When wildfire smoke blankets the air, homeowners often worry that running the air conditioning will draw harmful outdoor air inside. You should run your air conditioner, but only with specific adjustments and precautions. Your central heating, ventilation, and air conditioning (HVAC) system can be an effective tool for improving indoor air quality if configured correctly for smoky conditions. Utilizing the system to filter and recirculate indoor air is better for your health and comfort than leaving the unit off and letting your home heat up. Understanding how smoke infiltrates your home and how your system can be optimized is key.
Understanding Smoke Particulates and HVAC Interaction
Wildfire smoke presents a significant threat to indoor air quality because of its microscopic composition. The most concerning component is particulate matter known as PM2.5, which consists of particles 2.5 micrometers in diameter or smaller. These particles are tiny enough to penetrate deep into the lungs, potentially leading to respiratory and cardiovascular issues.
Even a home that feels sealed naturally exchanges air with the outdoors through small cracks, gaps, and leaks in the building envelope. This infiltration means that high concentrations of outdoor smoke will inevitably raise indoor PM2.5 levels. Standard residential HVAC systems are primarily designed for temperature control, not specialized air pollution removal.
When a typical system operates, it cycles indoor air through a filter before cooling or heating it and returning it to the living spaces. If the system includes a fresh air intake or ventilation mode, however, it can actively pull smoky air from outside, increasing the concentration of pollutants indoors.
Operational Settings for Central Air Systems
The most important step for those with central air is preventing the system from drawing smoky outdoor air into the home. While many central HVAC systems recirculate indoor air, some have a fresh air intake damper for ventilation. If your system has this feature, you must locate and manually close the outdoor intake damper to block the entry of polluted air.
During a smoke event, adjust the fan setting on your thermostat from “Auto” to “On” or “Continuous.” The “Auto” setting only runs the fan when the system is actively heating or cooling, stopping air filtration once the desired temperature is reached. Switching to “On” ensures the fan runs constantly, continuously pulling indoor air through your filter to remove particles, even when the compressor is off.
This continuous operation maximizes the number of times the air in your home passes through the filter, actively cleaning the indoor environment. Some thermostats offer a “Recirculate” or “Max Cool” setting, which serves a similar function by ensuring the system primarily cycles and filters the air already inside the house. Keeping windows and doors tightly closed is essential to reduce the amount of contaminated air your system must clean.
Filtration Strategies for Cleaner Indoor Air
To effectively reduce smoke particulates, the standard low-efficiency filter in your central system should be upgraded. Filter efficiency is measured by the Minimum Efficiency Reporting Value (MERV), with higher numbers indicating better filtration. To capture the fine PM2.5 particles found in smoke, aim for a filter rated MERV 13 or higher.
A MERV 13 filter captures at least 50% of particles in the 0.3 to 1.0 micron size range, including a significant portion of smoke particles. While MERV 16 filters offer maximum protection by removing 85% to 95% of particulate matter, installing a filter with a higher MERV rating can restrict airflow. This restriction places strain on your HVAC unit, especially in older systems.
You must check your system’s specifications or consult a professional to ensure it can handle the increased pressure drop from a high-efficiency filter. If your central system cannot accommodate a MERV 13 or higher filter, supplemental air purification is necessary. Portable room air purifiers equipped with High-Efficiency Particulate Air (HEPA) filters are an effective alternative. True HEPA filters are certified to capture a minimum of 99.97% of particles at 0.3 microns, making them excellent at removing smoke from individual rooms.
Special Considerations for Window and Portable AC Units
Window air conditioning units operate differently from central systems and require specific attention during smoke events. Many window units have a vent or damper that can be opened to bring in fresh outdoor air; this must be fully closed or sealed to prevent smoke entry. Inspect and seal any gaps between the unit and the window frame using foam or tape to ensure an airtight seal.
Portable air conditioning units present a different challenge depending on their design. Single-hose portable ACs vent exhaust air outside, which creates negative pressure inside the room. This pressure difference can inadvertently draw unfiltered, smoky outdoor air into the home through small cracks and openings.
For single-hose units, it is best to avoid using them entirely during heavy smoke, or to use them sparingly. Dual-hose portable ACs are more suitable for smoky conditions because they use one hose to draw in outdoor air for cooling the condenser and a second hose to exhaust the resulting heat. This design minimizes the negative pressure effect on the indoor air. Regardless of the type, ensure the window venting kit is sealed tightly to the window opening to prevent smoke infiltration.