Combustion air from ceiling space

[Mech]

2018 IFGC - New construction.

To utilize indoor combustion air, there must be 50 cu.ft. of space volume for every 1,000 BTU/hr input rating. (304.5.1)

Can the ceiling be omitted in a janitor closet to utilize the air space between the first floor office area ceiling and the second floor deck as combustion air?



Thanks in advance.

 

[classicT]

Interesting question. Couple thoughts come to mind.

First, I am not sure the proposed would comply with the high/low opening requirement of 304.5.3.1.

PA-FGC (2018 IFGC) 304.5.3.1 Combining Spaces on the Same Story​

Where combining spaces on the same story, each opening shall have a minimum free area of 1 square inch per 1,000 Btu/h (2200 mm2/kW) of the total input rating of all appliances in the space, but not less than 100 square inches (0.06 m2). One permanent opening shall commence within 12 inches (305 mm) of the top and one permanent opening shall commence within 12 inches (305 mm) of the bottom of the enclosure. The minimum dimension of air openings shall be not less than 3 inches (76 mm).​

Second, I would be concerned that you are creating a plenum. While not using forced air, the intent to draw air through this concealed ceiling space for use as combustion air is unique.

 

[Yankee Chronicler]

You're asking if you can use fir first floor ceiling plenum as the source for combustion air? That only gives you an opening at the top -- how do you fulfill the opening at the bottom?

How big is the building? Is it all one tenant/occupant space, or are there multiple tenants, with full-height walls separating them?

Where's the furnace and/or water heater you need combustion air for?

 

[klarenbeek]

Like everyone else said, you don't have the bottom opening. Both the bottom and top openings need to open into the same adjacent space, so not possible to use the space above the lid. To use the one opening from a different level, you would have to go all the way through to pull air from the floor above. Even if you could count the space above the lid but below the floor, it would have to be a VERY large area to provide any meaningful amount of combustion air, depending on how tall that space is.

 

[Yankee Chronicler]

Interesting question. Couple thoughts come to mind.

. . .

Second, I would be concerned that you are creating a plenum. While not using forced air, the intent to draw air through this concealed ceiling space for use as combustion air is unique.

If there's a drop ceiling, this proposal isn't creating a plenum, it's trying to use the ceiling plenum as the source for combustion air. Aside from the high/low openings issue, the next question is whether both supply and return air are ducted, or is the ceiling plenum used as a plenum return. If it's a plenum return, then opening the janitor's closet ceiling to the plenum would result in the return air system pulling air out of the janitor's closet rather than introducing combustion air into it. And I don't think you would want the air from a janitor's closet to be recirculated, anyway (although I guess it's not prohibited by the IMC). Even if it's allowed, it's going to mess up the system balancing.

 

[Mech]

I do not think Section 304.5.3.1 will not work either as there is no low opening. However, there is no high wall opening either; the ceiling would be removed and airflow would be vertical, not horizontal.

It seems more suitable, to me, to consider Section 304.5.3.2 Combining spaces in different stories as the ceiling in the janitor's closet (water heater location) would be removed and the 4+ ft mechanical space above the ceiling would be the space from which combustion air is drawn. The ceiling space is part of the same story, not another story, so perhaps this section does not apply. The commentary is silent about utilizing the ceiling space.

I am hesitant about utilizing the ceiling space. As classicT pointed out, this could be a plenum and part of me wonders if the wiring and piping above the ceiling would then need to be plenum rated.

I had suggested water heaters with combustion air intake capabilities. The GC/Architect countered by asking me about utilizing the ceiling space for combustion air.

The building is a car dealership. The water heater is in a janitor closet accessed from an unenclosed corridor/hallway/passageway that serves the janitor closet and two toilet rooms. The corridor is open on both ends. Its only function appears to be that it hides these three rooms from the showroom. I could theoretically place grilles high and low into this corridor, but then the IBC and IMC prohibit using a corridor as a supply, return, exhaust, relief or ventilation ducts unless outside air is supplied to the corridor at a rate greater than makeup air taken from the corridor.

