Heat Commander in Series as Add-on Furnace?

[JRHAWK9]

This works for some...but the way some systems are laid out it can't be done (correctly) without going to powered dampers...mine was one of them.
The other thing people need to keep in mind is keeping air flow available to the wood furnace when the power goes out...especially if it happens right after loading a big load and the fire is well established, to the point of being "on cruise control"...the furnace is still gonna get hot, but without some gravity air flow things can get extremely hot, including the ductwork...that's why the CTC specs on ductwork.
Anyways, gravity dampers can block gravity air flow...again, depends on your system though.

yep, everyone's setup differs. I don't even have my wood furnace hooked up the return duct. Allows both my wood and LP furnaces to run at the same time if needed. I have a power open/spring close damper in my supply line connecting the wood furnace to the main supply duct and a gravity damper in the plenum of my LP furnace. The way mine is setup works fantastic for me, however, may not for someone else.

 

[KC Matt]

Absolutely agree that you need dampers on the return side. They don't need to be powered though. I found some Dayton vertical install dampers that work great and have a spring to adjust the tension. They open with minimal air flow from the air handler on the furnace, and close when the HC kicks on. The interlock makes sure that both blowers don't turn on at the same time. Thankfully this makes for a simplified install. Although I really like the configuration you have. If I had to go that route I would probably get an arduino board setup with relays to control everything. I have done a couple projects like that. I have an arduino that controls a solid state relay for my beer brewing electric setup :D.

I just googled arduino board and immediately recognized it's a better solution than I have and also above my abilities. If mine ever gets reconfigured I'll look into those more. There are so many improvements I could make to my system if I knew how to use something like this rather than simple relays. Thanks for the suggestion.