# Wood Furnace Duct Work vs. Existing Duct



## Skunkman (Aug 24, 2012)

Hi Folks,
Found this site the other day as I was searching the net for info. on the hot blast type wood furnaces.
I would appreciate your comments/suggestions regarding my challenge.

I have a free standing stove in a finished basement that I have been using for ~8 years. I want to switch over to a furnace type stove because the 90 degree heat in the basement is a killer. I have a 1900 sq. ft. rancher with a hot air NG furnace. The furnace is on one side of the house and my wood stove is on the other side, ~60 feet apart.

My wood stove is located at the end run of my current duct work. I am trying to think a little outside of the box on creativity because I really don't want to add another chimney on the other side of my house.
I recognize what I say below is not perfect, but I am looking for a way to spread the heat in the home without a major renovation.

I feel like I can run a new wood stove furnace supply duct work into the end of my current return side duct. I know this would go against all installation instructions because you just aren't suppose to do that. The reason as I understand it, is so you don't burn up the filter on the return box at the furnace not to mention an uneven heat distribution. I'm reluctant to tie the wood furnace duct work into the main furnace distribution duct work and feed it backwards, for fear of the gas furnace and wood furnace fighting each other on distribution if they happen to run at the same time.

However, I am thinking that the majority of the heat that I would generate would escape via the return ducts upstairs and wouldn't be more than 75 degrees by the time it reached my filter at the main furnace there by reducing any fire risk. Does this seem doable? 

I would appreciate your feedback or even tell me that I'm crazy, I'm okay with that too.

BTW, I really like this website and will be a routine visitor.

Thanks,
Skunk


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## velvetfoot (Aug 24, 2012)

I'm totally no expert, but just to give this thread a bump and ruminate a bit...

So, the return air path now will be natural flow from each room through an open door and then down the basement stairs, through the wood furnace and then through the NG furnace (PS:  Unless you get free wood, Is burning wood even cost effective for you, since NG is pretty cheap, they say).  It sounds like you're proposing to change the way that the furnace was designed to work.

Could you put some additional ductwork in the basement?  I don't know about return air, louver's in basement door?  Is the basement finished?

Sorry I couldn't be more help.


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## blades (Aug 25, 2012)

Note your duct work from a solid fuel appliance will need a minimum 2" clearance from combustible materials for 10 feet and the 1" there after, national code. Check with your Ins  and local people first before expending funds.


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## Clarkbug (Aug 25, 2012)

So your existing NG furnace and woodstove are currently in the same basement, just on opposite sides?  Is your return ducted, or just open to the basement? 

If possible, why not just use a fan to move the heat off of your current woodstove and let the fan on the furnace carry it upstairs?  If your run your furnace fan when the woodstove is fired up, it would move that 90 degree heat from the basement all over your house. 

And if none of the above is possible, then never mind my ramblings


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## daleeper (Aug 25, 2012)

First, welcome to Hearth.com.

Now, to answer your question and ask more questions, I'm not sure I get which one of your ducts you are wanting to tie in to, but I would be asking a knowledgeable furnace man for help. I would not want to tie the heat output directly into the existing return for many reasons, one being it is not designed to handle the heat that will come out of the wood furnace. Another problem is, depending on the system design, the duct usually gets smaller the farther it is from the furnace, and you would be hooking in on the small end. You don't want two furnaces kicking on at the same time or fighting one another either. If you have the space to put a new chimney up where the existing furnace is, and you have the space in the basement to put the wood furnace, get it moved down there, that is your best option.

Have you considered using a wood burning boiler and use a heat exchanger in your existing furnace air handler? The boiler can be placed where the wood stove is now if you don't mind the look, which I am guessing is the case because you are planning to put a furnace there now, or that boiler can be put in an outbuilding away from the house, and the hot water plumbed into the heat exchanger in the furnace, and even keep the current wood stove in place for emergency use.

Do you have a wood furnace in mind that you are considering installing? I would be studying the installation manuals for ideas on how it could be hooked into your system. Some of them have different options.

I assume you are wanting to keep the gas furnace for backup heat?

What room is above the wood stove now? Maybe a separate but minimal set of ducts would work on your primary rooms upstairs?

I am assuming the room in the basement is 90 degrees because you are running it hot in order to get some of that heat upstairs?

I really don't think you should do what you are thinking about here. Give us a few more details, and maybe some of us could come up with some good alternative options for you.


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## ROVERT (Aug 25, 2012)

With natural gas heat, I doubt you are going to save enough to recover the costs of a wood furnace or boiler in a reasonable amount of time.

As long as local code does not prohibit it, I would consider installing a return grille near your wood stove and using your furnace fan to circulate the heat out of the basement and throughout the house. If the return trunk is adequately sized near your wood stove you could use this as your primary return, closing off the other return/returns. You might need to keep the basement door open to allow the cold air to fall back down the stairs. It's not going to work as well as a furnace in terms of even heat distribution, but you might be surprised. It would certainly cost very little to try compared to a wood furnace.


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## Skunkman (Aug 29, 2012)

Hi Folks,

Thanks so much for your replies! Thanks for the welcome!

I have tried utilizing my current return by cutting off the return grills upstairs and adding one near the current wood stove in the ceiling. I have left basement door open and I even cut another hole in the floor above and near the current stove and used fans to circulate the air. It's still brutal.
It is a finished basement with a bedroom downstairs. Last year was the first year to have a bedroom down there, hence the idea of trying to get more of that hot air upstairs. Literally can not stand being in the basement to watch TV in the winter due to the heat build up.

The way my basement is currently laid out, I can not add anymore ductwork tactfully down there as it would not look nice in the finished basement along with many other challenges.

