Combustion Air Question - Conroe TX

[Cowboy81]

Quick Scernio - We have 8 furnaces in an equipment closet that are on one solid horseshoe platform. I need to know if according to code my combustion air has to be within 12" from the hard floor, or 12" within 12" of the platform. I know that in a normal closet (reach in type with nothing but the furnace in it) the answer would be within 12" of the platform, because there is no hard floor, but my GC is saying that I have to bring it down to the floor. I have already passed all my local inspections, but have to have a state inspection because of the building type (retirement center). Any insite would be much appreciated because I don't have my code books with me (if possible send where in what code book your answer is based on.)

[Jerry Peck - Codeman]

Your question:

I need to know if according to code my combustion air has to be within 12" from the hard floor, or 12" within 12" of the platform.

Confirming the above scenario and expanding on it: I am making a presumption that all air is being taken from outdoors based on your stated scenario above.

Your statement of other conditions:

I know that in a normal closet (reach in type with nothing but the furnace in it) the answer would be within 12" of the platform, because there is no hard floor,

That is because the platform the furnace is setting on is "the floor" of that closet.

but my GC is saying that I have to bring it down to the floor.

Your GC is correct as the combustion air opening is required to be within 12 inches of "the floor". This is because the code states the following:
- From the 2006 IMC (International Mechanical Code) - note that in Texas various AHJ may be using different editions, with the 2006 International Codes adopted for statewide application where there is no local AHJ or the local AHJ has not adopted a different code, yes, that makes "the correct" answer more complicated but the following should be correct with very little, if any, differences between editions. (underlining and bold are mine)
- SECTION 703
- - OUTDOOR AIR
- - - 703.1 All air from the outdoors. Where all combustion and dilution air is to be provided by outdoor air, the required combustion and dilution air shall be obtained by opening the room to the outdoors. Openings connecting the room to the outdoor air shall comply with Sections 703.1.1 through 703.1.4.
- - - - 703.1.1 Number and location of openings. Two openings shall be provided, one within 1 foot (305 mm) of the ceiling of the room and one within 1 foot (305 mm) of the floor. (Jerry's note: Read this as "of the floor of the room" as stated for the ceiling. "The room" is being opened to the outdoors, not "the space" where the furnace is located.)
- - - - 703.1.2 Size of direct openings. The net free area of each direct opening to the outdoors, calculated in accordance with Section 709, shall be a minimum of 1 square inch per 4,000 Btu/h (550 mm2/kW) of combined input rating of the fuel-burning appliances drawing combustion and dilution air from the room.
- - - - 703.1.3 Size of horizontal openings. The net free area of each opening, calculated in accordance with Section 709 and connected to the outdoors through a horizontal duct, shall be a minimum of 1 square inch per 2,000 Btu/h (1100 mm2/kW) of combined input rating of the fuel-burning appliances drawing combustion and dilution air from the room. The cross-sectional area of the duct shall be equal to or greater than the required size of the opening.
- - - - 703.1.4 Size of vertical openings. The net free area of each opening, calculated in accordance with Section 709 and connected to the outdoors through a vertical duct, shall be a minimum of 1 square inch per 4,000 Btu/h (550 mm2/kW) of combined input rating of the fuel-burning appliances drawing combustion and dilution air from the room. The cross-sectional area of the duct shall be equal to or greater than the required size of the opening. (Jerry's note: This is the section you would apply as you are referring to vertical ducts.)

To determine the size of each opening you would need to take the total Btu/hr input rating of all 8 furnaces and divide by 4,000 to find how many square inches each opening needs to be. Let's presume an example of 8 150,000 Btu/hr input rated furnaces:
150,000 X 8 = 1,200,000 / 4,000 = 300 square inches each opening or a 15 inch by 20 inch opening for each opening
- I keep stressing each opening as some installers calculate it out as each opening then apply it as combined opening area.

Hopefully that clears up the location of the combustion air openings and why -because they are for "the room", which also means "the floor of the room".

I am also contacting a friend in Texas for additional input.

[Nolan Kienitz]

Just for the sake of additional information I would ask your GC for the precise basis of his comment to you. I'm not saying he is wrong, but rather so you can both be on the same song-sheet about what is being discussed.

Based upon what I've read the combustion air vent would have to be at the hard-floor level. As Jerry noted in a typical closet there is air coming into the space below the deck the units are sitting on as the space below the deck is often a return air plenum chamber.

If you have passed all local code inspections from City of Conroe and you have yet to pass a state inspection ... I would endeavor to find out precisely what they will be inspecting for (in advance) so that you and the GC can be sure everyone is on the same page.

I am also an ICC-certified code inspector and agree with Jerry and your GC that the vent needs to be at ground level.

Again ... communication is key. Get with your GC to learn where he is coming from. Is it experience or has a code inspector mentioned something? I'm surprised that your GC is saying this needs to occur, yet you indicated you have passed all local code inspections.

Did someone miss something or is the state inspection different?

[Cowboy81]

Hey gentlemen, Thanks for your input in this matter. We found a way to get our combustion air all the way to the floor - Not exactly how we wanted to do it, but it will work..


Brandon