When the AQI spikes across the Inland Northwest, your HVAC system is either helping or hurting. Here’s what MERV rating actually means, whether you need an air purifier, and how to prep your system before smoke season peaks.
Late July through September, most of us in the Inland Northwest get used to checking the AirNow map before deciding whether to open a window. Spokane’s air quality can shift from “good” to “unhealthy” in the span of a day depending on which direction a fire is burning, and once it does, your home’s HVAC system becomes one of the most important tools you have for keeping indoor air breathable — for better or worse, depending on how it’s set up.
Why Indoor Air Matters as Much as Outdoor Air During Smoke Events
A common misconception is that once you’re inside with the windows closed, you’re safe. That’s only partly true. Wildfire smoke particles (PM2.5) are small enough to work their way into homes through gaps around doors and windows, attic vents, and any HVAC intake that’s pulling in outdoor air. A standard low-grade furnace filter — the kind rated MERV 4-6 that came with the house — does very little to stop particles that small. Meanwhile, if your system is cycling air properly with the right filtration, it can actually reduce indoor smoke exposure significantly compared to outdoor levels. The EPA’s guidance on indoor air quality covers this in more depth if you want the full technical background.
Understanding MERV Ratings (Without the Jargon)
MERV (Minimum Efficiency Reporting Value) measures how effectively a filter captures particles of different sizes, on a scale from 1 to 20. Here’s what actually matters for wildfire smoke specifically:
Here’s the catch: you can’t just drop a MERV 13 filter into any furnace and call it done. Higher-MERV filters are denser, which means more resistance to airflow. Older or smaller blower motors can struggle to pull air through a high-MERV filter, which can reduce system efficiency, cause uneven heating and cooling, and in some cases stress the blower motor over time. This is exactly the kind of thing worth a quick call before smoke season, not during it — we can tell you what the highest MERV rating your specific system can handle without a fight.
What to Do Before the Smoke Rolls In
1. Get your filter situation sorted now, not mid-emergency. If your system can support it, upgrade to MERV 11–13. If you’re not sure what your system can handle, that’s a five-minute question for a technician, not something to guess on.
2. Have a spare filter (or three) on hand. During heavy smoke weeks, filters clog faster than normal — sometimes in half the usual time. Running a system with a maxed-out filter restricts airflow and reduces the system’s ability to filter anything.
3. Check your fresh-air intake settings, if your system has them. Some HVAC setups include a dedicated outdoor air intake for ventilation. During heavy smoke events, this typically should be closed or minimized — a technician can walk you through your specific system’s settings.
4. Seal obvious gaps. Weatherstripping around doors and windows isn’t an HVAC fix, but it directly affects how much smoke infiltrates regardless of what your filter is doing.
5. Consider a standalone air purifier for high-use rooms. For bedrooms and living rooms, a portable HEPA air purifier sized to the room’s square footage can supplement whole-home filtration, especially in homes where the ductwork or blower can’t support a high-MERV filter.
Should You Run the Fan Continuously During Smoke Events?
Running your HVAC system’s fan on “on” instead of “auto” keeps air continuously cycling through the filter rather than only when heating or cooling is actively running, which can meaningfully improve filtration during a multi-day smoke event. The tradeoff is higher electricity use and faster filter loading — worth it during a genuinely bad air quality stretch, less necessary on a moderate day. Check the current AirNow reading for Spokane before deciding; if it’s in the “unhealthy for sensitive groups” range or worse, continuous fan operation with a good filter is generally worth the tradeoff.
Ductless Mini Splits and Smoke Filtration
If your home (or a specific room) is conditioned with a ductless mini split rather than central air, filtration works differently — most ductless indoor units have their own washable pre-filters, which are good for dust and larger particles but generally not rated for fine PM2.5 the way a MERV-rated central filter is. If wildfire smoke is a recurring concern for a specific ductless-conditioned room, a dedicated portable air purifier in that space is usually the more effective add-on than relying on the mini split’s built-in filter alone.
Commercial Buildings Have Additional Considerations
Restaurants, offices, retail spaces, and medical facilities have their own air handling requirements during smoke events, often involving rooftop unit fresh-air dampers, filtration banks, and in some cases separate makeup air units. If you manage a commercial building in Spokane, Coeur d’Alene, or the surrounding area, it’s worth a proactive check of your system’s smoke-season readiness before AQI spikes rather than during one — our commercial service team can assess your specific rooftop or split system setup.
Is a Whole-Home Air Purifier or Media Air Cleaner Worth It?
Beyond a higher-MERV filter, some homeowners choose to add a dedicated whole-home air cleaner installed directly into the ductwork — a step up from filtration alone, using either a deeper-pleat media cabinet or, in some cases, additional electronic filtration. These tend to make the most sense for households with someone managing asthma, COPD, or another respiratory condition where smoke season is a recurring, serious concern rather than an occasional inconvenience. They’re a bigger upfront investment than a filter upgrade, but they don’t carry the airflow-resistance tradeoff of cramming an oversized filter into a standard slot, since they’re sized into the system properly from the start. If this is something you’re considering, it’s worth a conversation during your regular system maintenance rather than a rushed decision in the middle of a smoke event.
A Pre-Smoke-Season Checklist
- ☐ Confirm the highest MERV rating your system’s blower can handle
- ☐ Install a fresh MERV 11–13 filter (or highest supported rating)
- ☐ Stock extra filters for faster-than-normal replacement during heavy smoke weeks
- ☐ Identify and close any dedicated outdoor air intakes during smoke events
- ☐ Check weatherstripping on doors and windows
- ☐ Consider portable HEPA purifiers for bedrooms, especially for household members with asthma or respiratory conditions
- ☐ Bookmark AirNow.gov for real-time local AQI
When to Call Us
If you’re not sure what filter your system can actually support, if your system seems to be struggling with airflow already, or if you want a full indoor air quality assessment before smoke season peaks, that’s exactly the kind of call worth making in July rather than during a red-AQI week in August. We can also talk through whole-home air filtration upgrades if you want something beyond what a furnace filter alone can deliver.
Schedule an indoor air quality assessment or call (509) 534-0547.