# Plywood heating duct ?



## willem (Jan 5, 2007)

All references to heating duct materials I have come across use either sheet metal, fiberboard or flex duct, although I have never seen a building code specifying duct materials explicitly.

Is there anything in the building codes that prohibits the use plywood for heating ducts ?


----------



## trollmastergeneral (Dec 19, 2006)

You can use floor joists for cold air return.this very common. When it comes to supply air there is a requirements. materials must be "ul aproved ". there are thickness requirements. The man will keep you down :no: by saying it is a "combustable material"


----------



## joasis (May 28, 2006)

I have done remodels that indeed used floor joists for supply and returns....not something I would do. We ended up placing HE equipment and flex duct.


----------



## willem (Jan 5, 2007)

I understood that UL 181 deals with requirements for ducts.
One of the issues is flame spread and smoke development.

Plywood would not qualify as it has a flame spread index > 20 (the limit for class I).

Does anyone know whether the flame spread requirements apply to the inside alone or both inside and outside of a duct ?

Would it be OK to line a plywood duct with sheet metal ?

Some flexible ducts are lined with plastic, wouldn't this raise issues with smoke / flame spread ?


----------



## #CARRIERMAN (Oct 18, 2006)

Hi willem

You asked kind of a sticky question there. The flame spread rating on any material is a basic burn through or volitility rating. This is where they come up with the fire ratings from. There is more to do with it than this, but that is the basic skinny on it. As far as linning the plywood, I would not reccomend anything but sheet metal. But that would just be a waste of plywood unless you are using this area for storage and are adding the plywood to keep from destroying the duct. Sheet metal duct is more of a permanent structure than the other forms of ductwork. But keep in mind everything has a usable life.

Good luck
Rusty


----------



## troubleseeker (Sep 25, 2006)

willem said:


> All references to heating duct materials I have come across use either sheet metal, fiberboard or flex duct, although I have never seen a building code specifying duct materials explicitly.
> 
> Is there anything in the building codes that prohibits the use plywood for heating ducts ?


It's not the plywood specifically that is prohibited, it is flammable materials with specified flame spread rates, and plywood with all of its glue certainly is one of those materials. You could line it with sheet metal and seal all the joints, but why not just use proper ductwork? Using floor/wall cavities is a extremely inefficient thing for either supply or return. You will be paying for the pennies saved now with dollars to operate your system more than necessary for many years in the future.


----------



## diywoman (Jan 4, 2007)

A quick interjection...I have my duct work coming along the underside of my floor joist..I was tracing it the other night and found that my return was just open to above the ducting and a hole has been cut into the top of the duct work to allow for the air. I thought this looked ridiculous, but are you saying that it's common?:confused1: I'm trying to find ways to make my heating more efficient. My gas bill is ticking me off. Personally I don't like furnaces because they heat areas equally. I prefer to be able to control each room separately. When I'm asleep at night I don't want my kitchen and living room the same temp as the bedrooms...argh. So anyways, Any helpful hints would be wonderful. "If life were like that" I'd have my whole system set-up automatic...so that at night the vents in the living room and kitchen would close partially to allow less air through, and vice versa when needed. But genious ideas always cost $$$ so now I'm back to the real world and will continue to seal up cracks, windows and all that crap.
:tank: Fort Knox this baby!!

DIYwoman


----------



## Gredal (Sep 29, 2010)

*Plywood ducts would allow mold to grow*

I believe one problem with plywood that no one has mentioned is the possibility of mold growth in plywood. I have take lots of ducts apart and you see lots of signs of moisture and mold from wet dust bunnies...I would expect this to be worse with wood. The cold air returns that I have seen use metal for the horizontal surfaces (between joists) this would help with the moisture.


----------



## hvaclover (Oct 2, 2008)

it's plywood! 
The furnace can burn wood.

Wouldn't a simple "no" have sufficed?:huh:


----------



## beenthere (Oct 11, 2008)

Well maybe 3 years ago when this thread was started. But not today. :whistling2:


----------



## hvaclover (Oct 2, 2008)

OK, now that I've been had, i'm gonna a grouch all night not that anybody would notice the difference.


----------

