Do I need MERV 13 when I replace only the furnace?

Not from the alteration rules, which stop at Section 150.0(m)10. A complete new system or an entirely new duct system does carry the filter requirement.

Is a nominal 20 by 25 filter really 500 square inches?

For this calculation, yes. The appendix multiplies the nominal length by the nominal width and compares the product with the minimum face area.

MERV 13 Filter Sizing for Title 24

A nominal 2-inch filter is sized by the system designer. A 1-inch filter needs a nominal face area of at least the design airflow divided by 150 feet per minute, times 144 square inches. Either way it is MERV 13 or better, reachable, sealed against bypass, and labeled with its design airflow and maximum clean pressure drop.

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Does an existing house reach this rule?

Not through an air handler swap alone. For altered components Section 150.2(b)1 cites Section 150.0(m)1 to 10 and stops short of the filter paragraph, (m)12. The filter rule enters with a complete replacement system, an entirely new duct system, an addition, or a ventilation filter that an earlier permit made subject to it.

Sizing arithmetic for 1-inch filters

Face area in square inches is airflow in cfm, divided by 150, times 144. Take a 3-ton system at 350 cfm per ton, 1,050 cfm in all, split by the designer between returns of 600 and 450 cfm. The first needs 576 square inches and the second 432. A nominal 24 by 24 filter (576) clears the first. A 20 by 25 (500) clears the second but would fail the first, because each filter is judged against its own share, nominal length times nominal width.

Where 2-inch filters and the tables fit

A nominal 2-inch filter takes its allowed pressure drop from the system design, so no face-velocity check applies.
A 1-inch filter must be rated at 25 Pa (0.1 inch of water) or less when clean.
Tables 150.0-B and 150.0-C give a grille-area route instead, such as 900 square inches of filter grille for a 3-ton system with two returns.

Proof on site

The filter is MERV 13, or on the AHRI 680 route removes at least 85 percent of 1.0 to 3.0 micron particles and 50 percent of 0.30 to 1.0 micron ones. The maker's label shows its efficiency and pressure drop. Each filter location needs a permanent, legible label with design airflow and maximum clean-filter pressure drop. On a face-velocity design a missing label fails that indoor unit. Gaskets close gaps around the filter, and the owner must be able to reach it.

See the ADU filter page for ductless units. Airflow figures come from airflow verification, the new-duct trigger from duct replacement, and balanced units from HRV and ERV. Background is on whole-house ventilation, and reports start at residential.

Common Title 24 Questions

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