Back Flow Dampers

[Ohio Dave]

Do backflow dampers have to be automatic (motorized) or can they be manual? Where are some good places to buy them from and good manufactures. I will need one to go between the fuel oil furnace and where I duct in the wood furnace. My old set up didn't have one.

Thanks
Dave

 

[Whitespider]

They don't have to be motorized to be automatic... a simple flapper door that opens and closes from air flow would be non-motorized yet automatic (i.e., when the fuel oil furnace blows the flapper would blow open, when the wood furnace blows the flapper would blow closed).
You can find some on-line, but the odds of finding an "exact fit" will likely be long, just to many variables... most are fabricated on-site. You could probably take measurements and go to any HVAC installer and have one made.

Here's the one I made for my gas furnace when I installed the wood furnace... mounted just above the heat exchanger.
Mine opens by air flow if the gas furnace blower starts up, closes by gravity. I installed a similar one on the wood furnace blower to cancel any back flow through wood furnace if the gas furnace blower runs. With my setup both can be running at the same time... the "flappers" just equalize where no back flow happens in either direction.

 

[Whitespider]

 

[anlrolfe]

Ah, I see. I was thinking this was asking about flue gas/smoke.
Do you have to do anything special with the supply side/cold air return?

 

[Whitespider]

I'll address this in the AM... when less whiskey is involved.
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[fordf150]

Do a search....There was a pretty in depth thread that talked about this last fall I think. I'm on a phone that is just to slow to search and post a link to it

 

[Whitespider]

Do you have to do anything special with the supply side/cold air return?

When two or more forced air heating/cooling appliances, with separate blowers, are connected to the same plenum/ducting and supply/return each should have a backflow/backdraft damper. The purpose of the dampers is to block the air from flowing through the non-active appliance(s) in a reverse direction. When/if that happens the air won't flow to the room vents, it just circulates in a short path through the non-active appliance(s) which may/can overheat and damage the active appliance... and the area you're trying to heat/cool receives zero or reduced heated/cooled air. The forced (pressurized) air, or at least the bulk of it, will take the path of least resistance... normally the shortest path depending. No matter what path is the least resistance, without backflow dampers some air will still reverse flow through the non-active appliance... reducing the active appliance efficiency and effectiveness.

Because it's blocking the "reverse flow through" the damper can (technically) be placed on either the plenum or return side... but only one per appliance is required. If the reverse flow is blocked... the flow is blocked... it don't matter where the blockage is. In my system the gas furnace backflow damper is on top of the heat exchanger inside the plenum, and the wood furnace backflow damper is under the heat exchanger built into the blower housing. Both open by air flow (pressure), and both close by gravity. Meaning, if neither appliance is active, both dampers are closed waiting to be opened by their respective blower... and, both appliances can be active at the same time if wanted or required.
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[fordf150]

http://www.arboristsite.com/community/threads/wood-furnace-to-duct.265430/ here is one of the threads i was thinking of.