# installing a second thermostat for wood furnace



## bgleich (Feb 7, 2012)

Hello everyone,
As the new guy, I expect to get tossed around a bit but here goes. Like many people have posted, I am adding a wood furnace ducted to my existing propane furnace. I would like to use my furnace blower to move the air because the fan on the wood furnace is not big enough. The guy that installed the A/C unit has told me the following; Buy a second "heat only" thermostat. Tap into the red wire and the green wire on the old thermostat. Run those 2 wires to the new thermostat and attach to the R and G terminals. Set the old thermostat at 65 degrees and the new thermostat at 72 degrees. At 70 degrees only the furnace blower will turn on. At 65 degrees, the orginal thermostat will kick in and the propane furnace will light. After reading some of the posts, I have my reservations about whether or not it will work. Can someone help with an explanation that the simplest of minds will understand? Thanks, Bob


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## MNGuns (Feb 8, 2012)

That's how I have mine set up. The LP thermostat works as a safety in the event that the boiler goes out. I keep it set at 58. The OWB thermostat is set at 72 or so, and kicks on the blower only as needed.


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## bgleich (Feb 8, 2012)

Thanks for the reply. I hope to hear from others but if not, I will run with it


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## beerman6 (Feb 8, 2012)

Been a few years since I hooked mine up,but that seems right.


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## bgleich (Feb 8, 2012)

Hopefully the stove will arrive today and I can get it installed over the weekend. I think I am going to go to town and get a "heat only" tstat, pigtail wire it into the existing tsat and test it. My belief is that if I mount the tstat, move the mercury switch to a high position calling for heat, the furnace blower should turn on. There are no wires making contact to the yellow terminal so the gas furnace should not turn on nor should the A/C pump. If it doesn't work, I will let you guys know and hopefully a solution can be achieved. Stay warm.


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## CrappieKeith (Feb 8, 2012)

I've said what I've said thinking that you are just looking for some extra CFM in air movement.
I may not have read it right ,but I'm calling a foul.
Thermostats are for calling for heat to be made...fan and limit switches run blowers.
Before you just start wiring stuff together you may want to get an electrician out there....OMG ...what are you people thinking...I understand trying to save a buck.

Stat wiring is only 24 volts but fan & limit switches are 115 volt ...be very careful.

If you are hooking to the same ducting that your A/C is in you'll want to change out your drip pan under that a-coil if the pan is plastic.... The wood heat might melt it and start a fire.
All of the ducting must be made of metal.


After reading your last post....you are heading for trouble.


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## MNGuns (Feb 8, 2012)

CrappieKeith said:


> I've said what I've said thinking that you are just looking for some extra CFM in air movement.
> I may not have read it right ,but I'm calling a foul.
> Thermostats are for calling for heat to be made...fan and limit switches run blowers.
> Before you just start wiring stuff together you may want to get an electrician out there....OMG ...what are you people thinking...I understand trying to save a buck.
> ...



Sorry Keith...I read the post wrong and ASSumed he was working with an OWB.


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## beerman6 (Feb 8, 2012)

:big_smile: and I just followed MN...


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## CrappieKeith (Feb 8, 2012)

*stop...drop...and roll*

No prob guys...I just saw a guy running to a cliff like the buffalo used to do without a clue to what he was doing.

There is a few issues here to think about. Another one that came to mind...is a back flow damper going to be needed?
It sort of sounds like a parallel install so the answer is yes. You want the heat to flow all in the same direction...down stream.
A more powerful blower on a gas furnace could cause an issue with the wood furnace blower without proper ducting directing everything.How about just getting the right sized blower for the furnace and forget about all of this cross wiring...you know do it right.Naaaawwwwww that would be too easy.......let's cut corners and create issues....

The OP is invited to call me so we can discuss his setup.


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## MNGuns (Feb 8, 2012)

Now now, I do stand by my wiring. A thermostat wired to the low voltage control board on forced air furnace to cycle the blower is an acceptable practice. Wiring direct to the blower and by-passing the control board and relays involved would perhaps be a bad thing.

In addition, with a forced air furnace, and and HX as used in an OWB there is no more chance of melting anything than there is with the LP or Gas portion of the furnace in use. With an Add-On wood burning furnace, as I understand the OP has, this may be different.


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## CrappieKeith (Feb 8, 2012)

MNGuns said:


> Now now, I do stand by my wiring. A thermostat wired to the low voltage control board on forced air furnace to cycle the blower is an acceptable practice. Wiring direct to the blower and by-passing the control board and relays involved would perhaps be a bad thing.
> 
> In addition, with a forced air furnace, and and HX as used in an OWB there is no more chance of melting anything than there is with the LP or Gas portion of the furnace in use. With an Add-On wood burning furnace, as I understand the OP has, this may be different.



