Airflow

Airflow offset

The difference between the air supplied to a room and the air returned or exhausted from it, in cubic feet per minute. A room with more supply than return pushes the difference out through its gaps and runs positive, and a room with more exhaust than supply draws the difference in and runs negative.

Also called volumetric offset, exhaust offset, supply offset

Direct answer

What is an airflow offset?

An airflow offset is the gap between a room's supply airflow and its return or exhaust airflow, and it equals the air leaking through the room's gaps. Sized from leakage with Q = 2610 × A × √ΔP, the airflow offset is what produces a room's pressure differential.

Airflow offset: how it works

Air the HVAC system puts into a room and does not take back out has to leave another way, and it leaves through door gaps and penetrations. Room pressure rises or falls until that leakage matches the offset exactly. Because leakage grows with the square root of pressure, doubling the offset in a given room raises its pressure differential fourfold, and halving the leakage area lets the same offset produce four times the differential.

Where you meet the term

  • In a balancing report, as the difference between measured supply and return or exhaust airflow for each controlled room.
  • In laboratory and isolation room controls that track exhaust and hold supply a fixed amount below it.
  • In a cleanroom suite, where each room's offset is reduced by transfer air arriving from a cleaner neighbor.

How to check it

  1. Compare supply and exhaust on the balancing report. The difference between measured supply and measured return or exhaust for the room is its offset, and its sign should match the intended pressure direction. Owner check
  2. Plot pressure against offset at commissioning. A technician sets several offsets, records the steady room pressure at each, and fits the room's effective leakage area from the results. Technician
Read this twice

Airflow offset: what it is not

  • An airflow offset is not a fixed share of supply. Its correct size depends on the room's leakage area and pressure differential, not on how much air the room receives.
  • A larger offset does not fix a leaky room efficiently. Sealing the leak raises pressure for no added air, while raising the offset costs filtered, conditioned makeup air.

Airflow offset questions

Where does the 2610 in the leakage equation come from?

It is the standard-air velocity relationship, 4005 times the square root of pressure in inches of water column, multiplied by a discharge coefficient of about 0.65 for a sharp-edged gap. The result gives leakage in cfm when the gap area is in square feet.

What is transfer air?

Transfer air leaves one controlled room through its gaps or a transfer grille and enters the next room down the pressure cascade. It counts in the receiving room's balance, so that room needs less offset of its own.

Does an airflow offset change when filters load?

It can. If supply airflow falls as its filters load while exhaust holds steady, the offset shrinks and room pressure falls, which is why critical rooms control supply and exhaust to measured airflow rather than to fixed damper positions.

Where this definition comes from

Written from standards body publication, government guidance material held in the HVAC Bench evidence record. Where a document limits a definition to certain models, that limit is repeated here rather than generalised across a brand.

  • ASHRAE Handbook, FundamentalsASHRAE · standards body publicationASHRAE reference volume carrying the psychrometric relationships, the climatic design condition tables and their annual percentile basis, duct and pipe fitting loss coefficients, the bin and degree-day methods used for seasonal energy estimation, the table of heat gain from occupants by activity including light bench work at 275 BTU per hour sensible and 475 latent, the standard-air sensible and latent heat factors, and the orifice flow relationship used to estimate airflow through openings and gaps from pressure difference.
  • Guidelines for Environmental Infection Control in Health-Care Facilities, Appendix B: AirCenters for Disease Control and Prevention · government guidanceCDC table of ventilation parameters: airborne infection isolation rooms with at least 12 total and 2 outdoor air changes per hour, air movement into the room, a pressure differential of at least 0.01 inches of water gauge (2.5 pascals), and all air exhausted directly outdoors unless recirculated within the room through HEPA filters; protective environment rooms with at least 12 air changes per hour, positive air movement, and HEPA filters of 99.97 percent efficiency at 0.3 micrometers.
  • ISO 14644-3:2019, Cleanrooms and associated controlled environments, Part 3: Test methodsInternational Organization for Standardization, via ANSI · standards body publicationTest methods for cleanroom performance: air pressure difference, airflow volume and velocity, airflow direction and visualization, recovery time using the 100 to 1 ratio with a 10 to 1 ratio added in the 2019 edition for less clean rooms, installed filter system leakage, containment leakage, temperature, and humidity. Recovery testing is optional and its acceptance is agreed between customer and supplier.