Only 3,300 cu ft of space is required for combustion air. The volume above the ceiling provides more than this amount. The showroom easily exceeds 120,000 cu ft, but the janitor closet is not immediately adjacent to the showroom.

I may suggest a small 25 cfm fan with ductwork to take air from the showroom and dump it into the janitor closet when the water heater burner is active.

 

[Mech]

The ceiling is not a return air plenum. The office spaces utilize drop ceiling tiles. The two toilet rooms border the janitor closet on three sides and the toilet rooms utilize gyp bd ceilings. The corridor on the 4th janitor closet wall utilizes a drop ceiling.

 

[ICE]

I seldom know what code applies to any given location location. The question posed by the OP is whethyer a single combustion air opening to an appliance is possible. It is the upper opening that the OP wants to use and take the combustion air from a space that is between a eiling and a floor above the ceiling. This is California Mechanical Code:

701.6.2 One Permanent Opening Method. One permanent opening, commencing within 12 inches (305mm) of the top of the enclosure, shall be provided. The appliance shall have clearances of at least 1 inch (25.4mm) from the sides and back and 6 inches (152 mm)from the front of the appliance. The opening shall directly communicate with the outdoors or shall communicate through a vertical or horizontal duct to the out-doors or spaces that freely communicate with the outdoors (see Figure 701.6.2) and shall have a minimum free area of the following:

(1) One square inch per 3000 Btu/h (0.0007 m2/kW) of the total input rating of all appliances located in the enclosure.
(2) Not less than the sum of the areas of all vent connectors in the space. [NFPA 54:9.3.3.2]

Apparently a bunch of other codes would not allow this. I say apparently because I am often wrong and just plain confused.

 

[Yankee Chronicler]

It seems more suitable, to me, to consider Section 304.5.3.2 Combining spaces in different stories as the ceiling in the janitor's closet (water heater location) would be removed and the 4+ ft mechanical space above the ceiling would be the space from which combustion air is drawn. The ceiling space is part of the same story, not another story, so perhaps this section does not apply. The commentary is silent about utilizing the ceiling space.

Since the ceiling cavity is not a different story, there's no "perhaps" involved. The section discussing combining spaces in two stories clearly does not apply.

What's your role, if I may ask? You have mentioned an architect, so you are apparently not the architect. Are you the code official? If so, I would tread very lightly when it comes to suggesting how to comply with the code. Building Officials have to be very careful about that, because all too often we may "suggest" something as one way of complying, then a building owner goes out and does what we suggested -- sort of, but in a half-assed way that doesn't meet code -- and then when we reject it they complain that they did what we "told them to do" and then we still wouldn't accept it.

 

[Yankee Chronicler]

The ceiling is not a return air plenum. The office spaces utilize drop ceiling tiles. The two toilet rooms border the janitor closet on three sides and the toilet rooms utilize gyp bd ceilings. The corridor on the 4th janitor closet wall utilizes a drop ceiling.

So a car dealership uses a ducted return air system? I'm amazed.

 

[klarenbeek]

You say that you can't take combustion air from the "corridor" because the IMC prohibits it, but that code section addresses "air movement in egress elements". From your description, it sounds like the hallway is basically a short wall that was put up to hide the doors to the closet and restrooms? Not sure that would be considered an egress element at that point, its providing "egress" from the showroom to the showroom. Also, if it opens to the full showroom on both ends its for all intents and purposes part of the showroom. The showroom should be getting supplied with plenty of occupancy air that will be more than sufficient for combustion air.

 

[Mech]

Yankee Chronicle: My company is reviewing someone else's design and then signing and sealing the plans. I am trying to accommodate their design if permissible by code. I am asking questions because I will be asked these same questions and I want to definitively tell the client yes, no, or I do not know but it does not sound like a good idea.

One of our clients specifically does not want plenum ceilings in their non-dealership buildings because they do not want to deal with ratings for plenums.

klarenbeek: Thank you for your corridor insight. I still do not have my head wrapped around corridors and that section. For some reason it just does not click in my head. And yes, there is plenty of space volume and outside air being supplied to the showroom, lounge, and retail parts areas that are all open to each other.