I don't have any "REAL" expierence when it comes to OWB's so I'm giving you the benefit of the doubt you know what the hay you are talking about.I do know that a stat can tell a blower to run....but in add on style fan & limit switches do that job....at least in my world.
I know with a wood furnace there is a danger in taking too much heat off of it cooling it down too much.


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## MNGuns (Feb 8, 2012)

CrappieKeith said:


> I don't have any "REAL" expierence when it comes to OWB's so I'm giving you the benefit of the doubt you know what the hay you are talking about.I do know that a stat can tell a blower to run....but in add on style fan & limit switches do that job.




And I don't know about add-ons, so I guess we make a pair..

My original post was made then deleted under the assumption that the OP was asking about an OWB setup.


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## CrappieKeith (Feb 8, 2012)

MNGuns said:


> And I don't know about add-ons, so I guess we make a pair..
> 
> My original post was made then deleted under the assumption that the OP was asking about an OWB setup.



OK...I'll bite on we make a pair thing...but then comes the next question...a pair of what?????:msp_sneaky::msp_tongue:

This oughtta get good!LMAO already!


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## bgleich (Feb 8, 2012)

All right boys, here is how it shakes out. I purchased a Claton wood furnace. The furnace is located in the garage, approximately 25 feet from the gas furnace, which is located in the crawlspace. I ran metal ducting from the wood furnace, not to be confused with a wood boiler. The warm air being pushed by the wood furnace blowers dumps into the return air plenum at the gas furanace (pre-filter). The warm air is then recirculated by the gas furnace blower. So, I took a second heat-only thermostat and mounted it on the wall 4 inches above the thermostat that operates the gas furnace. I pigtailed a red wire and green wire from the gas furnace thermostat and attached them to the second thermostat. I turned the gas thermostat down to 65 and set the wood thermostat at 70. It is working like a charm and is not sending any power to the A/C unit. The only forseen issue will be the fire dying down and the second thermostat calling for heat and there is none. This will cause the furnace blower to continue running until the gas thermostat calls for heat and lights the gas furnace. So, don't let the fire die down or set the thermostats 2 or 3 degrees apart to minimize run time. 

For the record, a second thermostat is not even needed because the wood furnace has twin 800cfm blowers producing plenty of air flow. Regardless, I thank each and everyone of you for your imput. Sparked some good debate. Bob


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## Stihl310 (Feb 8, 2012)

Did you check your AC to ensure it still works correctly? You may want to check, but I had to put an isolation relay in on my setup since I'm using two thermostats. There is the chance that both the heat and AC could be turned on at the same time. I believe on some setups the fan speeds vary based on whether the unit is calling for heat or cool. Just something to consider, I just about fried part of the board that controls the AC cause I thought I knew what I was doing. Lol

By the way, mine was not sending power to kick the relay in on my compressor either, but when I accidentally had both stats on heat I was doing some sort of wierd magical electrical back feeding that just about cost me $300. It's been 3 years since I did it so I'm not even real sure anymore as to what all was involved, just don't want to see anythng bad happen to anyone else.

Good luck.


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## bgleich (Feb 8, 2012)

I checked the A/C unit and the second thermostat is not sending any power. However, I did have to turn the second thermostat to off in order for the cooling side of the other thermostat to function. After that, I don't see any issues. Thanks for the heads up.


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## s219 (Feb 8, 2012)

You should make sure you have all the wiring straight in your head. But what it sounds like you're trying to do is have an auxilliary thermostat trigger the blower circuit. It would be like someone walking over and flipping the "fan" switch on the main thermostat, but instead of a person flipping the switch, the new auxilliary thermostat does it.

This is a fairly simple, low risk mod, and the line is low voltage so you're not actually controlling the blower power, just the trigger relay that switches the blower power. Just make sure you have all this figured out so you know what you're doing. I'd probably go look at the blower wiring on the furnace control panel to see what color wires in the main thermostat line control the blower. Then you know what to tap into.


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## homernelson (Oct 6, 2012)

*maby this will help*

new to the site, i just installed two owb and had a problem withe the thermostat turning on the ac unit, called homeywell and talked to the tec, came up with the old thermostat was out dated, installed two honeywell rth2300b programmable thermostates, on the back is a switch move it to heat pump setting, wired it r to r and g to g, to use the odwb thermostat both has to be on heat setting, to run ac on the mane thermostate the owb thermostat needs to be off. this got thes two units going.


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## dan88 (Jan 22, 2020)

i just installed an outdoor wood stove and i installed a new thermostat that i need help with as im trying to jump from the old one to the new one to turn the fan. The old one only has 2 wires running to it a black thats going to RH on the Thermostat and a white going to the W spot also is a jumper from the RH-RC need help


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