# IMC 403.2.1 Recirculation of Air



## RobbyO (Mar 31, 2016)

I have an existing building retrofit project where mechanical equipment serving the primary space is located in an ancillary space adjacent to the primary, with the ceiling of the ancillary space used as a return air plenum. The ancillary space has an ACT ceiling with no intentional openings between the return air plenum and the space below. So the plenum above the ancillary space is serving as a part of the duct system for the primary space.

As far as the IMC/IBC is concerned for occupancies, is the plenum in the ancillary space a part of the primary space or the ancillary space? The definition of a plenum states: 
*PLENUM. *An enclosed portion of the building structure, other than an _occupiable space_ being conditioned, that is designed to allow air movement, and thereby serve as part of an air distribution system.

Based on this definition, it seems as if a plenum is separate from the space it is located in.

Can the plenum above the ancillary space be considered part of the primary space with regard to occupancy and the other restrictions in section 403.2.1 for air recirculation?


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## JBI (Mar 31, 2016)

RobbyO - Welcome to the forum. The 'ancillary' space would either be considered 'incidental' or 'accessory' in most cases. Those are terms which are used in the Code(s).

Incidental Spaces are very specific and found in Chapter 5 of the IBC. There is a table for them that is 'all inclusive', as opposed to the 'include but not limited to' language used elsewhere. Accessory spaces are limited first by the area of the story on which they are located (10% or less aggregate). Accessory uses are also found in Chapter 5 of the IBC.

It sounds like your 'ancillary' space is an equipment room, which is typically an 'accessory' space unless the equipment rises to the level of the incidental use table.

More simply, it sounds like your 'ancillary' space is part of the primary occupancy.


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## steveray (Mar 31, 2016)

Are you asking if the plenum counts as outside air?


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## RobbyO (Mar 31, 2016)

JBI, unfortunately its a bit more complicated than simply an accessory mechanical equipment room.  What we have is mechanical equipment over a kitchen, which returns air from a conference space adjacent to the kitchen.  The partitions are full height between the two occupancies and the walls are rated (all openings currently have fire dampers).

The question is mainly with regard to IMC 403.2.1 item 3 which prevents recirculation of air from a space which has footnote b in Table 403.3. IMC 403.2.1 states: 
 *403.2.1 Recirculation of air. *

				The outdoor air required by Section 403.3 shall not be recirculated. Air in excess of that required by Section 403.3 shall not be prohibited from being recirculated as a component of supply air to building spaces, except that: 			  			 				1. Ventilation air shall not be recirculated from one _dwelling _to another or to dissimilar occupancies. 			  			 			  			 				2. Supply air to a swimming pool and associated deck areas shall not be recirculated unless such air is dehumidified to maintain the relative humidity of the area at 60 percent or less. Air from this area shall not be recirculated to other spaces where more than 10 percent of the resulting supply airstream consists of air recirculated from these spaces. 			  			 			  			 				3. Where mechanical exhaust is required by Note b in Table 403.3, recirculation of air from such spaces shall be prohibited. All air supplied to such spaces shall be exhausted, including any air in excess of that required by Table 403.3. 			  			 			  			 				4. Where mechanical exhaust is required by Note g in Table 403.3, mechanical exhaust is required and recirculation is prohibited where more than 10 percent of the resulting supply airstream consists of air recirculated from these spaces 			 			

The kitchen is equipped with its own dedicated exhaust and makeup air system, which exhausts all air supplied to the space. I believe where we might have an issue is that the return air from the meeting rooms is being introduced into the ceiling of the kitchen (is this considered "air supplied" to the kitchen?).

Thus my question about whether or not the plenum is a part of the kitchen or the conference space.  Based on the definition of a Plenum, it seems there may be some distinction between the plenum and the space it is located in.  The plenum is clearly enclosed on all sides by partitions that enclose the kitchen, but no air is transferred between the kitchen and the plenum, and it is serving as part of the duct system for the conference room.

Steve - No, that's not an issue.  We have a separate outside air system that provides ventilation air to the heat pumps.


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## JBI (Mar 31, 2016)

Your Op was much more vague about the 'ancillary' space...


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## RobbyO (Mar 31, 2016)

Correct, I was initially trying to get a response on how to treat plenums that span across multiple occupancies/spaces.


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## steveray (Mar 31, 2016)

If the plenum is not open to the kitchen it is not circulating air there IMO...


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## Gregg Harris (Mar 31, 2016)

I would agree


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## Paul Sweet (Apr 1, 2016)

I have seen very few acoustic ceilings that are airtight, so plenum air can mix with room air to a limited degree.  On the other hand, when the exhaust system is on the kitchen is probably under negative pressure, so much of your plenum air may end up being exhausted instead of returned to the air handler.


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## north star (Apr 2, 2016)

*& & &*

Gregg Harris,

Long time no see...

Good to see you back here ! 

*& & &*


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## klarenbeek (Apr 4, 2016)

Which version of the code are you under? 2012 IMC section 918.6 and previous editions prohibited taking return air from places with objectionable odors for forced air furnaces.  Like Paul sweet said, very few acoustical ceilings are airtight.  Even if there are no return openings in the ceiling, the kitchen smells will get in the airstream. But, it would only apply to a forced air furnace if you are under the 2012 or older codes. That section was moved to the generic duct section in the 2015, but the objectionable odors part was removed.  If it is a fuel-fired appliance you are replacing, the question is moot because fuel fired appliance can't be in a plenum, period.

However, new to the 2015 IMC, 602.1 requires air systems to be ducted from the boundary of the fire area served directly to the air handling equipment.  If you are under the 2015 IMC, based on what you said about the partitions being fire rated, the unit will have to be ducted from the dampered openings to the unit if this was a new install.  Being a retrofit, the IEBC might come into play depending on how extensive the work being done is


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