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]

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.

Rob,

Actually, the code does not require undercutting of the doors.

First, though, some questions:
- What grille opening sizes do you get when you measure them?
- What supply air cfm do you get when you measure them?
- When you find transfer ducts, how do you check them and against what do you check them?
- When you find transfer grilles, how do you check them and are they just grilles over openings through the wall and open to the inside of the wall stud cavities or is there something through the wall connecting one transfer grille to the other transfer grille?

The Residential code is the code you should be using for townhouses, not the Mechanical Code ... although it does not matter regarding the discussion as the requirements are the same in both, the proper code matters when writing things up for contractors, owners, and the like.

[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]

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.

"is it not required to have a 1" undercut at the doors along with the duct/vents?"

I'll simply my answer "the code does not require undercutting of the doors."

The Architect can require whatever the Architect wants to require. Once the Architect requires something and it is part of the approved documents, then it is required until and unless the Architect changes what they want - but that is not the same thing as saying that the code requires undercutting doors ... because the code does not require undercutting doors.

Just like the code does not require outswing exterior doors on dwelling units, however, if the architect draws and specifies outswing exterior doors, then that is the the approved documents require ... until the architect decides on something different.

One should not state that "the code requires" something just because an architect requires it on a specific project or a specific location in a specific project - "the architect" designed it and requires it ... the AHJ simply reviewed and approved what the architect specified, and that becomes part of the approved documents, and the construction shall match the approved documents - or - revised documents shall be submitted for review and approval.

The short answer is that the code does not require undercutting doors.

The long answer is that the code does not require undercutting doors ... unless that is part and parcel of the approved documents and/or part of an exception being used to get by with/without doing something else.