Wildfire Smoke and HVAC Filtration in Spokane | MERV-13, PM2.5 and ERV Intakes

PM2.5 refers to particulate matter with a diameter of 2.5 microns or smaller. For scale, a human hair is roughly 70 microns across. Particles this small stay suspended in air for long periods, travel enormous distances, and are small enough to penetrate deep into the respiratory system rather than being filtered out by the nose and throat.

Wildfire smoke is predominantly PM2.5, which is why it is both so visible as haze and so difficult to exclude from a building. Ordinary infiltration through a building envelope carries it indoors, and standard furnace filters do essentially nothing to capture it. During a significant smoke event, indoor particulate levels in an unprepared house track outdoor levels closely.

MERV stands for Minimum Efficiency Reporting Value and it rates a filter's ability to capture particles in specified size ranges. Higher numbers capture smaller particles more effectively. The ratings that matter in a residential context are these.

MERV-13 is the generally accepted practical minimum for meaningful wildfire smoke filtration in a residential system. It captures a substantial share of PM2.5 while remaining achievable in a properly designed residential air handler. Higher ratings capture more and impose considerably more resistance.

This is the part that gets skipped, and it is the reason so many filtration upgrades make systems worse. Every filter imposes resistance to airflow, measured as a pressure drop. A denser filter imposes more. The blower in your air handler has a finite capacity to overcome resistance, expressed as its rated performance against external static pressure.

A typical system is designed with a total external static pressure budget of around 0.5 inches of water column, and that budget has to cover the ductwork, the coil, the registers and the filter together. If the existing ductwork is already consuming most of it, there is no headroom left for a denser filter. Adding one anyway reduces airflow, which produces a cascade of consequences.

• Reduced airflow across the indoor coil lowers capacity in both heating and cooling.
• In cooling, low airflow can drop the coil below freezing and ice it up, which stops the system entirely.
• In heating, low airflow raises supply temperature and can trip high limit safeties.
• The blower works harder and draws more power to deliver less air.
• The system may short cycle, which is uncomfortable and hard on the equipment.
• Rooms furthest from the air handler suffer first and worst.

The correct approach is almost always to increase filter area rather than filter density in the same footprint. A four inch deep media filter has vastly more surface area than a one inch filter of the same face dimensions, which means it can achieve MERV-13 capture at a considerably lower pressure drop.

1. Measure total external static pressure on the existing system, with the current filter installed and again with it removed.
2. Establish how much of the budget the ductwork is consuming and how much headroom remains.
3. Where headroom exists, a deeper media cabinet with a MERV-13 filter is usually the answer.
4. Where it does not, address the duct restriction first, because filtration cannot be improved on a system that is already starved.
5. Consider a dedicated portable air cleaner for one or two rooms as a supplement, particularly bedrooms.
6. Re-measure static pressure after the change to confirm the system is still within its budget.

A four inch media cabinet is a physical modification requiring space at the air handler, which is why it needs planning rather than being a filter you buy at the hardware store. Where the space genuinely does not exist, portable air cleaners in the rooms that matter most are a legitimate and often overlooked answer.

Modern tight homes need mechanical ventilation, and in Climate Zone 5 homes above 1,500 square feet are required to have heat recovery or energy recovery ventilation. During a smoke event that ventilation system is bringing outdoor air into the house, which is precisely the air you are trying to keep out.

This does not mean switching ventilation off indefinitely. A tight house with no ventilation accumulates carbon dioxide, humidity and indoor pollutants, and the smoke event can last weeks. The correct approach is filtration on the outdoor air intake, so the ventilation system continues to work while the incoming air is cleaned.

• Confirm whether your ERV or HRV has provision for intake filtration, since many do.
• Check what filter rating the intake accepts, as it varies considerably by unit.
• Some units have a recirculation mode that can be used for short periods during peak events.
• Intake filters load quickly during a smoke event and need checking far more often than usual.
• If your unit has no intake filtration provision, that is worth addressing before the next season rather than during one.

While you are managing indoor air quality, the outdoor unit is drawing that same smoky air across its coil continuously. Fine ash deposits on and between the coil fins and acts as an insulating layer across the heat exchange surface, which reduces capacity exactly during the hot weather that typically accompanies smoke events.

This is the specific reason we recommend an autumn service visit in this area rather than a single annual one. Spring pollen loads the coil at one end of the season and late summer ash loads it at the other, and a coil cleaned in April is not clean by October. Rinsing the coil from the inside outwards, so debris exits the way it entered rather than being driven deeper into the fins, is what actually restores capacity.

1. Close windows and exterior doors and keep them closed for the duration.
2. Set the system fan to run continuously rather than only on a call for heating or cooling, so air circulates through the filter.
3. Check the filter far more frequently than usual, since it loads rapidly during a smoke event.
4. Check the ERV or HRV intake filter with the same frequency if you have one.
5. Run portable air cleaners in bedrooms and any room used for long periods.
6. Avoid adding indoor particulate from cooking, candles or vacuuming without a HEPA filter.
7. After the event passes, have the outdoor coil cleaned rather than assuming it recovered on its own.