# Help!! adding 2nd thermostat to control furnace fan



## rjbeery (Jan 5, 2009)

Hi guys,
I needs some detailed advice on how to hook up a 2nd thermostat to turn only the fan on on my furnace.

I currently have a propane forced air system, I have added a coal boiler, with a heat exchanger in the duct work. My pump will run continuously to circulate the water. I simply want to use my fan on my propane furnace to take the heat off of the heat exchanger.

I am not sure, but I think I can add a 2nd thermostat to simply turn the fan on and off. 

I will use my original thermostat and furnace as a backup and just set the temp lower in case I need the propane furnace to kick on. 

I have heard that I can possibly create a feedbacks that cause the A/C unit to run when you I dont want it, thus the need for the isolation relays

Please advise on how to hook up a relay if necessary and what wires I need to hook up to get this job complete. Thanks Bob ([email protected])


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## Labman (Jan 5, 2009)

In cooling mode, the green fan and yellow compressor relay wire are both connected to the red hot wire from the transformer. Power to the green wire would then back feed to the yellow and start the compressor. 

You might want to buy a 2 heat stage thermostat, Tie the green wire that activates the blower to W-1 and the white wire to the propane furnace to W-2. With the thermostat calling for heat, power will first flow to W-1 and then to the green wire. It will have no connection to the yellow. If the temperature drops to the second stage set point, power will then flow to the white wire starting up the propane furnace. 

If you go with a second single stage thermostat, you will need to jumper the red wire to its R or RH terminal, and the green to the W terminal. If you want the A/C to work next summer, you will have to move the green wire back to the G terminal, or just jumper it to the W terminal activating the blower.


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## 046 (Jan 5, 2009)

go to Grainger.. ask for a thermofan switch that flush mounts. 

before you can specify what temps for fan to kick on, what what temps for fan to kick off. you need a IR thermometer to find correct temps ranges for what area to mount switch. 

for instance... my fan kicks on at 110f and kicks off at 90f
my TF switch handles 15 amps so can hook directly to fan motor.


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## ckr74 (Jan 6, 2009)

If you use two thermostats an isolation relay will be needed. It's not that hard to wire up and you won't have to be switching wires at the end of the seasons. The green wire (fan) needs to be isolated or it will feed back to compressor.


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## mtfallsmikey (Jan 6, 2009)

I used a separate relay/transformer/t-stat, put the hot wire to the fan motor (choose your speed) across the relay terminals. Keeps everything separate, no problems.


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## CowboyAndy (Jan 6, 2009)

If it is a forced air system, then the fan hi/low/limit switch should take care of turning on and off the blowers. It would be a better option to run the system that way as opposed to a wall mounted tstat in the house... think about it this way. If you have coal burnin in the stove and set the tstat to say 70. What happens when the coal is not producing any heat anymore, but there is a demand for heat in the house... that blower is going to run and run and run and not be moving any heat...


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## ericjeeper (Jan 6, 2009)

*Just use a line voltage stat.*

Then there is no need for a transformer.
With your setup the HX is always going to be hot. So all you need is a stat to turn on and off the blower correct?


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## Labman (Jan 6, 2009)

That is a good point. If the furnace has a low limit switch,it would shut the fan off when there wasn't any heat. Newer furnaces don't have them, doing a timed cool down cycle. With the heat exchanger in a duct, I don't see how the furnace controls would start the blower when there was heat.


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## Butch(OH) (Jan 6, 2009)

There are a few different ways to do it depending on your system particulars that we don't know at this point. You may or may not need isolation relays, you may need to change your present thermostat. I can tell you what I have and how I did it.

My 10 year old gas forced air Amana Hi-ef controls the fan at the main control box with timers, there is no hi-low limit switch as the older units had to control the fan. The thermostat that controls the furnace has a "fan on" feature that basically connects terminal "R" and terminal "G" at the controller and gives the high fan speed. When we added our outdoor boiler all I did was wire in a second thermostat in series with that Fan-On loop . With the fan switched to On the second thermostat then starts the fan at the set temp. The original thermostat is left set a few degrees under and will kick on the gas burner if temps fall to the set point. Mine has run this way for three years and I have never had any issues with the air conditioner that some have with other systems. The only potential problem I have is that when the new thermostat calls for heat the fan will run no matter if there is heat in the boiler or not. It could be fixed with a temp sensitive relay plumbed into the boiler loop but we have not burned a cubit foot of gas in the three years we have owned the boiler so it is a non-issue with me at this point.

