For most homes with central heating and air, the best wildfire smoke filter is an HVAC filter rated MERV 13 or higher, run with the fan on, plus a portable HEPA air cleaner in the room where you spend the most time. The EPA recommends a MERV 13 or higher filter to remove fine particle pollution from smoke. The catch is that not every furnace blower can pull air through a dense filter, so the fit matters as much as the rating.
This guide covers what the ratings mean, how to tell whether your system can take a MERV 13 filter, how to run your equipment during a smoke event, and when a portable cleaner is the better tool. It is written for Colorado Springs homeowners, where smoke from Front Range fires can settle over the city for days.
TL;DR
- Choose a MERV 13 filter, or the highest rating your system can handle. The EPA and the CDC both give this guidance.
- A higher rating restricts airflow. Check your furnace manufacturer’s specifications, or have a technician measure static pressure, before you switch.
- During smoke, set the fan to “On,” switch to recirculate if your system has a fresh air intake, and replace the filter once it looks dark.
- Add a portable HEPA air cleaner for one room if your system cannot take a high-MERV filter or you want extra protection.
- Evaporative (swamp) coolers pull outdoor air in, so they need a different plan on smoky days.
Table of Contents
- Why smoke is a filter problem, not just a nuisance
- What MERV means and which rating to choose
- Will your furnace handle a MERV 13 filter?
- How to run your HVAC system during a smoke event
- When a portable HEPA air cleaner makes sense
- If you have a swamp cooler
- Protecting yourself beyond the filter
- Our perspective on smoke and Colorado Springs homes
- Get your system ready before the next smoke event
- FAQ
- Sources
Why smoke is a filter problem, not just a nuisance
Wildfire smoke carries fine particles that travel deep into the lungs. Those particles are far smaller than the dust and pollen a basic furnace filter was designed to catch. The fiberglass filters many homes still use mainly protect the blower from lint and large debris. They do very little against smoke.
That is why the guidance from public health agencies points to higher-efficiency filtration. The Colorado Department of Public Health and Environment advises installing the highest-efficiency filters possible for your system, with MERV 13 or higher being best when it is available and fits your system’s specifications. The same page says to run the system’s fan as often as possible and to limit outdoor air intake.
The filter is only one part of the job. Closed windows, a fan that keeps air moving through the filter, and a sealed return path all matter. We walk through each below.
What MERV means and which rating to choose
MERV stands for minimum efficiency reporting value. The scale runs from 1 to 16 for most residential and commercial filters, and a higher number means the filter captures more of the smaller particles. Smoke particles sit at the small end of that range, which is why a MERV 8 filter that works fine for household dust is not the right choice for a smoke event.
Several agencies give slightly different numbers, and the differences are worth understanding:
- The EPA says to choose at least a MERV 13 rating, or as high as your system fan and filter slot can accommodate, and to consult an HVAC technician about the highest-efficiency filter that works for your system.
- The CDC says to use MERV 13 or higher if your system can use it.
- The Colorado health department’s air quality page for fires says to consider upgrading to a MERV 11 or higher filter if your HVAC system can accommodate it.
Taken together, the message is consistent. Aim for MERV 13. If your system cannot handle that, go as high as it safely can, because even a MERV 11 filter does more against smoke than a basic one. ASHRAE says the same: if a MERV 13 filter cannot be accommodated, use the highest rating you can.
We cover the scale step by step in our guide to picking a home filter rating your blower can handle.
MERV 13 or HEPA?
True HEPA filters capture 99.97% of particles at the most difficult size to catch. They are the standard for portable air cleaners, but a residential furnace is not built to push air through one. A HEPA-grade media is dense enough that it would choke the airflow of a typical home system.
For that reason the two tools do different jobs. A MERV 13 filter cleans the air throughout the whole house every time the blower runs. A portable HEPA unit cleans one room very well. Our article on MERV 13 versus HEPA for homeowners explains how to combine them.
Will your furnace handle a MERV 13 filter?
This is the question that keeps many homeowners on a lower-rated filter, and it is a fair one. A denser filter creates more resistance to airflow. ASHRAE explains that higher filter efficiency raises pressure drop, which can reduce airflow through the system, increase the energy the fan uses to compensate, or both.
When airflow drops too far, a furnace can run hotter than it should. That can stress components and trip safety limits. Air conditioners are affected too, because reduced airflow across the coil lowers cooling performance. None of this is a reason to avoid better filtration. It is a reason to match the filter to the system.
Three things that decide the fit
The first is the filter slot. Standard 1-inch slots hold thin filters with less surface area, so a MERV 13 in that space moves air with more effort and clogs sooner. A deeper media cabinet, 4 inches or 5 inches, gives the same filter far more surface, which lowers resistance. If your return has a thin slot, a technician can often add a media cabinet that makes MERV 13 practical.
