Connecting Two Forced Air Systems - http://www.hvacmechanic.com/ Forums
Original message
| dougseals
 |  "Connecting Two Forced Air Systems"
, posted Wed 8 Oct 15:48  
This is bound to be a rather unique situation, and I hope I can describe it properly.
I have two separate forced air furnace systems in my house. I have a larger dual fuel (oil/wood) furnace that I installed a few years ago to replace an old oil unit, which is more than enough to heat the 2/3 of the house it's ducted to. The bedrooms in the back of the house are heated by a separate furnace with the appropriate supply/return ducts.
Since the wood unit is more than capable of heating the entire house, in order to save money on fuel this winter, I'm advocating connecting it to the second system to try to heat everything.
The unique construction of the house is such that the only easy solution would be to install a hot air supply duct from the wood furnace and connect it to the cold air return duct of the second unit. Then, I need to run a wire/relay between the two units, so that when the wood furnace fan turns on, the second furnace's fan also turns on, in order to draw the warm supply air into the second unit's return ductwork. Essentially, I'm hoping to use the entire second system to take a little heat from the first unit and warm the back of the house.
Does this sound like it should work? If so, the only thing I'm still unsure of is the wiring between the two furnaces.
Thanks for the help.
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| dougseals
| "Re(1):Connecting Two Forced Air Systems" , posted Tue 21 Oct 12:23
Just bumping this to the top, along with an update.
I attached a 6" round insulated duct from the wood supply duct on the one furnace, and ran that into the cold air supply of the second furnace. Just to see if it's even close to working, I just manually run the fan on the second unit when the wood fire is burning, and it definitely helps to heat the back of the house. Of course, it's not that cold outside yet, so I may still run a second 6" duct, and well as possibly installing an inline fan into one of these.
What I really need help with now is the wiring. I want to second furnace blower fan to turn on whenever the first unit's blower runs. Any ideas on just how I could accomplish that?
Thanks for the help.
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