I would love to add an outside wood boiler but don't have the funds...one kid in college and the other will be in college in two years. Although, running a water line to my current furnace and thru an exchanger would be great, don't know if it is affordable at this time.

I do want to keep the gas furnace as a back up....when I started with this set up, i was burning coal, but it has got so expensive, it is no longer feasible or financially the right fuel to use. I get all my wood for free, I put up about 7 cord or so a year and it is my excercise program as I do most of it by hand. I sit behind a computer most days so this is my relief.

Thanks for the duct work clearances! This has made a decision much easier as my return duct has no gap between comubustible material. My supply side does have a 1" gap the whole length and the 2"clearance for the first 10 foot will also be able to adhere too.

So, the way I look at it now, my choices are:
1. Outside boiler (did not want to spend that kind of money at this time)
2. Buy a furnace, place near existing furnace, but I will need to put up chimney (might as well buy the outside boiler then I think with that cost)

or, this is where I hope I have clarified some answers but still need help;

#3. Place the furnace where I have my current stove, run my supply line into the end of my gas furnace supply side (which is the smaller end of the duct work and is not the "way" to hook it up. But that might help get more heat upstairs?
If that works, I need to come up with a way to be able to turn off or redirect the heat supply from my wood furnace if the gas furnace comes on?
The only reason my gas furnace would come I think is when the wood furnace fire is dieing out right? I think I read where these furnaces, have a low temp setting where the blower turns off at 90F, sound familiar? I am thinking I would have my gas furnace set to come on around 62F which would mean that the wood furnace is going out anyway....make sense or not really?

So, just in case I lost you on that last part with many questions....is there any good reason (code or safety) why I can't feed my supply backwards even though it's not the best or most efficient set up?

Thanks again for your help. I have pushed off buying anything as of now until I do figure out exactly what I need to do and I always appreciate feedback from others.


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## ROVERT (Aug 29, 2012)

I don't know much about wood furnaces, but I think what you are describing is a bad idea. Duct work needs to be sized properly for a furnace. Tying in to the end of a run after it has been reduced down is likely not going to give you the air flow you need. 

Also, what kind of duct work do you have? It's my understanding that unless it is 100% metal you cannot hook up a wood furnace to it. That means if there is any fiberglass trunk or flex duct it's a no go.

Have you tried heating the basement to only 75 or 80 degrees w/ the wood stove and supplementing up stairs with the NG?

I still think there must be a way to get some of that heat out of the basement. What is the ceiling construction in the basement? Drop ceiling, drywall, insulation/ no insulation, etc?


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## Skunkman (Aug 29, 2012)

My duct is all sheet metal and I have a drop ceiling in the basement with no insulation. In addition to the return grill I added to the basement, I also have added flexible duct with an inline blower to the hole I had drilled in the floor to the main floor.

Thanks for your help.


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## stee6043 (Aug 29, 2012)

ROVERT said:


> With natural gas heat, I doubt you are going to save enough to recover the costs of a wood furnace or boiler in a reasonable amount of time.


 
This depends on a lot of things.  I have natural gas forced air heat and my gasifer boiler with storage will pay itself of this year, year 5 of operation.  Given it has a 20 year warranty I would say that 5 years is definitely a "reasonable" amount of time.  The only better investment I've seen in the past five years has been Apple stock.


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## ROVERT (Aug 29, 2012)

stee6043 said:


> This depends on a lot of things. I have natural gas forced air heat and my gasifer boiler with storage will pay itself of this year, year 5 of operation. Given it has a 20 year warranty I would say that 5 years is definitely a "reasonable" amount of time. The only better investment I've seen in the past five years has been Apple stock.


 
You're correct, there are many variables. I see he gets his wood for free, so that is a big plus. From what I can gather of his current system, it is going to be an expensive install. It may work out for him. I don't think tying in to the end of the current feed duct is a viable option.


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## daleeper (Aug 30, 2012)

Skunkman said:


> this is where I hope I have clarified some answers but still need help;
> 
> #3. Place the furnace where I have my current stove, run my supply line into the end of my gas furnace supply side (which is the smaller end of the duct work and is not the "way" to hook it up. But that might help get more heat upstairs?
> If that works, I need to come up with a way to be able to turn off or redirect the heat supply from my wood furnace if the gas furnace comes on?
> ...


 
I can't site you code that says you can't do what you are wanting, but I have looked at the manuals to some of the add-on furnaces available.  Blaze King specifically says not to install in a manner that would create a potential for reverse flow.  All others that I looked at including the Caddy manuals show the flow of air going from the gas furnace through the wood furnace before going to the supply duct.

You really need to get someone that is knowledgeable in furnace installs get involved with this.  My gut says it would be wrong to install your furnace as you suggest in #3, but I can't point you to code. On the other hand, I understand tight budgets, and if someone knowledgeable in installing one this way can get you set up safely, then get it done.  All I can see is problems.  Sometimes the cheap way is more costly in the long run.

Just because none of us here can quote you a reason not to do it this way, does not mean it is safe.


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## cstamm81 (Feb 11, 2014)

thread revival.... I installed an Englander wood furnace this very way a few years ago.  I fed my ductwork from the end of the trunk, but I shut off my oil burner whenever I use the wood furnace.  It works pretty well, but is not overly efficient.  A lot of heat is lost going into the plenum of the oil burner, and feeding the ducts backwards does not allow for good airflow.  Let me know if you ever went ahead with your install.


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## KindredSpiritzz (Feb 11, 2014)

Skunkman said:


> I want to switch over to a furnace type stove because the 90 degree heat in the basement is a killer.



Im thinking  you'll probably still end up with 90 degrees down there.  Least i do with my wood furnace, but i like it toasty warm down there. They radiate a lot of heat.


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