Give some more system info and one of the experts will give you the correct answer. I am no HVAC man, just know what worked on mine.


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## Slick (Jan 6, 2009)

Butch beat me to it...I just did exactly what the original poster is asking about last week but with my OWB not a coal burner...but same setup, heat exchanger in duct work with pump keeping it warm all the time, I just kick the fan on to heat house. 
I have a 3 year old propane forced air furnace with cental air hooked to it...I did what Butch just described, got a second thermostat but I unhooked the fan wire from my original thermostat and hooked it to my new thermostat...but hooked it to the heat output of the tstat...so the new stat think it's calling for heat but it's really just kicking the fan on...just like butch said only drawback is if the boiler is out of heat the fan keeps running trying to get heat from it until house gets low enough to trip the propane furnace...haven't had the AC compressor kick on and it does work....this weekend was away and fire went out...house got low, propane kicked on....fire restarted, heat in heat exchanger, house heated up, both stats kicked off....


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## mtfallsmikey (Jan 6, 2009)

ericjeeper's way is the simplest. If you have a newer gas furnace, or heat pump, you can fry the control board, at mucho $$ real quick if you mess up a low-voltage wiring attempt. I had no prob with mine, been doing HVAC work for...too long.


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## Slick (Jan 6, 2009)

I think that's what Butch and myself both said...just turn on the fan with a t-stat...just hook the fan to the heat output (rh or r I think)...so the stat thinks it's turning the heat on but it's really just turning the fan on...


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## rjbeery (Jan 6, 2009)

*more information on the system*

The coal is always going to be producing heat, I have a digital hi/low sensor on the boiler that will keep the water at lets say 200 degrees. There is always to going to be hot water circulating, I just need to know how to hook up the 2nd thermostat to kick the fan on and off, without the propane also kicking on.

Also, should I use a 2 stage thermostat, it is a 2 stage furnace. Does that influence the fan operation at all?

I've also heard of using a dual fuel thermostat to accomplish it and make it all automatic?


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## Labman (Jan 6, 2009)

Several of us have said about the same thing. Maybe it would be clearer with a little more background on thermostats. 

Thermostat 101

Usually there is a 24 volt AC transformer in the furnace with the secondary winding connected to a red wire running to the thermostat and a blue wire, common, to the gas valve, A/C relay, and fan relay. From the thermostat there will be white wire to the gas valve, yellow to the A/C, and green to the fan. The thermostat is wired to switch the power from the red to the white, yellow, and green as needed with the blue completing the circuit. Most thermostats and furnaces have the contacts labeled R, B or C, W, Y, and G for the corresponding wire colors. Many have RH and RC to allow separate transformers for heating and cooling. With just one red wire, jumper them. It may be wired to have the A/C control wires return to the furnace and its controls and then a pair of wires goes to the A/C unit. Internal wiring may replace the green wire if the thermostat does not give you the option of fan only or continuous fan. Digital or programmable thermostats may need the blue wire connected to them.

Yes, circuit boards are expensive if one goes bad. I paid about $300 for the one on my Bryant. I hadn't been messing with the wiring, I blame a lightening storm. However, like many expensive things, they have a cheap little blade fuses to protect them. It is very difficult to blow a circuit board with the breaker for the furnace off. 

My furnace is 2 stage, but I have the dip switches on the circuit board set to allow the furnace to decide which stage to run. If your furnace follows the wiring scheme above, likely it is set up the same way. If the thermostat is controlling both stages of the furnace, it might be just as easy to add a second thermostat. 

Either way, connect the new thermostat or W-1 to the green wire. That does give you the high speed on the blower. There are ways to use the low usually used with the heat, but you would need to give a lot more details on your system to do it. Leave the white wire on the old thermostat, or move it to W-2 if only using a single 2 stage thermostat. 

If you use a 2 stage thermostat, you can't have it switched to both heat and cool at once, thus insuring the power to the fan can't back feed to the compressor relay. With 2 thermostats, you could have one set on heat and the other on cool and do just that. You will need to be sure to shut off the new heat thermostat before setting the old one to cool. Chances are, long before you need A/C, you will quit building a fire in the boiler and shut the thermostat off so the blower doesn't run continously.


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## AOD (Jan 6, 2009)

This solution is so easy I don't know why you guys are making it more complicated. 

