Supply, return, exhaust and filtration: follow the air’s path

01

Start at the supply: delivered does not mean all-outdoor

A supply outlet tells you where air enters the room, not its complete origin. ASHRAE’s terminology allows supply air to include outdoor air, recirculated air, transferred air from another space, or a combination. Follow the path upstream to understand what is being supplied.

Cooling and bringing in outdoor air answer different questions. If a unit draws air from the room, cools it and blows it back, that described path is recirculation. Cooler air alone does not show that outdoor air has been introduced.

[1][2]

02

Return is a starting point, not a final destination

In ASHRAE’s 2019 interpretation, return air is air removed from a space to be recirculated or exhausted. Only the reused part is recirculated air. Exhaust is the path out of the building in the simplified example below.

A return grille therefore does not, by itself, mean that everything entering it goes outdoors. To distinguish the two branches, ask where the return path leads and whether any of that air is supplied again.

[1]

03

A filter answers ‘what is removed?’, not ‘where did the air come from?’

Particle filters and filters designed for particular gases do different jobs. The US EPA explains that air cleaners cannot remove all pollutants; a particle filter is not a general answer to gaseous pollutants. The word ‘filtered’ is incomplete without information about what the device is designed to capture.

Filtration can occur on a recirculating path or alongside an outdoor-air supply. Neither the presence of a filter nor a ventilation system means indoor smoking has been made harmless. EPA’s secondhand-smoke guidance says these methods may reduce exposure but do not eliminate it.

[3][4]

04

Worked example: a meeting room with a split return path

Fictional example: the building manager explains that air leaves a meeting room through a return path. Some is exhausted outdoors; the rest goes back to the unit, mixes with incoming outdoor air, passes through a filter and is supplied to the room. This is one possible arrangement, not a description of your building.

What can the reader now identify? The air leaving the room is return air. The branch leaving the building is exhaust. The reused branch is recirculation. The supply is the mixed air delivered back to the room. The remaining question is what the fitted filter is designed to remove—not whether the word ‘filter’ proves clean air.

A complete follow-up question is: ‘Could you show me where outdoor air enters this meeting room’s system, which return-air branch goes outside, and which is reused? What does the equipment documentation say the filter captures, and who can explain this route for this room?’

05

What to keep in mind

• Name the function by the air’s route, not the outlet’s appearance.
• A return path can lead to exhaust, recirculation or both.
• Filter capability and outdoor-air supply are separate pieces of information.

06

Common questions