ADU MERV 13 Filter Requirements
A ducted ADU system needs a MERV 13 or better filter when its ductwork runs more than 10 feet, and the supply side of a balanced ventilation unit needs one too. A ductless mini-split falls outside the rule. The filter slot must also be reachable, sized for the airflow and labeled.
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What brings the filter rule in
Section 150.0(m)12 covers systems that supply an occupiable space through ductwork over 10 feet, supply-only ventilation and makeup air, and the supply side of balanced HRV or ERV units. A new detached ADU and an addition both carry it through the mandatory measures. In a converted room it follows a complete replacement system or an entirely new duct system, while replacing a ventilation filter carries it only if an earlier permit required it. A ductless indoor unit has no duct run, so it misses the 10 foot trigger.
What qualifies as MERV 13
MERV 13 or higher by ASHRAE 52.2, or at least 50 percent efficiency in the 0.30 to 1.0 micron range and 85 percent in the 1.0 to 3.0 micron range by AHRI 680. The manufacturer's label must state efficiency and pressure drop.
Size the filter for a small system
Choose a filter at least 2 inches deep, sized by the designer, or a 1 inch filter at no more than 150 feet per minute face velocity with a clean pressure drop of 25 Pa (0.1 inch of water) or less. At 350 cfm per ton, a 1.5-ton unit moves 525 cfm, so 525 divided by 150 gives 3.5 square feet, or 504 square inches of nominal face area. Tables 150.0-B and 150.0-C offer a return grille route instead, with 500 square inches at 1.5 tons.
Who looks at it
The building inspector checks the MERV rating and thickness at final. The ECC-Rater inspects the filter device where the compliance documents call for it. On a face-velocity design the grille or rack must carry a sticker with the design airflow and maximum pressure drop; without it the indoor unit does not comply. Filters must be reachable by the owner and sealed against bypass.
Airflow numbers are on ADU airflow verification, ductless choices on mini-split options and the ventilation side on whole-house ventilation. The 10 foot figure also appears on residential duct leakage. Order from the ADU page.
Common Title 24 Questions
- Do ADUs need Title 24 in California?
- What plans are needed for an ADU Title 24 report?
- How much does an ADU Title 24 report cost?
- How long does an ADU Title 24 report take?
- Does a garage conversion ADU need Title 24?
- Does a JADU need a Title 24 report?
Frequently Asked Questions
Section 150.0(m)12 applies to systems supplying air through ductwork over 10 feet, so a ductless system is outside it. A ducted ventilation unit with a supply side is covered.
Yes, if it is sized for a face velocity of 150 feet per minute or less and its clean pressure drop is no more than 25 Pa at the design airflow.
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