When Not to Run a Whole-House Fan: Wildfire Smoke, Pollen, and Poor Outdoor Air

A whole-house fan is designed to pull in outdoor air through your open windows. At the same time, indoor air is exhausted through the attic. While this is helpful when the outdoor air is cool and clean, it isn’t as safe when the air is polluted by wildfire smoke, high pollen counts, smog, and dust on days with a high air quality index (AQI).

By working with Fox Family Heating & Air Conditioning, you can learn when whole-house fans are effective tools and when it is better to leave it turned off.

Why Outdoor Air Quality Matters With Whole-House Fans

A whole-house fan is not intended to filter incoming outdoor air, which is why outdoor air quality matters so much. If you’re running a whole-house fan during bad AQI days, you could end up pulling pollutants directly into your living spaces. Because you also have to keep your windows open when the whole-house fan is operating, it can lead to more pollutants in your home.

In comparison, heating, ventilation, and air conditioning (HVAC) systems can be set to recirculation mode. This allows the indoor air to be recirculated through the HVAC filters without intentionally pulling any outdoor air into the home. 

Do Not Run a Whole-House Fan During Wildfire Smoke

A whole-house fan should not be run during wildfire smoke because of the health risks involved. Instead, there are alternative things you can do to keep your family comfortable.

Why Smoke Is a Serious Indoor Air Risk

Wildfire smoke contains fine particles, including PM2.5 particles. When these pollutants are present, they can irritate your eyes, throat, and lungs. Unfortunately, wildfire smoke is especially hazardous for people who have asthma and respiratory issues. Because of how small the particles are, they can easily enter through vents, windows, cracks, and ventilation systems.

What To Do Instead

To protect your family’s comfort and health, use the following tips when wildfire smoke is present outside.

• Shut your windows and doors.
• Turn off the whole-house fan to keep outdoor air from being pulled inside.
• Set your fan to “On” or “Circulate” mode if your system has those options.
• Use a high-efficiency filter, such as MERV 13, if your HVAC system is designed to accommodate it.
• Turn on a portable HEPA purifier in your bedrooms or living room.

Avoid Running It on High-Pollen Days

We also generally recommend shutting your whole-house fan off on high-pollen days, especially if some of your family members suffer from allergies.

How Whole-House Fans Pull Allergens Indoors

To operate a whole-house fan, you must leave one or more windows open. This allows pollen to enter, and the fan can draw pollen throughout the home. As a result, whole-house fans can worsen asthma symptoms, allergies, sinus irritation, and indoor dust buildup.

Better Options During Allergy Season

Rather than use your whole-house fan during allergy season, we recommend the following alternatives.

• Always check your local pollen levels before you open any windows.
• During times with high pollen counts, keep your windows closed.
• Use HVAC filtration and portable air cleaners to filter out pollen.
• Replace your HVAC filter more frequently during heavy allergy seasons.

Skip Whole-House Fan Use During Smog, Dust, or Poor AQI

AQI is basically a system that measures and reports the levels of five different pollutants in the air. It is presented as a figure between 0 and 500 for your area, with 0 being the best air quality and 500 representing the highest pollution level.

When your air quality is smoky, dusty, visibly hazy, or otherwise unhealthy, you should avoid using the whole-house fan. Local conditions, such as construction dust, industrial odors, road traffic, chemical smells, and agricultural activity, can make your air quality worse than what the nearest AQI monitoring station reports. To protect your indoor air quality (IAQ), it’s a good idea to always check AirNow or a local weather app before you run the fan.

Whole-House Fan vs. HVAC Fan: Know the Difference

A whole-house fan is responsible for bringing outdoor air into your home. In comparison, the “HVAC fan/circulate” mode on your HVAC system is only moving indoor air through your filter. When outdoor air quality is poor, HVAC circulation is generally the safer option.

You can improve your IAQ by using a MERV filter that has a higher rating. However, you should check the manufacturer’s instructions for your unit to verify that it is compatible with higher ratings.

How To Decide If It Is Safe to Run Your Whole-House Fan

To determine if a whole-house fan is a good option right now, you can use the following checklist.

• Is the outdoor AQI at an unhealthy level or higher (above 100)?
• Is there any wildfire smoke, ash, or smoke odors present outside?
• Are pollen counts high?
• Is the air temperature warmer outside than it is inside?
• Is it unsafe to open your windows?
• Is there dust, exhaust, construction, or chemical odors present nearby?

If you answered yes to any of the previous questions, you should leave the whole-house fan off.

What To Do After Accidentally Running It During Bad Air

Sometimes, you may run your whole-house fan without realizing that the air quality was bad. When this happens, the following steps can help you alleviate IAQ issues.

• Turn your fan off.
• Close all of your home’s windows and doors.
• If it is an option, set your HVAC fan to recirculation mode.
• Turn on your portable HEPA air cleaners.
• Clean any settled dust in the room with a damp cloth.
• If smoke odors and poor airflow are recurring problems, schedule a professional HVAC inspection.

When You Can Use a Whole-House Fan Again

You can switch back to using your whole-house fan again if all of the following statements are true.

• The outdoor air quality has improved.
• Your area’s AQI is at an acceptable level.
• You no longer smell any smoke or ash odors.
• The visible haze is gone.
• Pollen levels are manageable.
• The outdoor temperature is cooler than the indoor temperature.

FAQs

Can I run a whole-house fan if I use an air purifier?

In general, you shouldn’t run the whole-house fan and air purifier at once. If the outdoor air quality is poor enough to require an air purifier, the whole-house fan should be shut off. Otherwise, the fan could bring in pollutants faster than the purifier can remove them.

Is a whole-house fan the same as air conditioning?

No, it is not. Whole-house fans ventilate using outdoor air. In comparison, ACs normally recirculate indoor air as they cool.

Should I run my HVAC fan during wildfire smoke?

Often, this is the best option. You should use a clean, appropriate filter for your HVAC system and set the system to recirculate indoor air.

What filter is best for wildfire smoke?

MERV 13 is generally the recommended option, but this is only if your HVAC system is able to handle it.

Should I open windows when outdoor air quality is bad?

No, you shouldn’t open windows and doors during bad AQI days because this allows wildfire smoke, pollen, and other pollutants to get inside your home.

Optimize Your IAQ

Whole-house fans are effective when the outdoor air is cool and clean. Because they ventilate using outdoor air, they aren’t ideal if there is smoke, pollen, dust, or other air quality issues present. On these days, setting the HVAC system to recirculate your indoor air, using a portable HEPA purifier, and closing your windows can help maintain your IAQ.

Need help finding the right IAQ equipment and filtration setup? Schedule an HVAC and indoor air quality inspection before smoke or allergy season.