Florida Building Code, Mechanical 601.4

[RobShepp]

Jerry,
Tom Glynn and I are having a discussion on the FBC, Mechanical 601.4, and we can't seem to agree on what the code is calling for. For fairness, I'll go ahead and say this is my interpretation. Hopefully Tom will come on and give his side.

2010 Florida Building Code, Mechanical 601.4 Balanced Return Air.
Restricted return air occurs in buildings when returns are located in central zones and closed interior doors impede air flow to the return grill or when ceiling spaces are used as return plenums and fire walls restrict air movement from one portion of the return plenum to another. Provisions shall be made in both residential and commercial buildings to avoid unbalanced air flows and pressure differentials caused by restricted return air. Pressure differentials across closed doors where returns are centrally located shall be limited to 0.01 inch WC (2.5 pascals) or less. Pressure differentials across fire walls in ceiling space plenums shall be limited to 0.01 inch WC (2.5 pascals) by providing air duct pathways or air transfer pathways from the high pressure zone to the low zone.

Exceptions:

1. Transfer ducts may achieve this by increasing the return transfer 11/2 times the cross sectional area (square inches) of the supply duct entering the room or space it is serving and the door having at least an unrestricted 1 inch (25.4 mm) undercut to achieve proper return air balance.

2. Transfer grilles shall use 50 square inches (322.6 cm2) (of grille area) to 100 cfm (of supply air) for sizing through-the-wall transfer grilles and using an unrestricted 1 inch (25.4 mm) undercutting of doors to achieve proper return air balance.

3. Habitable rooms only shall be required to meet these requirements for proper balanced return air excluding bathrooms, closets, storage rooms and laundry rooms, except that all supply air into the master suite shall be inlcuded.

Research has determined that closed doors have a Pascal rating of 7+, the code specifically states that undercutting of the door alone is not the remedy. The code requires undercutting of the doors and vents or return ducts to satisfy the code requirement. This came into question because I was inspecting a new townhome with a central located Air Handler, bedrooms had ductwork from the rooms to the hallway ceiling between the two bedrooms. The front bedroom had a grate over the door leading to the hallway, in both cases the door were between 1/2" and 3/4" undercut. The code does not say this satisfies the requirement, the code says if the return is central located and the return air is obstructed by doors, both undercutting of the doors by 1" AND return vents or ducts are required.

[Jerry Peck - Codeman]

[RobShepp]

I'll simplify my question. In a new structure that has return ducts/through vents(not leading back to the Air Handler, but the air space the handler is located in) in place to compensate for a central located Air Handler, is it not required to have a 1" undercut at the doors along with the duct/vents? At the point the return ducts/vents were installed, the exemption in the code calls for both. I didn't make the determination they were required, the Architect did. And once he did that, the undercut of the doors was required also. What we are seeing in the field is the return ducts/vents as desiged by the Architect...then the builder never undercuts the doors because he has no idea it's required.

[Jerry Peck - Codeman]