# Does return air have to equal supply air?



## MikeC (Feb 23, 2015)

As many of you know, I am new to all of this.  I am currently looking at a set of plans where a forced air system is supplying a lobby, and two offices.  All of these spaces share only 1 return air duct directly under the unit and in the lobby.  There are transfer ducts above the ceiling in the walls, but no mention of using this area as a plenum or any way to get the return air through the acoustic ceiling tiles and into that area.

I know that there is mention of pressure equalization for exhaust systems, but I cannot find anything that addresses the situation that I am looking at.  I looked in the energy code also, but couldn't find anything their either.

Can they just pressurize these offices, or do they need some way to return that air back to the system?  It doesn't seem very efficient to have supply with no return.

BTW, 2009 I-codes.


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## mtlogcabin (Feb 23, 2015)

403.1 Ventilation system.

Mechanical ventilation shall be provided by a method of supply air and return or exhaust air. The amount of supply air shall be approximately equal to the amount of return and exhaust air. The system shall not be prohibited from producing negative or positive pressure. The system to convey ventilation air shall be designed and installed in accordance with Chapter 6.


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## markw (Feb 23, 2015)

I think he is looking for forced air systems- IMC 09,sec.918 for return air and follow 602/607


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## MikeC (Feb 23, 2015)

Yes,  Mark is correct.  I just reviewed section 918 and see nothing that prohibits taking return air from just one area in this case.  I am still going to question it because of the transfer ducts above the ceiling between the offices and the lobby.  As of right now I haven't reviewed that area as a plenum.


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## mtlogcabin (Feb 23, 2015)

VENTILATION. The natural or mechanical process of supplying conditioned or unconditioned air to, or removing such air from, any space.

VENTILATION AIR. That portion of supply air that comes from the outside (outdoors), plus any recirculated air that has been treated to maintain the desired quality of air within a designated space.

It does not matter if it is air conditioning, a forced air furnace, heat pumps or anything else. If natural ventilation is not provided then a mechanical ventilation system is. Chapter 4 specifically section 403 is where you find the requirements based on the occupancy

403.7 Balancing.

The ventilation air distribution system shall be provided with means to adjust the system to achieve at least the minimum ventilation airflow rate as required by Sections 403.3 and 403.4. Ventilation systems shall be balanced by an approved method. Such balancing shall verify that the ventilation system is capable of supplying and exhausting the airflow rates required by Sections 403.3 and 403.4.

Difficult to balance a system without returns in each room that has a supply register


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## MikeC (Feb 23, 2015)

Thanks.  I wasn't considering the ventilation air to be conditioned air.  I knew it made sense to equalize the air, but I couldn't figure out where it was required by the code.  I am assuming that they are going to use the space above the dropped ceiling as a plenum, but there is no mention of anything other than acoustic ceiling tiles in the dropped ceiling.


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## tmurray (Feb 24, 2015)

Did they provide a reflected ceiling plan? Usually when they are using a ceiling plenum they put grills in the ceiling.


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## MikeC (Feb 24, 2015)

The only plan that shows the layout of the ceiling is the electrical.  It shows the grid for the ACT ceiling and the location of the lighting.  I will email the plans back to the engineer today and figure out how they plan on handling the return air.


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## north star (Feb 24, 2015)

*+ % + % +*

MikeC,

Is there any clearance underneath the doors ?

*+ % + % +*


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## MikeC (Feb 24, 2015)

I don't think clearance under the doors would deal with the volume of air.  The plans show 474 cubic feet per minute in the bigger office.


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## markw (Feb 24, 2015)

Mike are you working on ventilation/exhaust air?? If so I didn't catch that in the op- table 403 covers ventilation per occupancy-and directs you to chapter 6 for design of the system to CONVEY ventilation air. When dealing with a specific appliance-see chapter 9 or ACCA manual D if you want to design a duct system. You are spot on-Tell the PE to finish his work....


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## Paul Sweet (Feb 25, 2015)

An air handler can't create or destroy air, so return air + outside (ventilation) air +/- leakage has to equal supply air.

If there are transfer ducts, they are probably using the ceiling space as a plenum, which will restrict some materials from being used in the plenum.

A 3/4" door undercut can transfer about 100 CFM.  They will need transfer grilles in the ceiling for greater amounts, or air handling light fixtures..


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