Wildfire smoke has become a familiar part of summer in Central Oregon. Even when a fire is many miles away, changing winds can bring smoke into Bend and surrounding communities and keep outdoor air quality poor for days at a time.
Closing the windows and doors helps, but outdoor smoke can still find its way indoors. When that happens, a forced-air HVAC system can be an important part of reducing smoke particles inside the home.
The basic strategy is straightforward: limit how much outdoor smoke enters the house, filter the indoor air effectively, and make sure the HVAC system can handle the filter being used.
For many homes, that means using a higher-efficiency HVAC filter, circulating indoor air through the system more often during smoke events, and supplementing with a properly sized portable air cleaner where additional filtration is useful. EPA and Oregon health officials recommend these approaches during wildfire smoke events.
What Is in Wildfire Smoke?
Wildfire smoke contains a mixture of gases and very small particles. One of the main concerns is PM2.5, particulate matter with a diameter of 2.5 micrometers or smaller.
These particles are small enough to remain suspended in the air and can travel long distances from the fire itself. Oregon's Department of Environmental Quality tracks PM2.5 as a major component of wildfire-smoke air pollution.
For HVAC purposes, the important distinction is that particle filtration can remove a significant amount of the particulate matter in smoke, but a normal HVAC filter does not remove every component of wildfire smoke.
Smoke can also contain gases and odors. Standard particle filters are designed primarily to capture particles, not all gaseous pollutants.
That means a good filtration strategy can substantially improve indoor air during a smoke event without making the air completely equivalent to outdoor air on a clear day.
Can an HVAC System Filter Wildfire Smoke?
Yes, if the home has a forced-air heating or cooling system and an appropriate filter.
When the blower operates, household air passes through the HVAC filter before being circulated back through the duct system. A more efficient filter can capture substantially more of the fine particles associated with wildfire smoke than a basic low-efficiency filter.
EPA recommends a MERV 13 filter, or the highest-efficiency filter the system can accommodate, for wildfire-smoke filtration.
The words the system can accommodate are important.
Putting the highest-number filter you can find into every furnace is not necessarily the right approach. The filter still has to allow enough airflow for the HVAC equipment to operate properly.
What Does a MERV Rating Mean?
MERV stands for Minimum Efficiency Reporting Value. It is a standard used to describe how effectively an air filter captures particles of different sizes.
In general, a higher MERV rating means the filter captures a greater percentage of smaller particles.
For wildfire smoke, EPA recommends MERV 13 or higher when the HVAC system can accommodate it because filters in that range are substantially better at capturing the fine particles found in smoke.
But MERV rating is only one part of filter selection.
The size of the filter, its construction, the amount of filter surface area, the filter cabinet, the blower, and the rest of the duct system all affect how easily air can move through it.
A deep-media MERV 13 filter with a large surface area can behave very differently from a restrictive one-inch filter carrying the same MERV rating.
That is why we prefer to look at the system rather than simply tell every homeowner to buy the highest-rated filter on the shelf.
Why the Highest MERV Rating Is Not Always the Best Choice
Because an HVAC system needs both filtration and airflow.
A more restrictive filter can increase resistance to airflow. If the blower and duct system cannot handle that resistance, the HVAC equipment may not move the amount of air it was designed to move.
The goal is therefore not:
Install the highest MERV number possible.
It is to use effective filtration that is compatible with the HVAC system.
EPA gives essentially the same advice: use MERV 13 or the highest efficiency the system fan and filter arrangement can accommodate, and consult an HVAC professional if you are unsure.
In some homes, improving the filter cabinet or moving to a larger media filter can provide better filtration without creating unnecessary airflow restriction.
Does an HVAC System Bring Outdoor Smoke Inside?
Usually, a standard residential forced-air furnace or central air-conditioning system recirculates air that is already inside the home. It does not continuously pull large amounts of outdoor air through the furnace.
However, some homes do have dedicated outdoor-air intakes, ventilation equipment, or other systems that intentionally bring in outside air.
