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TI Remodel: return air requirements

Robert

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Jan 29, 2016
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274
Location
Pinole, CA
I am working on a TI (less than 1000 sq. ft) in which some walls move a couple feet and some office spaces get smaller or larger by 20 sq. ft. or so. No change to overall square footage and no natural ventilation. The existing single zone forced air system only had 2 return air ducts. Will I be required to add return air in each separate office space? Can louvered doors work as an alternative? I'd like to know the CMC section for more information, exceptions, etc. Thank you.
 

mark handler

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Oct 25, 2009
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So. CA
DO YOU WANT THE SYSTEM TO WORK PROPERLY?
Return air is critical for the performance of the HVAC system. Rooms without adequate return air can impede supply airflow due to overpressurization in the room, leading to comfort issues. Every room with a supply outlet must have a clear return air pathway. If not properly designed, Some rooms will be hot and others cold.
 

Robert

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Location
Pinole, CA
I am used to seeing central return air, perhaps one or two in an entire house, with each of the many bedrooms, bathrooms and similar having their own supply. The systems are balanced and comfortable. Is commercial different? A specific code section requiring each space to have its own return air? I do not need fire rated doors with weatherstripping so a louvered door or undercut door could be an option.
 

Paul Sweet

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Oct 17, 2009
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Richmond, VA
A 3/4" undercut will transfer a little over 100 cfm. Offices with more supply air than this will need transfer grilles or return grilles.
 

msjenkin

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Dec 20, 2020
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North Carolina
I am having a similar issue. Is this spelled out explicitly in the mechanical code? I cannot find it in the mechanical code
 

Robert

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Jan 29, 2016
Messages
274
Location
Pinole, CA
I am having a similar issue. Is this spelled out explicitly in the mechanical code? I cannot find it in the mechanical code
I could not find anything in CMC either, but a plan check response required us to do a return in each office. I should have asked for a code section. We ultimately provided "transfer ducts" in the ceiling of each office that shared the hallway/common area return air. Be careful though of noise transfer if offices are sensitive to that.
 

mark handler

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Location
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You did not say what type of TI this was?

OSHPA projects
Louvers are considered as supplemental to the required mechanical ventilation systems.

....and shall not be used to convey supply, return, transfer or exhaust air to or from any room if the corridor is required to be fire resistive construction per the California Building Code.

All supply-air, return air, and exhaust-air systems shall be mechanically operated and such systems for areas listed in Table 4-A shall be operated continuously. Natural ventilation through windows or other openings such as louvers will be considered as supplemental to the required mechanical ventilation systems.

No space above a ceiling may be utilized as an outside-air, relief-air, supply-air, exhaust-air, or return-air plenum.
 

steveray

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Nov 25, 2009
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Location
West of the river CT
IEBC

809.1 Reconfigured or converted spaces. All reconfigured
spaces intended for occupancy and all spaces converted to
habitable or occupiable space in any work area shall be provided
with natural or mechanical ventilation in accordance
with the International Mechanical Code.
Exception: Existing mechanical ventilation systems shall
comply with the requirements of Section 809.2.
809.2 Altered existing systems. In mechanically ventilated
spaces, existing mechanical ventilation systems that are
altered, reconfigured, or extended shall provide not less than
5 cubic feet per minute (cfm) (0.0024 m3/s) per person of outdoor
air and not less than 15 cfm (0.0071 m3/s) of ventilation
air per person; or not less than the amount of ventilation air
determined by the Indoor Air Quality Procedure of ASHRAE
62.

Do the louver doors go into a corridor?

1020.5 Air movement in corridors. Corridors shall not
serve as supply, return, exhaust, relief or ventilation air ducts.
 

Robert

Registered User
Joined
Jan 29, 2016
Messages
274
Location
Pinole, CA
IEBC

809.1 Reconfigured or converted spaces. All reconfigured
spaces intended for occupancy and all spaces converted to
habitable or occupiable space in any work area shall be provided
with natural or mechanical ventilation in accordance
with the International Mechanical Code.
Exception: Existing mechanical ventilation systems shall
comply with the requirements of Section 809.2.
809.2 Altered existing systems. In mechanically ventilated
spaces, existing mechanical ventilation systems that are
altered, reconfigured, or extended shall provide not less than
5 cubic feet per minute (cfm) (0.0024 m3/s) per person of outdoor
air and not less than 15 cfm (0.0071 m3/s) of ventilation
air per person; or not less than the amount of ventilation air
determined by the Indoor Air Quality Procedure of ASHRAE
62.

Do the louver doors go into a corridor?

1020.5 Air movement in corridors. Corridors shall not
serve as supply, return, exhaust, relief or ventilation air ducts.
We didn't end up using louvered doors for sound reasons, but used rigid duct from each office ceiling (suspended ceiling) into the adjacent larger common workspace, which is where the return air was located. But if this space was a corridor, could it be used as return air if it is not fire rated? This was a B occupancy, small enough that only 1 exit was required, hence corridor unrated.
 
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