Jim Baird Posted October 30, 2009 Report Posted October 30, 2009 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?
Brandon Whitmore Posted October 31, 2009 Report Posted October 31, 2009 Jim, Seeing as how we're close to Halloween, a scary picture is a must. To help prevent flame spread, I hope they added a gas / diesel mixture.
Jim Baird Posted October 31, 2009 Author Report Posted October 31, 2009 OK Brandon, Here's the spooky subject. I think it's treated plywood. Click to Enlarge 57.63 KB 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. Click to Enlarge 46.86 KB
hausdok Posted October 31, 2009 Report Posted October 31, 2009 Hi, 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? ONE TEAM - ONE FIGHT!!! Mike
Jim Baird Posted November 1, 2009 Author Report Posted November 1, 2009 Mike, 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.
Jim Baird Posted November 1, 2009 Author Report Posted November 1, 2009 PS, Then there is the question of R-value for plywood.
hausdok Posted November 1, 2009 Report Posted November 1, 2009 IRC definitely says that return air should not be taken from bathrooms, utility spaces, furnace rooms etc. Got that citation? I need to add it to my cheat sheet. OT - OF!!! M.
Jim Baird Posted November 1, 2009 Author Report Posted November 1, 2009 SECTION M1601 DUCT CONSTRUCTION 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 M1601.1.1(1). 3. Fibrous duct construction shall conform to the SMACNA Fibrous Glass Duct Construction Standards or NAIMA Fibrous Glass Duct Construction Standards. 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. SECTION M1602 RETURN AIR M1602.1 Return air. Return air shall be taken from inside the dwelling. Dilution of return air with outdoor air shall be permitted. 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. Exceptions: 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 24. 2. The room or space complies with the following requirements: 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). 2006 INTERNATIONAL RESIDENTIAL CODE® 339 DUCT SYSTEMS
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