Does my ADU need MERV 13 air filters?

It depends on the equipment. Section 150.0(m)12 of the 2025 Energy Code requires MERV 13 filtration for space-conditioning systems that supply air through more than 10 feet of ductwork, for supply-only ventilation and makeup air systems, and for the supply side of balanced systems such as HRVs and ERVs. A ductless wall-mounted mini-split paired with an exhaust-only bath fan generally falls outside the requirement.

Which systems are covered

• Ducted heat pumps, including concealed-duct mini-splits, when the ductwork exceeds 10 feet.
• Supply-only ventilation and makeup air systems that bring outdoor air into living space.
• Balanced ventilation (HRV or ERV), on the supply side.
• Evaporative coolers are exempt.

What the filter must meet

• Efficiency: MERV 13 under ASHRAE Standard 52.2, or an AHRI 680 rating of at least 50 percent in the 0.30 to 1.0 micron range and 85 percent in the 1.0 to 3.0 micron range.
• Size: for ducted heating and cooling systems, either a nominal 2-inch filter sized by the designer, or a 1-inch filter sized for a face velocity of 150 feet per minute or less with a clean-filter pressure drop of no more than 0.1 inch of water (25 Pa).
• Placement: recirculated and outdoor air must be filtered before it reaches the heating or cooling coil, with a limited exception for HRVs and ERVs.
• Access and labeling: filters must be accessible to the owner, and the filter location must be permanently labeled with the design airflow and the maximum allowable clean-filter pressure drop.
• No bypass: filter racks and grilles must be gasketed or sealed so air cannot slip around the filter.
• Product label: filters for ducted systems must carry the manufacturer's efficiency and pressure drop ratings.

Verification and access

The CEC's Compliance Manual lists verification of air filter device design, filter MERV rating and labeling among the items an ECC rater (formerly HERS rater) can be required to check. For balanced and supply-only ventilation, Section 150.0(o)1C also requires filters and recovery cores to be serviceable from occupiable space, a garage, a balcony or a mechanical closet, within 10 feet of the walking surface, or else meet added attic-access rules.

Why filter size matters

A dense filter crammed into a small slot restricts airflow and can drop a ducted system below the 350 cfm per ton it must show during testing. That is why the code ties filter size and pressure drop to the design airflow.

What it means for your mechanical plan

• Identify which systems need MERV 13 and show each filter location and nominal size.
• Note the design airflow and maximum clean-filter pressure drop for the required label.
• Put the filter grille or rack where it can be changed without climbing into the attic.

For the full design overview, see the ADU mechanical guide; for testing and ventilation, see HERS (now ECC) testing and ADU ventilation requirements.

Sources: energy.ca.gov, energy.ca.gov, energy.ca.gov.