals did you hook it up to on the new thermostat? You need to use R (Rh if it also does cooling) and W.   You can check to see if you have the furnace hooked up correctly by removing the red and green wires at the thermostat and touching them together.


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## beanbreeder (Oct 4, 2014)

davem3261 said:


> What terminals did you hook it up to on the new thermostat? You need to use R (Rh if it also does cooling) and W.   You can check to see if you have the furnace hooked up correctly by removing the red and green wires at the thermostat and touching them together.


I hooked the G from the old one to G on the new and RH on the old to R on the new. My new one doesn't have an RH. It will only be for heat though. Will continue to use the old for cooling. Should I put the G into the W on the new one then? Thanks for the help.


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## davem3261 (Oct 4, 2014)

You need to use the w terminal on the new tstat.  W does heating, G does the fan.   When you touch the red and green wires on your new tstat together did the furnace fan come on?


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## beanbreeder (Oct 4, 2014)

davem3261 said:


> You need to use the w terminal on the new tstat.  W does heating, G does the fan.   When you touch the red and green wires on your new tstat together did the furnace fan come on?


That is a eureka moment, makes total sense.  Thanks.  

So, yes when I touch the two wires, the fan comes on. I wired to the W and now works like I thought it should.....other than the fan on the AC comes on as well.  The first thermostat is set on heat, so not sure why the AC fan would come on.


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## davem3261 (Oct 4, 2014)

The ac fan is was you are trying to control with your second thermostat isn't it?


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## beanbreeder (Oct 4, 2014)

davem3261 said:


> The ac fan is was you are trying to control with your second thermostat isn't it?


No, I'm trying to control the furnace fan only so it blows air across the exchanger from my wood boiler. However, both fans come on now, and I'm not sure how to not get the AC fan to come on.


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## davem3261 (Oct 4, 2014)

I'm having a hard time imagining what your heating cooling system looks like.   Do you have a hot air furnace with an A
 coil in it?


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## beanbreeder (Oct 4, 2014)

davem3261 said:


> I'm having a hard time imagining what your heating cooling system looks like.   Do you have a hot air furnace with an A
> coil in it?


Yes, I have an a coil in the plenum above the furnace. Then below that coil and above the furnace is a flat coil.  The flat coil is circulating water from the outdoor boiler. The fan in the furnace should kick on to blow air across it to get heat. That fan does now kick on, but the AC unit fan that is outside also kicks on.....which is where I'm now confused.


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## davem3261 (Oct 4, 2014)

I gotcha.   Your turning on the condenser outside.   Somehow your sending 24v on the Y which is cooling.  How exactly did you install the 2nd thermostat?   Did you put it right next to your original thermostat and run a wire for one to the other?


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## beanbreeder (Oct 4, 2014)

I put the second one right above the first and yes, just ran two small pieces of thermostat wire to it from the first.  

Not sure how sending juice to the Y. There is a jumper wire between RC and RH on first thermostat, but only the blue wire to the Y terminal. Both the condenser and the furnace fan have never come on together before.


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## bwise.157 (Oct 4, 2014)

beanbreeder said:


> I put the second one right above the first and yes, just ran two small pieces of thermostat wire to it from the first.
> 
> Not sure how sending juice to the Y. There is a jumper wire between RC and RH on first thermostat, but only the blue wire to the Y terminal. Both the condenser and the furnace fan have never come on together before.


Try disconnecting the Y terminal, and kill the breaker for your A/C.  That should prevent the voltage from getting to the A/C unit.  Short term fix until you can figure it out.  I did something similar last winter and it worked.


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## beanbreeder (Oct 4, 2014)

bwise.157 said:


> Try disconnecting the Y terminal, and kill the breaker for your A/C.  That should prevent the voltage from getting to the A/C unit.  Short term fix until you can figure it out.  I did something similar last winter and it worked.


Thanks, that's a good option and may be my solution until someone more experienced can take a look at it.


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## STIHLY DAN (Oct 4, 2014)

You are back feeding Y when you energize G. How is G tied in on both t-stats?


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## STIHLY DAN (Oct 4, 2014)

Throwing the disco on the condenser will make it not run, but your contactor will stay energized and likely burn out.


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## beanbreeder (Oct 4, 2014)

There is one wire coming from the furnace (think it's light brown, but am color blind  ). I have then added a wire to run up to the second thermostat which I have connected to W. 

I have now thrown the breaker and disconnected the Y on the main thermostat. After only throwing the breaker there was still a buzzing outside, assuming the contactor.  After disconnecting the Y though, it is now silent outside.


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## STIHLY DAN (Oct 5, 2014)

Good, that means you where back feeding Y on the T-stat. If you want that to end you need to add A isolation relay at the furnace. Maybe a second relay to the low speed fan. What you are doing now is powering hi speed fan for heat, You want low speed which comes on when your gas furnace turns on.


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## davem3261 (Oct 5, 2014)

Turn the power back on to the condensor.  Turn your original thermostat fan to the on position.  If just the fan comes on the issue is with the new thermostat.   Double check you are on the correct terminals.   If the ac came on again then it was never hooked up correctly from the get go.   Note what color wires are on each terminal at the thermostat.   Now go to the furnace and find the terminal strip where the low voltage goes in.  Usually have to take out a couple screws to get to it.   In there you should have an 18-2 that goes to the Condenser as well.   Check to see the wiring matches the tsats.   You can always take a pic and post it if you can't figure it out.


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## Newvigasowner (Oct 10, 2014)

I'm gonna follow this because I'm about to do the same thing shortly. Quick question though aren't u gonna want a circulation pump to start at the same time as the furnace fan once thermostat calls for heat?


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