Getting Furnace fan to run with wood furnace

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freeburn

freeburn

Feeling the Heat
Jan 5, 2008
391
USA
I looked all over, and I never got any real specifics. So that I don't destroy the electronics in my gas furnace, I need some counsel here. My wood furnace is ducted into the ductwork through the plenum so that there is no possibility of blowback and the gas furnace can run at the same time as the wood furnace. The one thing is it just doesn't get the heat to the outermost parts of the house. What is the easiest way to wire up a wood furnace to activate the gas furnace blower at the same time? My fan limit switch is too low on the gas furnace to heat up enough from the wood heat going into the plenum, (I do have another fan limit switch). I've read three things a person can do, but I need some help understanding how they work or if you know of another way of doing it.

1. Hooking up to the "r" and "g" on the gas furnace relay, but from where?

2. Using the extra fan limit switch, explain where and how without frying something on the gas furnace or causing the gas furnace not to operate as it's supposed to?

3. Using some sort of additional relay. (I'd rather not do this one if possible)

Yes, I realize that I can manually turn on the fan w/ T-stat, but would rather not if I didn't have to. Trying to make this as automatic as possible.
 
Did you install in series? I did what you want to do when I switched my furnace install to series a few years ago. I have a 90% propane furnace that only the central furnace fan pushed heat from (Series). I ran my hot in the lower left on my Limit/control, removed the jumper in the center of the limit/control and ran the 2 wires on the right top and bottom to r and g on the circuit board of the furnace. When my woodfurnace called for heat, the control kicked on the fan of the central furnace. When then the thermostat called for heat on the propane furnace, the relays in the circuit board of the propane furnace automatically kicked off the fan, and would cycle the propane to kick on the burner and produce heat. When that was met, the fan would shut off unless the woodfurnace still had a demand for heat. Worked well and was very efficient in removing heat. By doing this, you have a system that works in harmony, kinda like a multi fuel furnace. My only issue was when you didn't burn wood, the central furnace would run longer because you had to heat the whole firebox up, before the heat would go through the ductwork. In that situation you would need a damper to bypass the woodfurnace when running propane only, and for a/c also. Because of that, I would never own a multifuel furnace. It takes alot of energy to heat up a cold mass of steel and firebrick to then get heat into the ductwork, wasting fuel. If you kept a fire in the unit, then I wouldn't worry about it. I just can't tell you that your furnace will automatically switch on and off when called for heat.

http://www.inspect-ny.com/heat/Fan_Limit_Switch_Installation.htm

Look at the low voltage diagram.
 
I added a second thermostat located directly above my existing one, just one of the cheap ones. I then ran a jumper from the red and green wires from the existing thermostat up to the new one. this lets me set my existing thermostat that controls the gas furnace at 55 degrees, and the one above is at 72 and controls th just the blower in the gas furnace which blows through my HX for the boiler. hope this helps
 
I've got a relay on my existing furnace with separate t-stat as well as separate fan limit. Could I just run a jumper from that relay "r" & "g" terminal to the gas furnace relay "r" & "g" essentially doing the same thing as fabguy was saying but in a different place? or would that then cause the wood furnace blower to run when the gas furnace t-stat fan kicked in for a/c or summer time?
 
Oh, and I'm not in series, but parallel. The above way did not work.

I ran my hot in the lower left on my Limit/control, removed the jumper in the center of the limit/control and ran the 2 wires on the right top and bottom to r and g on the circuit board of the furnace.

Could I just connect my two wires on the right top and bottom fan limit to the r & g on my gas furnace so that it would only go on when the wood furnace did? That would be low voltage wouldn't it? Then would THAT cause the wood furnace fan to go on when the fan was on the the gas furnace?
 
I'm not sure. I do know if you run the central furnaces fan when the blower of the woodfurnace is running, it will temper the air lowering its temp. If the temp in the woodfurnace is high enough, then running the central furnaces fan may work. Some people have done it and have decent results. Is your blower on your furnace not adequate?
 
The blower on my wood furnace is supposed to be adequate, but I could get much more of the heat out into the house with the aid of the gas furnace blower. The wood blower does a good job of making the air seep out of the registers, but the furnace fan would make the warm air go into the house, and much faster. I may just wire up another thermostat into the existing gas furnace "R & G" and use it that way. I don't want to fry any relays or expensive circuit boards. Otherwise, the manual fan works too.
 
I see a couple of ways to do it mine is running a thermo for the boiler wired directly to the gas furnace it runs the fan and humidification wired by my hvac guy and he used a couple of those clamp on relays to run the humidifier and prevent the ac from kicking in.
The nice part is with the temp set 5/7 degrees cooler on the gas furnace thermo it kicks in automatically if I run out of wood.
The other thing you could look at is one of those in duct fan boosters to up the airflow.
 
What sort of relays are you referring to? Do they have any model numbers on them or anything?
 
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