forced air wood stove/LP ducting question

[mmead1014]

I typed this up quickly i hope it makes since.

If the supply of a forced air wood furnace were tied into the return side of an LP forced air furnace, would the hot air from the wood furnace damage the LP furnace? The return from the house would go into the return of the wood furnace and the supply end of the wood stove would go to the return of the LP furnace and then supply the house. I know I just repeated myself I just want to make sure my question is clear.

I'm asking, because I'm having problems getting enough heat from my wood furnace. I'm contemplating (for a few reasons) ducting the wood stove supply directly into the LP furnace return. My hope is that the fan from the LP and the wood furnace combined would move more air at a lower temperature.

 

[laynes69]

You can damage your LP furnace. Give some details on your home, woodfurnace and ducting and we can give you some things to look for.

 

[begreen]

That is usually frowned upon. What do the furnace docs say? What wood furnace make/model?

 

[jeff_t]

Forced air furnace blowers are designed to push cool air thru the heat exchanger. Feeding it hot air could cause premature failure, especially as far as bearings are concerned. But, I know there are lots set up like that.

Tell us more about the wood furnace you have and how it is currently set up.

 

[mmead1014]

Thanks for the help.

This could get long

I my house is 2000 sq ft, two stories, and a full unfinished basement. It was built in 2010 and it’s a modular. I’m not exactly sure what the insulation rating is but I’m under the impression that it’s average for a home of that year ( I know that’s generic but it’s the best I’ve got), the basement is superior insulated walls and stays warm even when the wood stove isn’t going. The ducting for the first floor is in the basement and the second floor is fed by a main trunk that goes up to the attic and branched down thru the bedroom ceilings. There is one small return in the upstairs hall way and a larger one in the kitchen. All the main ducting is insulated fiberboard and then 6 inch flexible ductwork to the room vents.

I’ve tied the wood stove duct into the system directly above the supply on the LP furnace. The LP ductwork goes straight up, branches left and right at the basement ceiling and then continues straight up to the attic. The supply form the wood stove is 90 degrees from the vertical LP supply and attached at the point where it branches left and right. The return from the wood stove is ducted out of the return from the LP. I have two square spring loaded louvers on the top of each heating unit. This stops short cycling when one system is operating and the other is not.

When the LP is on by itself it pressurizes the system nicely and I get good air flow around the house. But the wood stove hardly gets any flow to the second floor. On cold days (below 20 degrees) it’s hard to keep the house above 68. My wife and small children like 70. When it’s frigid (as it’s been for the last month) It’s almost impossible to heat with wood. That being said I’ve gone thru almost 6 cords so far this year. I’m on track to be way over 7 by years end.

I’m convinced that there is too much turbulent friction for the wood stove and that’s why isn’t not moving. So if I were able to move air with the LP fan and the wood stove I should be able to burn at a lower temp just move more air. My LP can be rewired High, Med, and Low speeds and my wood stove has 2, 800 cfm blowers. Duct work to and from wood stove is around 200 square inches inside

LP is a Ducane CMPA100U4
Wood is Clayton 1602m

Sorry for the length but I need to fix this. Any advice would be appreciated.

 

[jeff_t]

Have you tried to run the lp furnace blower while the wood furnace was burning? I was worried that it would overpower the two 550 cfm fans on my wood furnace, but it was actually the opposite. It seemed to create a venturi effect, and spread the heat around nicely.

My furnace could make a lot of heat, but the blowers are inadequate. The main trunk would be quite hot, enough that I couldn't hold my hand on it comfortably. With the big blower running, it cooled off more like I expected it to be.

I think it was @laynes69 that went to extensive effort to make his Hotblast flow more air.

 

[mmead1014]

Yes I have tried, and unfortunately it does over power the wood stove. I was hoping when I did all the duct work is past fall that it would work just like you described. But the house actually cools down...

 

[begreen]

Make sure the back draft damper in the LP furnace plenum is working correctly. If not there could be a loop where the wood furnace is sucking its own hot air back through the return. With 2 800 cfm fans (1600 cfm) one would think the Clayton should have enough cfm to deliver strong air flow but perhaps they are only 800 cfm at a much lower static pressure?

 

[laynes69]

The Clayton has a large plenum opening. Like mentioned above, the blowers should be large enough if dampers are in place. Is your woodfurnace ducted into your returns? That's a large furnace which supply more than enough heat for your home. Can you post a few pics of your setup?

 

[mmead1014]

Yeah. I had that thought too. I cut a port above each louver so I can check their function once a week. So far no problems.

As for static pressure I can't speak to that. I have a friend that does* forced hot air. He calculated duct size and* bought all of it. Almost 100 feet total from wood stove to LP and back.

And I thought 1600 cm would have been more than enough. I believe the stove is rated for up to a 2800 Sq ft house.

 

[mmead1014]

The Clayton has a large plenum opening. Like mentioned above, the blowers should be large enough if dampers are in place. Is your woodfurnace ducted into your returns? That's a large furnace which supply more than enough heat for your home. Can you post a few pics of your setup?

Yeah i can get a few pics. You're going to have to excuse the untidy basement.

 

[mmead1014]

I just got off the phone with my wife (shes at home, i'm working). It's 5 degrees and windy. She cant get it above 65 in the house with the wood. she had to damp the stove down and start the LP.

I know I'm not supposed to, i know the manuals say not to. But i really want to try ducting the supply into the return on the LP the. I'd like to know if any one has actually tried this and if it really hurt the LP unit.

I'm going to take some pictures tonight or tomorrow night. If i switch the ducting it's not going to be for a week or two. So i can still be talk out of doing something stupid.

 

[mmead1014]

Ok I'm officially desperate... I just got a propane bill today for the month. 333 dollars. I've got on demand hot water, gas stove and clothes dryer. But in the summer clothes , hot water, and stove is 45 a month. The rest of that is LP for the last month. I've been burning wood all month. I'm here are pics.