On the thermostat subbase there are typically 4 wires, red, white, yellow and green. Red is incoming power, white is power to activate heat, yellow is power ro activate cooling and green is power to activate fan. In cooling mode the thermostat automatically sends power don the green AND yellow wires. In manual fan mode, only the green wire is powered. The yellow wire must be powered in order to power the contactor inside the A/C compressor unit. Therefore, you can jump power between the Red and Green at any time for fan operation, which is exactly what the Fan switch on the wall t-stat does. All you need to do is wire a cheap, simple thermostat parallel to that switch, or between the Red and Green wires. Cooling WILL NOT energize unless power is sent down the Yellow wire. This is all separate from any control on the furnace, weather it is a fan center, control board or whatever from any type of integrated furnace control. If you want to be extra safe, just turn off main power to the A/C unit before you mess with wiring. If you accidentally send power down the yellow wire, all it will do is make the contactor click on and off, nothing else will happen.


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## Labman (Jan 6, 2009)

I am in the process of changing from an old mechanical Robert Shaw thermostat to LUX 9000TX programmable. I put my ohmmeter leads to the Y & G terminals with the fan switch in Auto. They are connected in Heat, Off, and Cool. Thus if you applied voltage to the green wire, it would also flow to the yellow, and the condenser relay coil. Turning the fan to On disconnects them. I missed that point in my above posts. 

Thermostats have 3 different switches in them. They may not all work like my old Robert Shaw.


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## rjbeery (Jan 14, 2009)

*thermostat/circuit board labels*

Okay guys I'm going to get this up and running this weekend. The furnace is a Lennox G61MPV.

Can someone give me some basic knowledge on what all following terminals are for or run?

G
Y1
W1
R2
C3

Thanks Bob


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## mtfallsmikey (Jan 15, 2009)

G-fan
Y1-condenser unit contactor
W1-some uses as a common, other 24v. leg to burner, condenser, etc.
R2-??...heat?...Honeywell uses RC=cooling, RH=heat
C3-common, or neutral

Not all mfr. of equip./t-stats follow a "standard".


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## Medman (Aug 31, 2009)

*Thanks for the information*

Newbie here. I thought I had it all done then i noticed the AC compressor running. Damn I thought but the posts here were right on the money. The whole tstat wiring took me about 10 min. working great with out the AC running. Thanks again.

Tim


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## Stihl310 (Aug 31, 2009)

Slick said:


> Butch beat me to it...I just did exactly what the original poster is asking about last week but with my OWB not a coal burner...but same setup, heat exchanger in duct work with pump keeping it warm all the time, I just kick the fan on to heat house.
> I have a 3 year old propane forced air furnace with cental air hooked to it...I did what Butch just described, got a second thermostat but I unhooked the fan wire from my original thermostat and hooked it to my new thermostat...but hooked it to the heat output of the tstat...so the new stat think it's calling for heat but it's really just kicking the fan on...just like butch said only drawback is if the boiler is out of heat the fan keeps running trying to get heat from it until house gets low enough to trip the propane furnace...haven't had the AC compressor kick on and it does work....this weekend was away and fire went out...house got low, propane kicked on....fire restarted, heat in heat exchanger, house heated up, both stats kicked off....



Can a potential problem be created if you OWB drops temp and the fan keeps running, then the temp get's low enough that the propane stat kicks in as well... Would this "overload" the wiring for lack of better words??

Also I have heard that you want to make sure that your heat and AC don't accidentally get turned on at the same time since you will be running two different stats. 

If your having doubts about doing it yourself I would just hire someone that knows what they are doing. It's better than dropping $300 on a new circuit board...


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## mtfallsmikey (Sep 1, 2009)

Stihl310 said:


> Can a potential problem be created if you OWB drops temp and the fan keeps running, then the temp get's low enough that the propane stat kicks in as well... Would this "overload" the wiring for lack of better words??
> 
> Also I have heard that you want to make sure that your heat and AC don't accidentally get turned on at the same time since you will be running two different stats.
> 
> If your having doubts about doing it yourself I would just hire someone that knows what they are doing. It's better than dropping $300 on a new circuit board...



That's why my preferred method is to totally isolate the OWB t-stat/circ./fan wiring with a separate transformer/relay setup, and use it to power the fan. The smoked circuit board deal is how I came about getting the old gas furncae I now use as a heater in my shop...and the boards are not getting cheeper eiether!


