# Help Me Understand How My Furnace/Wood Furnace Work Together



## MinnesotaNative (Oct 3, 2014)

We just moved into this home and I am not familiar with the workings of the wood furnace. The primary furnace is LP gas, variable speed. The wood burner says "Hot Shot" on the door, I'm not sure if that's the name. It appears to be in good working order. After close inspection of the unit, piping and chimney, I went ahead and built a small fire in it. The draft is excellent, zero smoke comes out anywhere and it is pulling air up the chimney very well.

I see how it ties in to the ductwork but I'm not sure how it works with the thermostat that runs the gas furnace. There is some sort of electronic box on the manifold above the wood furnace that I assume does something but I have no idea really how they work together. (It has a white button on it that switches it between auto and manual) I was thinking that maybe, once the temp dropped below the thermostat setting, the gas furnace would only run the fan if the wood furnace was hot and kick the gas burners on once it dropped below a certain temp but I'm not sure. Any ideas or more info I can offer that will help figure it out?

(sorry for the sideways pics, I'll try to fix them


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## brenndatomu (Oct 3, 2014)

That box on the plenum (manifold) is your limit switch, or fan control in your case. It will kick the gas furnace blower on (which is ducted through the wood furnace too) once the air above the wood furnace gets hot enough, and then off when the air cools. The blower may cycle on/off alot when burning wood . You will find adjustable pins under that cover, they likely are set at 150* on and 110* off, or thereabouts. Sometimes you can limit blower cycling by adjusting the on/off temps just a bit, or setting the blower motor to run slower/faster.
It looks like the wood furnace is totally manually controlled, no thermostat. You control the temp in the house by how much wood you load and how far open the air control damper(s) are on the front of the firebox.
My best guess from lookin at your pics and not actually being there to inspect anyways...never heard of a Hot Shot before.


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## MinnesotaNative (Oct 3, 2014)

View attachment 140228
View attachment 140228



brenndatomu said:


> That box on the plenum (manifold) is your limit switch, or fan control in your case. It will kick the gas furnace blower on (which is ducted through the wood furnace too) once the air above the wood furnace gets hot enough, and then off when the air cools. The blower may cycle on/off alot when burning wood . You will find adjustable pins under that cover, they likely are set at 150* on and 110* off, or thereabouts. Sometimes you can limit blower cycling by adjusting the on/off temps just a bit, or setting the blower motor to run slower/faster.
> It looks like the wood furnace is totally manually controlled, no thermostat. You control the temp in the house by how much wood you load and how far open the air control damper(s) are on the front of the firebox.
> My best guess from lookin at your pics and not actually being there to inspect anyways...never heard of a Hot Shot before.



You're correct on the wood furnace being manually controlled. There is nothing on it aside from the box on the plenum. Thanks for the heads up on how it works. It didn't kick the fan on at all until I pulled the button on it to the "auto" setting, then the fan kicked up without the gas burners lighting and heated the house up pretty well.

Is there any way to adjust the speed of the blower motor on the furnace? I'm not familiar with the variable speed style furnace and am not aware of any way to adjust fan speed. It just runs at slow speed all the time and then kicks up to a higher constant speed when the wood furnace plenum gets hot enough. (It ramps it self up and down as needed when making its own heat or AC)


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## laynes69 (Oct 3, 2014)

You have a series install, where the woodfurnace is in series with the central furnace. I had this setup for 5 or so years. Mine was wired from the limit/control to the motherboard on the central furnace. When the on temp     was hit, the limit would energize the central furnace and run the fan only for the woodfurnace. If the woodfurnace would burn out, then the thermostat in the home would cycle the central furnace. Now if the woodfurnace was running the blower on the central furnace and couldn't keep up, there was a built in relay in the motherboard on the central furnace that would automatically fire the burners of the central furnace if called for by the thermostat upstairs.

With your limit control leading to the central furnace, it may be wired like I had mine. The blower should not run constantly. Only when the woodfurnace gets hot enough to energize the circuit board.


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## MinnesotaNative (Oct 4, 2014)

Thanks for the reply laynes69. It sounds like mine might be wired just as you describe, seems like a good set up.  The little test fire that I built yesterday sure did a heck of a job of heating the house anyway once the furnace fan started pushing the warm air through the ducts!

With my furnace the blower does run constantly but only at a low setting, just enough to move a bit of air. This is nice when there is a gentle fire going in the wood burner as you get a nice gentle heat from the registers. All in all it seems like it should do a good job of heating and make pretty good use of whatever wood I burn this winter.


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## STIHLY DAN (Oct 4, 2014)

You pulled the auto which is manual fan on. Put it back in and let it get up to temp. The fan should turn off and on at the beginning but as the furnace gets nice and hot it should stay on longer. It takes awhile for all that mass to heat up, maybe 1/2 hr from cold.


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## MinnesotaNative (Oct 9, 2014)

STIHLY DAN said:


> You pulled the auto which is manual fan on. Put it back in and let it get up to temp. The fan should turn off and on at the beginning but as the furnace gets nice and hot it should stay on longer. It takes awhile for all that mass to heat up, maybe 1/2 hr from cold.



The button on the plenum says "push/manual" and pull/auto". I am assuming that it should be in the "pull/auto" position if I want the blower fan to respond to temp changes in the wood furnace, no?

Inside the electric box on the plenum there are adjustable tabs and the dial says off/on/off. Where should I set these tabs start with?


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## j7art2 (Oct 9, 2014)

I have a VERY similar setup to this as well that hasn't been used in 10+ years in my new house.

I cleaned the chimney, replaced the firebrick, gaskets, and piping to the flue, and I can't figure out the sequence of how to get the heat transferred properly into the house. In all of my tinkering, I have successfully also managed to disable my normal furnace, so now I don't have good heat coming from the wood furnace and no heat coming from the regular furnace. Great way to start a 30 degree morning.

I don't want to hijack your thread, but I'm hoping that we can both learn from each other and the others here and get both of our wood furnaces going -- especially since I've already harvested two face cords of seasoned wood, and spent 30 hours working on cutting new custom fire brick and re-piping this old beast.


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## STIHLY DAN (Oct 9, 2014)

MinnesotaNative said:


> The button on the plenum says "push/manual" and pull/auto". I am assuming that it should be in the "pull/auto" position if I want the blower fan to respond to temp changes in the wood furnace, no?
> 
> Inside the electric box on the plenum there are adjustable tabs and the dial says off/on/off. Where should I set these tabs start with?



Pull push depends I guess on what type you have. Easily figured out by testing it when cold. Not sure about the 2nd off, There should be at least two temps. 1 that turns the fan on say 140* another to turn off say 110* So when the plenum gets heat up to 140* fan turns on until the temp drops to 110. Drill a hole near the switch and take a temp as your unit warms up, see how far it goes. You may just need to replace the stack switch. Just keep in mind a cold furnace burns a while before the temp gets up there.


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