Horizontal Furnace - How long for supply air plenum?

[612Rob]

I was looking at the possibility of mounting a high efficiency furnace horizontally in a garage as opposed to a unit heater at a customers request. I've heard guys talk about a minimum length for the supply air plenum but do not see anything in the manual for the unit I was considering.

Is there a standard minimum and what are the reasons?

[cuchulain]

Depends on the number of ducts coming off of it, as well as available room. The more ducts the longer the plenum. Ours are a min 4' long, I know the pre-fabricated ones the supply houses carry are only 3'.

[shaka]

We leave at least a foot at the end
To build static and more balance output.

[gravity]

i can speak for rheem. they want at least an 18" plenum before any bends, takeoffs, etc...

turning vanes and proper take offs are the only way to go.