The second is the blower motor. Variable-speed blowers with ECM motors can raise their output to compensate for resistance, while older fixed-speed motors cannot. Many newer furnaces have the variable-speed type, including the two-stage variable speed, condensing, and modulating furnaces we install.
The third is the condition of the ductwork. Undersized or leaky ducts already work the blower hard, and adding resistance on top of that is where problems start.
How a technician checks it
The reliable method is to measure static pressure, which is the resistance the blower works against. The Building America Solution Center notes that after installation the total static pressure measured with a manometer should equal no more than 0.4 or 0.5 inches of water column. It also says system designers are supposed to account for a filter’s initial pressure drop when sizing equipment and ductwork.
A common rule of thumb, reported in ACHR News, is that the filter’s pressure drop should not exceed 20 percent of the fan’s rated maximum static pressure. On a gas furnace rated for 0.50 inch of water column, that means a filter drop of no more than 0.10 inch.
You can also check the manufacturer’s side. Bryant’s air filter guidelines advise checking your system manufacturer’s specifications and the filter’s pressure drop information before moving to a high-efficiency filter. The nameplate or manual for your furnace usually lists its maximum static pressure.
If you want this done properly, our technicians can look at the filter slot, blower type and duct condition during a furnace tune-up or service visit and recommend the highest rating that suits your equipment.
How to run your HVAC system during a smoke event
The right filter does little if the system is set up to pull smoky air into the house. These steps follow the guidance from the CDPHE, the EPA and the CDC.
Keep the fan running
Switch the thermostat fan from “Auto” to “On.” The EPA advises running the fan continuously so fine particles keep passing through the filter, and the CDPHE says to run it as often as possible. On Auto, the blower only moves air when the system is heating or cooling, which leaves long gaps with no filtration at all.
Continuous fan operation uses some extra electricity. Systems with ECM blower motors use much less than older motors, which is one more reason to know what is in your furnace.
Close off outdoor air
Close windows and doors. If your system has a fresh air intake or damper, close it or set the system to recirculate. The CDC says to do this when a system has an outdoor intake. The American Lung Association gives the same direction for air conditioners, advising the recirculation setting so the unit does not pull outside air into the room.
Also keep fireplace dampers closed, and avoid adding to indoor pollution. The EPA and the CDPHE both list smoking, gas or wood-burning stoves, frying food, burning candles and aerosol sprays as activities to limit.
Check the filter more often
A smoke-loaded filter clogs faster than a normal one. The EPA says to replace the filter when it looks dirty, dark brown or black, which may be sooner than the manufacturer’s schedule. A clogged filter also brings back the airflow problem described above, so it protects your equipment as well as your lungs.
A practical routine is to look at the filter every few days while smoke is present, and to keep a spare on hand so you are not shopping during an air quality alert. Our furnace filter schedule lays out normal replacement intervals by filter type and household.
Keep the outdoor unit and return clear
Ash and debris can settle on the outdoor condenser. Gently keep the area around it clear, and do not run a hose at high pressure into the coil. If the system seems to be struggling after a heavy smoke period, a service visit is a better step than guessing.
When a portable HEPA air cleaner makes sense
A portable cleaner is the right addition in three cases. Your home has no forced-air system, your system cannot take a high-MERV filter, or you want extra protection in a bedroom or living room. The EPA states that a portable air cleaner can be used when you do not have a forced-air furnace or air conditioner, or to supplement forced-air filtration.
The CDPHE suggests creating a Clean Air Room. This is one room, usually a bedroom, with doors and windows closed and a portable cleaner running. It concentrates your filtration in the space where you rest, and it is easier to keep clean than the whole house.
What to look for
- Size the unit to the room. The EPA says to look for a “tobacco smoke” Clean Air Delivery Rate that is at least two-thirds of the room’s floor area in square feet. A 150-square-foot bedroom needs a rating of at least 100.
- Avoid ozone. The EPA advises choosing models certified by the California Air Resources Board, which produce little or no ozone, a harmful pollutant.
- Plan for the filters. Smoke loads them quickly, so budget for replacements.
The American Lung Association also recommends HEPA air cleaning devices for added protection from soot and smoke. Running both the HVAC filter and a portable unit in the main living space gives you two layers.
For homes where a permanent solution makes more sense, we also install whole-home air purification. Our page on air purification and UV systems in Colorado Springs describes the options, and our guide to indoor air quality beyond the filter explains how they fit with filtration.
If you have a swamp cooler
Evaporative coolers are common in Colorado Springs because of the dry climate, and we install and service them. They work by drawing outdoor air through wet pads and into the house. That is efficient on a normal summer day, but on a smoky day it brings smoky air inside. Filterbuy’s Colorado Springs smoke page notes this and suggests a window air conditioner or portable filtration during smoky periods.
