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plywood distribution box.

Jim Baird

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Every house shows me something I have never seen.

This one showed me a plywood section of an air handling system.

A box right past the supply end of the unit that has ducts mounted on three of its faces.

Site-built ingenuity, no?

Is there any way that plywood could pass muster on flame spread, smoke development, etc?

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OK Brandon, Here's the spooky subject. I think it's treated plywood.

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It is built below the floor level attached to this unit, which, by the way, has no secondary condensate line. It's return end is up, with a return grille on the wall above a monster jet tub.

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I didn't bother to look it up but the first question that comes to my mind is why would the presence of a plywood plenum connected to an electric forced air heating system that's incorporated into the middle of a flexible duct system, that's also made from flammable materials, be any different than using panned joists for part of the system?

I'm not saying it's logical from a common sense viewpoint, but aren't electric furnaces the exception to the rule for locations relative to bathrooms?



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I did look up the IRC sections on duct materials, which only says that they must meet smoke development and flame-spread testing standards, and does mention that gypsum is OK for use in a plenum.

IRC definitely says that return air should not be taken from bathrooms, utility spaces, furnace rooms etc.

I did not research this unit's instructions, but do know it was installed by the owner/builder and did not get code review. To me it looked like a "creative" adaptation. I have seen a few up/down flow units but this one looked to me like it should have been installed horizontally with a pan beneath it.

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M1601.1 Duct design. Duct systems serving heating, cooling

and ventilation equipment shall be fabricated in accordance

with the provisions of this section and ACCA Manual D or

other approved methods.

M1601.1.1 Above-ground duct systems. Above-ground

duct systems shall conform to the following:

1. Equipment connected to duct systems shall be

designed to limit discharge air temperature to a maximum

of 250°F (121°C).

2. Factory-made air ducts shall be constructed of Class 0

or Class 1 materials as designated in Table


3. Fibrous duct construction shall conform to the

SMACNA Fibrous Glass Duct Construction Standards

or NAIMA Fibrous Glass Duct Construction


4. Minimum thickness of metal duct material shall be as

listed in Table M1601.1.1(2). Galvanized steel shall

conform to ASTM A 653.

5. Use of gypsum products to construct return air ducts

or plenums is permitted, provided that the air temperature

does not exceed 125°F (52°C) and exposed surfaces

are not subject to condensation.

6. Duct systems shall be constructed of materials having

a flame spread index not greater than 200.

7. Stud wall cavities and the spaces between solid floor

joists to be used as air plenums shall comply with the

following conditions:

7.1. These cavities or spaces shall not be used as a

plenum for supply air.

7.2. These cavities or spaces shall not be part of a

required fire-resistance-rated assembly.

7.3. Stud wall cavities shall not convey air from

more than one floor level.

7.4. Stud wall cavities and joist-space plenums

shall be isolated from adjacent concealed

spaces by tight-fitting fire blocking in accordance

with Section R602.8.



M1602.1 Return air. Return air shall be taken from inside the

dwelling. Dilution of return air with outdoor air shall be


M1602.2 Prohibited sources. Outdoor and return air for a

forced-air heating or cooling system shall not be taken from the

following locations:

1. Closer than 10 feet (3048 mm) to an appliance vent outlet,

a vent opening from a plumbing drainage system or

the discharge outlet of an exhaust fan, unless the outlet is

3 feet (914 mm) above the outside air inlet.

2. Where flammable vapors are present; or where located

less than 10 feet (3048 mm) above the surface of any

abutting public way or driveway; or where located at

grade level by a sidewalk, street, alley or driveway.

3. Aroom or space, the volume of which is less than 25 percent

of the entire volume served by such system. Where

connected by a permanent opening having an area sized

in accordance with ACCA Manual D, adjoining rooms

or spaces shall be considered as a single room or space

for the purpose of determining the volume of such rooms

or spaces.

Exception: The minimum volume requirement shall

not apply where the amount of return air taken from a

room or space is less than or equal to the amount of

supply air delivered to such room or space.

4. A closet, bathroom, toilet room, kitchen, garage,

mechanical room, furnace room or other dwelling unit.

5. A room or space containing a fuel-burning appliance

where such room or space serves as the sole source of

return air.


1. The fuel-burning appliance is a direct-vent

appliance or an appliance not requiring a vent in

accordance with Section M1801.1 or Chapter


2. The room or space complies with the following


2.1. The return air shall be taken from a

room or space having a volume exceeding

1 cubic foot for each 10 Btu/h (9.6

L/W) of combined input rating of all

fuel-burning appliances therein.

2.2. The volume of supply air discharged

back into the same space shall be

approximately equal to the volume of

return air taken from the space.

2.3. Return-air inlets shall not be located

within 10 feet (3048 mm) of any appliance

firebox or draft hood in the same

room or space.

3. Rooms or spaces containing solid-fuel burning

appliances, provided that return-air inlets are

located not less than 10 feet (3048 mm) from

the firebox of such appliances.

M1602.3 Inlet opening protection. Outdoor air inlets shall be

covered with screens having openings that are not less than

1/4-inch (6 mm) and not greater than 1/2-inch (12.7 mm).



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