During a wildfire-smoke event, EPA recommends setting systems with outdoor-air capability to recirculate or reducing the outdoor-air intake when appropriate.
This is one reason it helps to know how your particular home is ventilated. A conventional furnace, a system with a fresh-air intake, an energy-recovery ventilator, and an evaporative cooler all behave differently.
If you are not sure whether your HVAC system brings in outdoor air, we can help identify what you have.
Should You Run the HVAC Fan More During Smoke?
Running the blower more often can increase filtration because the filter only removes particles while air is actually passing through it.
EPA recommends running the central-system fan as often as practical during smoky conditions when an appropriate high-efficiency filter is installed. Setting the thermostat fan from Auto to On can provide more continuous filtration.
There is a tradeoff: running the blower continuously uses additional electricity, and not every system needs to run that way all season.
We generally think of continuous or extended fan operation as a tool for periods when outdoor smoke is significantly affecting indoor air, rather than an automatic year-round setting for every home.
Some modern equipment and controls also provide circulation modes that periodically operate the blower without running it continuously.
How Often Should You Change the Filter During Wildfire Smoke?
A filter that collects more particles will eventually become dirty.
During prolonged wildfire smoke, filters can load much more quickly than they do under ordinary conditions. EPA and Oregon guidance recommend checking filters frequently and replacing them when they become visibly dirty rather than assuming the normal replacement interval still applies.
This is particularly important when the system is being run more often specifically for filtration.
A heavily loaded filter can become restrictive and reduce airflow.
There is no single calendar schedule that works for every house. Filter size, filter type, pets, household dust, system runtime, smoke severity, and other conditions all affect how quickly a filter needs attention.
Central HVAC Filters and Portable Air Cleaners Can Work Together
A good HVAC filter treats air moving through the central system and can help reduce particle levels throughout the area served by that ductwork.
A portable air cleaner serves a smaller area directly.
These approaches do not have to compete with each other.
During severe smoke events, a homeowner might use a good central HVAC filter for whole-home filtration while also using a portable air cleaner in a bedroom, living area, or another room where people spend significant time.
EPA specifically recommends this combination as an effective strategy for wildfire smoke.
For some homes, especially those without central forced-air equipment, portable air cleaners may be the primary filtration method.
How to Choose a Portable Air Cleaner for Wildfire Smoke
For wildfire smoke, look for an air cleaner designed to capture fine particles and sized appropriately for the room.
One useful specification is Clean Air Delivery Rate, or CADR. EPA recommends choosing a smoke CADR appropriate to the room size; as a general guide, it recommends a smoke CADR of at least about two-thirds of the room's square footage for a typical eight-foot ceiling.
A unit that is too small for the room may have difficulty cleaning the air quickly enough to make a meaningful difference.
EPA also recommends selecting air cleaners that produce little or no ozone.
Portable filtration is particularly useful for creating a cleaner-air room during heavy smoke, such as a bedroom or primary living space.
What About HEPA Filtration?
HEPA filtration can be very effective at removing fine airborne particles.
True HEPA filters are common in portable air cleaners, which is one reason those devices can be helpful during wildfire smoke.
A standard residential furnace, however, is not normally designed to accept a true HEPA filter simply by sliding one into the existing filter slot. HEPA filtration generally creates more airflow resistance and requires equipment designed around it.
For a typical central HVAC system, a compatible MERV 13 or similar high-efficiency media filter is usually a more practical approach.
If a homeowner needs filtration beyond what a normal filter cabinet can provide, we can look at whole-home filtration equipment designed specifically for that purpose.
Does Running the Air Conditioner Filter Smoke?
Air conditioning and filtration are related, but they are not the same thing.
An air conditioner cools the house by removing heat. It does not automatically remove smoke simply because the air is getting colder.
The filtration happens because the HVAC blower moves air through the system's filter.
A home with a good filter and plenty of blower runtime may receive meaningful particle reduction. The same air conditioner operating with a very low-efficiency filter may provide much less filtration.
This is why upgrading the filter and understanding how the fan operates can matter during smoke season.
