Every August and September, wildfire smoke settles over Pierce County for a few days to a couple of weeks. AQI numbers go from good to unhealthy for sensitive groups to unhealthy to very unhealthy. Your HVAC system is either your best defense or a straight pipe pumping smoke through your home. Here is exactly what to do.
The 24-Hour Playbook When Smoke Hits
- Close every window and exterior door. Weather-strip any gaps you can. Basements, garages, and doggie doors leak more than you think.
- Set your HVAC fan to ON, not AUTO. The blower runs continuously and pushes air through your filter constantly instead of only when heat or cool is calling.
- Change your filter or install a MERV 13. A MERV 11 removes some PM2.5 (the harmful smoke particulate). A MERV 13 removes most. If your system tolerates it, MERV 13 is the wildfire filter.
- Close fresh air ventilation. If you have an ERV, HRV, or fresh air intake damper, close or bypass it. Otherwise you are actively pulling smoke inside.
- Run standalone HEPA units in bedrooms. Even one Coway or Winix in the master bedroom overnight helps.
- Monitor with a $100 PM2.5 sensor. A PurpleAir or Awair tells you if your defenses are working.
Understanding MERV Ratings for Smoke
| MERV | Smoke Removal (PM2.5) | Airflow Impact | Recommended Use |
|---|---|---|---|
| MERV 8 | Minimal | Low restriction | Not adequate for smoke |
| MERV 11 | Partial | Moderate | Baseline for allergy or smoke-prone |
| MERV 13 | Good (up to 90% of PM2.5) | Higher restriction; check static pressure | Best practical option for whole-home |
| HEPA | Excellent (99.97% of 0.3 micron) | Too high for main duct; bypass only | Standalone units or bypass loop |
The catch: throwing a MERV 13 in a system designed for MERV 8 chokes airflow, spikes static pressure, and can trip safety switches or damage the blower motor. If your system has a big media cabinet or was installed with high MERV in mind, you are fine. If it is a 1-inch slot on a 20-year-old furnace, verify with a tech before upgrading.
Standalone HEPA Units: Sizing and Placement
A standalone HEPA does not need a duct system. It sits in a room, pulls air through a HEPA filter, and puts it back out. During smoke events, put one in every occupied bedroom overnight. Sizing:
- Small bedroom (100 to 150 sq ft): CADR around 130 or more
- Master bedroom (200 to 300 sq ft): CADR 200 to 300
- Open living area (400 to 500 sq ft): CADR 350 or higher, or use two units
Good brands include Coway, Winix, Blueair, and Levoit. Expect $150 to $600 depending on room size.
The DIY Corsi-Rosenthal Box
If store-bought HEPAs are sold out (they will be), you can build a Corsi-Rosenthal box: four MERV 13 filters taped into a cube around a 20-inch box fan. Total cost around $80 to $120. Effective and covered by university studies during the 2020 to 2023 wildfire seasons.
What About “Fresh Air” Fans?
Attic fans, whole-house fans, and window fans PULL OUTSIDE AIR IN. During a smoke event they are the worst thing you can run. Kill power to them for the duration of the event.
After the Smoke Clears
- Change your filter. It is loaded.
- Rinse the outdoor unit coil (soot deposits reduce efficiency).
- Vacuum or wipe surfaces that were exposed. Smoke leaves particulate residue.
- Wash sheets and pillowcases; smoke particles adhere.
- Consider a slightly higher MERV as your new standard if you can tolerate it.
Health-Sensitive Households
If someone in your home has asthma, COPD, heart disease, or is pregnant, take smoke seriously. Consider:
- Multiple HEPA units, not just one
- N95 masks when you have to go outside
- Postponing outdoor exercise
- Monitoring indoor PM2.5 continuously
- Talking to your doctor about medication adjustments in advance of smoke season
Frequently Asked Questions
Should I run my AC or heat pump during smoke events?
Yes if you need cooling. It runs the blower and filter the same as fan-only mode, plus it dehumidifies. Just keep the outdoor unit running (do not shut it off); the outdoor coil does not care about smoke.
Is smoke bad for my HVAC equipment?
Not usually. The outdoor coil gets a light layer of soot; a spring cleaning takes care of it. Indoor components see less exposure. The biggest risk is a clogged filter starving the system for airflow.
Can I upgrade my system before smoke season?
Yes. If your home cannot tolerate a MERV 13 in the current setup, adding a 4-inch media cabinet or a HEPA bypass gives you serious smoke defense. Best time to do this work is spring or early summer, before the season starts.
What about “smart” purifiers with wildfire modes?
They are fine but the underlying filter is what matters. A HEPA is a HEPA whether it has wifi or not. Do not overpay for connectivity if the filter is the same.
Does GreenFlow do smoke-season HVAC prep?
Yes. We can install higher MERV filter cabinets, add HEPA bypass loops, and evaluate whether your existing system can handle upgraded filtration.
Questions? Real people answer.
Call GreenFlow Heating & Cooling. No pushy sales pitch, just a technician who can look at your system.