There are two ways to satisfy the Energy Code's HVAC airflow and fan-watt-draw requirement: run the on-site diagnostic test, or size your returns to the prescriptive tables and skip it. This tool covers the table path — what it requires, what disqualifies it, and the exact duct and grille sizes for your system.
Both paths end the same place — a system the Rater can sign off. They differ in what gets verified: a measured airflow result, or a designed-to-table layout.
Measure it on site. The Rater verifies the real numbers with instruments.
Design it to the table. Hit the duct and grille sizes and you're exempt from the airflow & fan-watt diagnostic.
Pick the cooling capacity and return configuration, then confirm the layout gates. The readout gives your required sizes — or tells you to test.
Reference charts
Minimum sizes that grant the airflow & fan-watt exemption. Diameters are nominal; filter-grille area is total gross (nominal) area. Both ducts may be equal diameter on the larger systems.
| Cooling capacity | Return duct min. nominal diameter | Min. total filter-grille gross area |
|---|---|---|
| 1.5 ton | 16 in | 500 in² · e.g. 20×25 |
| 2.0 ton | 18 in | 600 in² · e.g. 20×30 |
| 2.5 ton | 20 in | 800 in² · e.g. 20×40, or 2× 20×20 |
| Cooling capacity | Return duct #1 min. dia. | Return duct #2 min. dia. | Min. total filter-grille area |
|---|---|---|---|
| 1.5 ton | 12 in | 10 in | 500 in² |
| 2.0 ton | 14 in | 12 in | 600 in² |
| 2.5 ton | 14 in | 14 in | 800 in² |
| 3.0 ton | 16 in | 14 in | 900 in² |
| 3.5 ton | 16 in | 16 in | 1,000 in² |
| 4.0 ton | 18 in | 18 in | 1,200 in² |
| 5.0 ton | 20 in | 20 in | 1,500 in² |
Single nominal diameter per duct — there is no separate flex-vs-rigid column. The same nominal diameter governs whether the run is flex or smooth metal.
Hitting the table sizes is necessary but not sufficient. The physical run has to obey every rule below, or the prescriptive exemption is off and the system goes to diagnostic testing.
A system with zoning dampers can't use these charts at all. It must pass the RA3.3 airflow & fan-watt diagnostic instead.
The return duct for each filter grille must not exceed 30 linear feet. The limit is per run, not for the system total.
Each return run may contain no more than 180° of total bend — the equivalent of two 90° elbows. More than that, and you test.
If the return duct contains more than 90° of total bend, at least one of those bends must be a rigid metal elbow — not flex — to keep the bend from choking airflow.
Each filter grille must be labeled to disclose its design airflow and a maximum clean-filter pressure drop of 0.1″ water column (25 Pa) at that airflow, per ASHRAE 52.2 / AHRI 680.
The air filter must be MERV 13 or better (ASHRAE 52.2). This is a standalone mandatory requirement — it applies whatever airflow path you choose, not just the table path.
The numbers the table path lets you skip. Worth knowing — when a layout fails a gate above, this is the fallback, and these are the targets the Rater measures to.
Distinct from the refrigerant-charge airflow floor (300 CFM/ton altered, 350 CFM/ton new), which is a separate verification.
Gas-furnace air handlers built before July 3, 2019 may use 0.58 W/CFM. Procedure: Reference Appendix RA3.3.
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