will this add on furnace ducting idea work?

In my basement, I have a forced air LP furnace with a 24"x8" X 14' main trunk duct that tappers down to 8"x18" for the next 20'. there is 6" round ducting going from the main trunk to all the rooms in my 1 1/2 story house on a full basement (1000sft per floor).
the furnace has a 8"x18" cold air return ducting that runs in each main room of the upper two floors.

The LP furnace is located in the center of north wall. the main trunk and main cold return ducts, run down the middle of the house to the south wall, where I would like to put a wood add on furnace because of a chimney chase that runs to the top of the peak of the house.

My question is, can I run the hot air from the wood furnace into the the 8"x18" heat duct trunk that is opposite end of the house to the LP furnace if I use a dampener on one each furnace? (I would also duct the wood furnace cold air return into the LP return.)

what would happen if both furnace fans ran at the same time blowing at each other?

would I need to put a 45* bend in the wood furnace duct work plenum? or could I just go with a 8"x18 plenum in the 8"x18 duct?

At the end of the main trunk there is 2, 6" ducts coming out of it, so when I would tie in into it with the wood furnace it would like a "T" with the 2 6" ducts being the upper left part of the "T". Would that create a problem of the hot air going one way more the the other? or would it even out?

If the way I'm describing this seem confusing think of like this 2 furnaces sharing the same duct work just like a normal add on install just the wood furnace would be at the other end of the house blowing towards the furnace.

adkdadto4 said:

You really need to consult the owners manual for the wood-add-on furnace stove...

Do you have that?

Click to expand...

I have looked at it on line. this is not going to be a typical install because of the wood furnace chase being on the opposite side of the house then the LP furnace because of chase location. If I did it the way they show I would have 36' of duct work just to get to the LP plentum and then the air would come all the way back 36' to the last 3, 6" register ducts. This would dubble the length of the duct work, but I don't know maybe it would work fine.

BeGreen said:

Normally the supplementary wood furnace is placed close to the primary furnace to take advantage of tapping into the adjacent supply and return plenums. This keeps the new wood furnace ducting to a minimum. You are correct about an additional 72' of ductwork being excessive. With the price of sheetmetal, it would also be expensive. What is the LP furnace connected to? Does it have it's own chimney or is it a high-efficiency unit vented out PVC pipe?

What you are proposing could work, but would require electric dampers to avoid having a loop through the opposite furnace when running. The risk with this setup would be a failure of one of these dampers. If a damper stuck closed it could cause the furnace to short cycle and the ductwork to get pretty hot. So safety circuits would be critical. Also, the trunk ductwork coming from the wood furnace should be about 6" away from wood for the first 10 ft.. That might be an issue if it has to tie into the 8 x 18" ducts right away.

Click to expand...

LP is a PVC. I could run 6' of horozontal duct 18" from the floor joist from the wood furnace then up into the main trunk.

BeGreen said:

Also, due to the reversed nature of the ductwork, the air flow is likely to be poor out of the ductwork takeoffs at the LP furnace when the wood furnace is running. What rooms are above the area where the wood furnace would be located? What is above the LP furnace area?

Are there any other options being considered like a wood stove on the 1st floor? Would running the wood furnace as independent system with it's own ductwork be an option?

Click to expand...

there is another option check out my new post called "new idea for wood furnace istall"

could I run a 12” duct from the wood furnace in the basement straight up the chase to the main floor and then up another floor to the master bedroom.
my chase runs outside my house from the basement to a couple feet above the roof line. the chase is 7’ wide by 3’ deep and left of center of the 26’ wide part of my 26’x38’ house. The main floor is a open floor plan with a small bathroom being the only enclosed room which is on the far wall from the chase.

as far as the upstairs (26’x38’) the master bed room is 1/2 the size of the hole floor and the other 1/2 is a bathroom and a 7’x9’ open stairwell and two spare room that don’t need much heat.

the top of the stairwell has a ceiling fan as do the 3 bed rooms. the door are always open on all the bedrooms. all 4 rooms doors meet at the stairwell.

This would be an easier way to do the install but would it work well?
I also could run the LP on circulate, if that would help.

BeGreen said:

Are you dead-set on putting in a wood furnace? Is a woodstove on the first floor an option?

Click to expand...

no place for wood stove.

what I'm going to do now, is just install the furnace in the basement with a pelnum blowing towards a LP cold air return. I will make it so when there is heat from the wood furnace, it will run the fan on both the LP and wood furnace. The basment can be as warm as it can get because we never use, it so if it 100* in the basment, 80* on the main floor and 70* on the upper floor thats just fine. I also have a big ceiling fan in the middle of the house above the stair well on the upper floor that I will run in reverse to help the natural convection.