How can my ADU's HVAC and ventilation be designed for wildfire smoke?

Start with the filtration the 2025 Energy Code already requires, then design so occupants can limit smoky outdoor air during a smoke event. Ducted systems and supply or balanced ventilation get MERV 13 filters by code, and the required label on the ventilation switch itself tells occupants they may turn it off when outdoor air quality is very poor.

What the code already gives you

• MERV 13 on ducted and supply systems. Section 150.0(m)12 requires MERV 13 or equivalent on ducted systems with more than 10 feet of ductwork, on supply-only ventilation and on the supply side of HRVs and ERVs.
• A switch occupants understand. Section 150.0(o)1J requires the whole-dwelling ventilation on-off switch to carry this label, or equivalent text: "This switch controls the indoor air quality ventilation for the home. Leave switch in the 'on' position at all times unless the outdoor air quality is very poor."
• Intake location. Outdoor air intakes must comply with California Mechanical Code Section 402.4.1, and the CEC advises locating them away from known contaminant sources.

How each ventilation strategy behaves

• Exhaust-only (a continuous bath fan): simple, but replacement air leaks in unfiltered through gaps in the building.
• Balanced ERV or HRV: outdoor air enters through a filter that must be MERV 13 on the supply side, and the system can be switched off during heavy smoke.
• Central fan integrated supply through a ducted air handler: outdoor air passes the system's MERV 13 filter, and Section 150.0(o)1B requires a motorized damper that closes the outdoor air duct when ventilation is not called for.

Ductless systems

Wall-mounted mini-split heads recirculate room air and typically use washable screen filters rather than MERV 13 media, so they are not built to clean smoke from the air. If your ADU is ductless, plan for a portable air cleaner. The U.S. EPA recommends placing one where people spend the most time, choosing a unit with a known clean air delivery rate (CADR) and making sure it does not produce ozone.

Practical design moves

• Use a 2-inch or deeper filter slot sized for the airflow, so a MERV 13 filter does not choke the system.
• Put filters where they are easy to reach; the EPA advises changing filters more often during heavy smoke.
• Give occupants a clearly labeled way to pause ventilation and close outdoor air dampers.
• Think twice about an evaporative cooler as the main cooling in smoke-prone areas; the EPA notes it can bring more smoke inside.
• Build a tight envelope, which reduces unfiltered leakage.
• If your lot is in a Fire Hazard Severity Zone, coordinate vent and exhaust terminations with the wildfire-resistant construction rules that apply there.

What it means for your mechanical plan

The plan should identify the ventilation strategy, filter locations and ratings, dampers and switch labels, and any dedicated receptacle you want for a portable air cleaner. See ADU ventilation requirements and what mechanical plans include.

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