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## Slick (Sep 1, 2009)

I guess it's possible to overload some wiring...but as I said it's happened that the fan was funning (OWB thermostat calling for heat) and the temp got low so the propane stat called for heat...worked just as I would want...propane kept the house warm until I heated up the OWB water again and the house came up to temp. 
I'm not exactly sure how the AC and heat would come on at the same time? The propane stat controlls both of those...the same way it always has....all I did was remove the fan wire from the propane stat and move it to the owb stat...the propane furnace still kicks on the fan when it goes to heat or AC, honestly not sure how it does that without the fan wire, I'm thinking it's internal to the furnace? But I can't just turn on the fan from that original propane stat anymore...that wire is now on the owb stat which thinks it's turning the heat on....maybe I just got lucky


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## Butch(OH) (Sep 1, 2009)

Slick said:


> I guess it's possible to overload some wiring...but as I said it's happened that the fan was funning (OWB thermostat calling for heat) and the temp got low so the propane stat called for heat...worked just as I would want...propane kept the house warm until I heated up the OWB water again and the house came up to temp.
> I'm not exactly sure how the AC and heat would come on at the same time? The propane stat controlls both of those...the same way it always has....all I did was remove the fan wire from the propane stat and move it to the owb stat...the propane furnace still kicks on the fan when it goes to heat or AC, honestly not sure how it does that without the fan wire, I'm thinking it's internal to the furnace? But I can't just turn on the fan from that original propane stat anymore...that wire is now on the owb stat which thinks it's turning the heat on....maybe I just got lucky



Could be lucky, if so than I am too When I was installing my OWB I consulted a friend who's job is in HVAC and he said I could not wire in a second 'stat because he had tried it on a OWB install and the A/C unit tried to run and he ended up installing isolation relays etc to get rid of a back feed and make it all work. I kept looking at drawings and playing with my "fan on" switch and thinking in my simple mind,, the A/C isn't trying to run and the board doesn't smoke when I turn the fan on, how is the board going to know if I am flipping a switch or there is another 'stat wired in series to do it for me? So I tried it and was not amazed when it worked just the same as doing it manually. My HVAC buddy still says it cannot work but has been for 3 seasons, LOL. I am surely not smart enough to know why it works on some but not others but in reading these posts for several years at least part of the problems lies when someone tries to parallel another thermostat in the circuit rather than in series and that's an entirely different deal,, I think


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## Stihl310 (Sep 1, 2009)

Slick said:


> I guess it's possible to overload some wiring...but as I said it's happened that the fan was funning (OWB thermostat calling for heat) and the temp got low so the propane stat called for heat...worked just as I would want...propane kept the house warm until I heated up the OWB water again and the house came up to temp.
> I'm not exactly sure how the AC and heat would come on at the same time? The propane stat controlls both of those...the same way it always has....all I did was remove the fan wire from the propane stat and move it to the owb stat...the propane furnace still kicks on the fan when it goes to heat or AC, honestly not sure how it does that without the fan wire, I'm thinking it's internal to the furnace? But I can't just turn on the fan from that original propane stat anymore...that wire is now on the owb stat which thinks it's turning the heat on....maybe I just got lucky




I meant to watch from accidentally turning the AC on, on the old T-stat and still having the 2nd t-stat calling for heat. I know, it sounds stupid, but I'm sure it could happen, then you would get a signal calling for Hi-speed fan on the ac and a signal calling for low speed fan on the heat... That could be bad.


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## mtfallsmikey (Sep 1, 2009)

Slick said:


> I guess it's possible to overload some wiring...but as I said it's happened that the fan was funning (OWB thermostat calling for heat) and the temp got low so the propane stat called for heat...worked just as I would want...propane kept the house warm until I heated up the OWB water again and the house came up to temp.
> I'm not exactly sure how the AC and heat would come on at the same time? The propane stat controlls both of those...the same way it always has....all I did was remove the fan wire from the propane stat and move it to the owb stat...the propane furnace still kicks on the fan when it goes to heat or AC, honestly not sure how it does that without the fan wire, I'm thinking it's internal to the furnace? But I can't just turn on the fan from that original propane stat anymore...that wire is now on the owb stat which thinks it's turning the heat on....maybe I just got lucky




Yes it is possible...ask me how I know! I've smoked a few 'stats and transformers in my day by mis-wiring. The problem comes up when you add another t-stat, control a fan,etc.....then exceed the current rating of the low-voltage transformer. Most residential transformers are only rated at or around 40 VA (volt-amps) then, the printed circuits boards may have a voltage reduction circuit, which reduces the voltage to 5 volts for the PC components. A lot of control functions are done by solid-state components vs. traditional relays, limit switches, etc...There may be auxiliary contacts on the board to control something else, like a humidifier/electronic air cleaner,  that could be used.....I guess.


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