How do the altered-system and new-system numbers differ?

An altered system under Section 150.2(b)1Fiia needs 300 cfm per ton, and if it cannot get there it may use the remedial route. A complete new system under Section 150.0(m)13 needs 350 and is corrected until it passes.

Is airflow tested on a mini-split?

No. The airflow requirement attaches to systems that use forced-air ducts, so a ductless unit is verified for charge but not airflow.

Airflow Verification: When Is It Required Under Title 24?

A complete new system with new ducts must show 350 cfm per ton at the return grilles plus a fan watt limit. An altered system with replaced refrigerant parts must show 300 cfm per ton, tied to the charge test. A furnace-only swap, a ductless unit, or equipment extended to an addition needs no airflow test.

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Match the job to the number

Entirely new system or new duct system. Section 150.0(m)13, reached through Section 150.2(b)1C or 1Diia: 350 cfm per ton in every control mode, with 0.45 W/cfm or less for a gas furnace air handler and 0.58 for other types.
Replaced refrigerant parts. Section 150.2(b)1Fiia: 300 cfm per ton, or 250 on a small-duct high-velocity system, in the zones where charge is verified.
No airflow test. Equipment extended to an addition, a furnace swapped without cooling parts, and any ductless unit.

Worked numbers for a 3-ton unit

A nameplate reading 036 means 3 tons. A new system needs 1,050 cfm with fan power at or below 472.5 watts on a gas furnace, or 609 watts on another air handler. An altered system needs 900 cfm.

Skipping the measurement

In place of measuring, a conventional ducted system with no zoning dampers can be inspected against the return-duct tables, 150.0-B and 150.0-C. For two returns, a 3-ton system takes 16 and 14 inch ducts with 900 square inches of filter grille, and a 3.5-ton system two 16 inch ducts with 1,000. Each return run is at most 30 feet with no more than 180 degrees of bend, documented on CF2R-MCH-28-H.

How the reading is taken

Filter installed and registers open, a flow hood is read at each return grille and the figures are added, or a fan flowmeter matches the supply plenum pressure. When the meter cannot reach that pressure, Appendix RA3.3 scales the reading by the square root of the pressure ratio. At altitude or far from 70 degrees F, gauges can read 15 percent off. Multispeed systems are read at top compressor and fan speed.

If the reading falls short

A new system is corrected, then retested. An altered system can follow the installer's certified remedial steps, which the ECC-Rater reviews. A failed result makes that rater the Rater of Record for the retest. The forms are CF2R-MCH-22-H for fan efficacy and CF2R-MCH-23-H for airflow.