Will a Particle Filter Remove Smoke Odors and Gases?
Particle filtration can reduce the particulate portion of wildfire smoke, but odors and gaseous pollutants are more complicated.
MERV and HEPA filters are primarily particle filters. They are not designed to remove all gases or odors.
Activated carbon and other sorbent materials can help with certain gases and odors, but their effectiveness depends on the amount and type of media used. A thin carbon layer added to a disposable filter should not be assumed to remove all wildfire-smoke gases.
EPA similarly distinguishes particle filtration from technologies intended to address gaseous pollutants.
If smoke odor is the main concern, it is important to understand that simply increasing the MERV rating does not necessarily solve it.
Keeping Outdoor Smoke Out Still Matters
Filtration works better when the home is not constantly filling with new smoke.
During periods of poor outdoor air quality, Oregon and EPA guidance recommends keeping windows and doors closed when it is safe to do so, using recirculation where applicable, and limiting activities that introduce additional particles indoors.
Once outdoor air quality improves, opening the house and ventilating can help clear pollutants that remain indoors.
Homeowners should also pay attention to other sources of indoor particles during a smoke event. Cooking at high temperatures, burning candles, fireplaces, smoking, and other activities can add particulate matter at exactly the time you are trying to keep indoor levels low.
Why Some Homes Let In More Smoke Than Others
Older Central Oregon homes can allow more outdoor air to enter through gaps and leakage in the building envelope.
No HVAC filter can stop smoke before it enters through a crack in a wall or around a poorly sealed opening. It can only filter that air after it reaches the HVAC system.
Air sealing can therefore be part of improving a home's overall ability to resist wildfire smoke, particularly when smoke intrusion is a recurring problem.
EPA notes that newer and tighter homes generally do a better job of limiting outdoor particle infiltration than leakier homes.
That does not mean a homeowner needs to rebuild the house to improve indoor air. It simply explains why two neighboring homes using similar filters may experience smoke very differently.
Prepare Before Wildfire Smoke Arrives
The middle of a heavy smoke event is not the easiest time to discover that your filter is dirty or that every local store has sold out of replacements.
Before wildfire season, it can be useful to know:
- What filter size your system uses
- What MERV rating is appropriate
- Whether you have replacement filters available
- Whether the thermostat can circulate the fan independently
- Whether the home has an outdoor-air intake
- Where a portable air cleaner would be most useful
EPA recommends preparing filtration equipment before smoke events when possible because filters and air cleaners can become difficult to find during widespread smoke.
For Central Oregon, that is practical preparation rather than an unusual emergency measure.
How Deschutes Approaches Wildfire-Smoke Filtration
We do not think the answer is simply to install the thickest or highest-rated filter available.
We want to know what HVAC system you have, how the filter is installed, how much airflow the equipment needs, and what you are trying to accomplish.
For one home, an appropriate MERV 13 filter may be a simple improvement.
Another system may benefit from a larger media-filter cabinet that provides more filter surface area while maintaining good airflow.
A homeowner with a particularly important bedroom or living area may also benefit from portable HEPA filtration during smoke events.
The goal is to improve particle filtration without creating another HVAC problem in the process.
We can also help homeowners understand whether their system has an outdoor-air intake, how to use the blower for additional filtration, and how often a smoke-loaded filter may need to be checked.
The Bottom Line
Your HVAC system can help reduce wildfire-smoke particles inside your Central Oregon home, but the filter and the system need to work together.
EPA recommends MERV 13 filtration, or the highest efficiency your HVAC system can appropriately accommodate. Running the blower more often during smoky conditions can increase filtration, and a properly sized portable air cleaner can provide additional help in rooms where cleaner air is especially important.
At the same time, filtration is only part of the strategy. Keeping smoke from entering the house, reducing indoor particle sources, checking filters more frequently, and understanding the limits of particle filtration all matter.
If wildfire smoke regularly affects the air inside your home, contact us. We can look at your existing HVAC system and help you understand what filtration options make sense without compromising the airflow the equipment needs.