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Sheet T24-8  ·  §150.0(m)13  ·  Tables 150.0-B / 150.0-C

Return Duct
Sizer

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.

The decision

Two ways to comply

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.

Option A

Diagnostic test

Measure it on site. The Rater verifies the real numbers with instruments.

  • Airflow confirmed at ≥ 350 CFM/ton through the return grilles, in every control mode.
  • Fan efficacy confirmed at or below the W/CFM cap (fan watt draw).
  • Flow hood, plenum pressure matching, or fan flow meter — per Reference Appendix RA3.3.
  • Works for any system: zoned, multi-return, or over 5 tons.
Verified by · ECC (HERS) Rater — RA3.3
Option B — this tool

Prescriptive sizing

Design it to the table. Hit the duct and grille sizes and you're exempt from the airflow & fan-watt diagnostic.

  • Size return duct(s) + filter grille(s) per Table 150.0-B (single) or 150.0-C (two).
  • Available only to standard ducted systems with no zoning dampers.
  • Single return: 1.5–2.5 tons. Two returns: 1.5–5.0 tons. Outside that, you test.
  • Layout rules apply — length, bends, metal elbow, filter. See below.
Verified by · ECC (HERS) Rater — RA3.1.4.4 & RA3.1.4.5
It's an exception, not a loophole. Sizing to the table doesn't remove the Rater — it swaps the airflow/fan-watt test for Rater verification of the return-duct design (RA3.1.4.4) and the air-filter device (RA3.1.4.5), documented on form CF3R-MCH-28-H (single-family) or LMCV-MCH-28-H (multifamily). Any zoning damper, an oversized system, or a layout that breaks a rule below sends you back to the diagnostic path.
Interactive

Size it for your system

Pick the cooling capacity and return configuration, then confirm the layout gates. The readout gives your required sizes — or tells you to test.

Nominal cooling capacity
Return configuration
Layout gates — all must hold
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—

Reference charts

The prescriptive tables

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.

Table 150.0-B · Single Return Duct

One return · 1.5 – 2.5 tons

Not permitted above 2.5 tons or below 1.5 tons — those systems use two returns or the diagnostic test.
Cooling capacity Return duct min. nominal diameter Min. total filter-grille gross area
1.5 ton16 in500 in² · e.g. 20×25
2.0 ton18 in600 in² · e.g. 20×30
2.5 ton20 in800 in² · e.g. 20×40, or 2× 20×20
Single-family Table 150.0-B. Low-rise multifamily uses Table 160.3-A with identical values.
Table 150.0-C · Two Return Ducts

Two returns · 1.5 – 5.0 tons

Two independent return runs. Combined filter-grille area is the total across both grilles.
Cooling capacity Return duct #1 min. dia. Return duct #2 min. dia. Min. total filter-grille area
1.5 ton12 in10 in500 in²
2.0 ton14 in12 in600 in²
2.5 ton14 in14 in800 in²
3.0 ton16 in14 in900 in²
3.5 ton16 in16 in1,000 in²
4.0 ton18 in18 in1,200 in²
5.0 ton20 in20 in1,500 in²
Single-family Table 150.0-C. Low-rise multifamily uses Table 160.3-B with identical values. No 4.5-ton row — for a capacity between listed sizes, use the next-larger row.

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.

Don't skip these

Critical layout constraints

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.

Disqualifier

No zoning dampers

A system with zoning dampers can't use these charts at all. It must pass the RA3.3 airflow & fan-watt diagnostic instead.

§150.0(m)13C · §160.3(b)5L
Disqualifier

30-foot length limit

The return duct for each filter grille must not exceed 30 linear feet. The limit is per run, not for the system total.

CF3R-MCH-28-H · table header note
Disqualifier

180° bend limit

Each return run may contain no more than 180° of total bend — the equivalent of two 90° elbows. More than that, and you test.

CF3R-MCH-28-H · table header note
Build rule

Metal elbow over 90°

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.

CF3R-MCH-28-H · table header note
Build rule

Filter grille ≤ 0.1″ w.c.

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.

§150.0(m)12Biv · §150.0(m)12Div
Build rule

MERV 13 minimum filter

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.

§150.0(m)12C · §160.2(b)1B
If you test instead

What the diagnostic path checks

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.

Airflow rate
Cooling airflow  ≥ 350 CFM/ton
Small-duct high-velocity  ≥ 250 CFM/ton
Measured in every control mode at the return grilles.

Distinct from the refrigerant-charge airflow floor (300 CFM/ton altered, 350 CFM/ton new), which is a separate verification.

Fan efficacy — watt draw
Gas-furnace air handler  ≤ 0.45 W/CFM
Other air handlers  ≤ 0.58 W/CFM
Small-duct high-velocity  ≤ 0.62 W/CFM

Gas-furnace air handlers built before July 3, 2019 may use 0.58 W/CFM. Procedure: Reference Appendix RA3.3.

Prepared by

Prepared by
Ben Long · Certified HERS/ECC Rater
Cell
401-714-8043
Email
[email protected]
Sheet
T24-8 — RETURN DUCT SIZER