The practical approach is to shut the cooler off when smoke arrives and keep windows closed, then use a portable HEPA cleaner in your Clean Air Room. If you want cooling and smoke protection in one system, a refrigerant-based air conditioner or heat pump with a MERV 13 filter is built for that job. Our air conditioner and heat pump options start at $5,500 for a 14 SEER2 single-stage air conditioner.
Protecting yourself beyond the filter
Indoor measures reduce exposure, but they do not replace judgment about your own health. If you must go outside, the CDPHE advises limiting your time outdoors, reducing physical activity and wearing an N95 or KN95 mask. The American Lung Association adds that ordinary dust masks and cloth coverings do not filter the smallest particles, and that N95 masks may not fit children or be easy to use for people with lung disease. If you have asthma, COPD or another lung or heart condition, talk with your doctor about an N95 mask and about your own smoke action plan.
Check the local air quality index before you decide how to spend the day. When it is poor, plan to stay inside with your filtration running.
After the smoke clears, odor can linger in fabrics and furnishings even when the air tests clean. Resources on removing smoke smell from your house cover cleaning methods, and keeping the HVAC fan and a fresh filter in service helps carry airborne odor out of the living space.
Our perspective on smoke and Colorado Springs homes
We work on Front Range systems every week, and the pattern we see is simple. Most homes were set up for dust and pollen, not for smoke. The filter is a one-inch fiberglass panel, the fan sits on Auto, and nobody has thought about the system until the sky turns orange.
Our view is that the best smoke plan is set up before the season. Know what filter slot you have. Know whether your blower is variable speed. Keep a spare MERV 13 or the highest rating your system accepts. At our altitude, systems already run differently than they do at sea level, so we match the filter to the equipment instead of putting the densest filter into every home.
We believe in flat-rate pricing and clear explanations, so you know what you are getting. Our NATE-certified technicians can check static pressure, recommend a filter and a media cabinet if it helps, and add whole-home air purification when you want a permanent solution. When a system is older and cannot be fitted for better filtration, we say so and show you the replacement options plainly.
Owner
Get your system ready before the next smoke event
Strong Heating And Cooling is a veteran-owned HVAC company in Colorado Springs with over 40 years of combined experience and NATE-certified technicians. We can check your filter slot and blower, recommend the right filter rating, and set up air purification for your home.
If your furnace is past the point where upgrading the filter makes sense, our published pricing helps you plan. A new 80% AFUE single-stage furnace is $3,900, and our higher-efficiency models with variable speed blowers start at $4,500 on our furnace pricing page. You can also see everything we offer on our services page, or start from our home page.
FAQ
What MERV rating should I use for wildfire smoke?
Use MERV 13 or higher if your system can handle it. The EPA and CDC both give this guidance. If your furnace or air conditioner cannot take MERV 13, use the highest rating it can accommodate. The Colorado health department names MERV 11 as a minimum worth considering.
Can a higher MERV filter damage my furnace?
A filter that is too restrictive can cut airflow, raise heat exchanger temperatures and trip safety limits. The solution is to match the filter to the system, not to avoid better filtration. A technician can measure static pressure and tell you the highest rating that fits.
Should I run my HVAC fan on “On” or “Auto” during smoke?
Use “On.” The EPA and the CDPHE advise running the fan continuously or as often as possible so air keeps cycling through the filter. If your system has a fresh air intake, close it or switch to recirculate.
Do I still need a portable air cleaner if I have a good HVAC filter?
It is not required, but it helps. The EPA says a portable cleaner can supplement forced-air filtration, and it is the main option for homes without central air. Choose a CARB-certified model with a CADR sized to the room.
How often should I change the filter during a smoke event?
Look at it every few days and replace it when it turns dark brown or black. The EPA notes this may be sooner than the manufacturer recommends.
Sources
- Preparing for Smoke and Heat, US EPA
- What is a MERV rating?, US EPA
- Indoor Air Filtration, US EPA
- Reduce Your Smoke Exposure, US EPA
- Wildfire Smoke Preparedness Program, Colorado Department of Public Health and Environment
- Indoor air quality after a fire, CDPHE
- Safety Guidelines: Wildfires and Wildfire Smoke, CDC
- Wildfire Smoke and Lung Health, American Lung Association
- High-MERV Filters, Building America Solution Center
- Filtration and disinfection FAQ, ASHRAE
- Confronting Restrictive Air Filters, ACHR News
- What is a MERV Rating? Air Filter Guidelines, Bryant
- Colorado Springs wildfire and smoke page, Filterbuy
- How to Remove Smoke Smell From Your House
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