Indoor air quality in the Puget Sound — what your HVAC actually does

Indoor air in the Puget Sound has two seasons of trouble. From late June through early October, wildfire smoke from Eastern Washington and British Columbia can push outdoor PM2.5 into the unhealthy range for days at a time. From November through March, persistent damp and limited ventilation push indoor humidity well above what most homes are designed to handle. A residential HVAC system has tools for both — but only if it is set up right.

What your filter is actually doing

The single biggest lever for indoor air quality in a typical Puget Sound home is the filter sitting in your return-air grille or the cabinet on the side of your furnace. The MERV rating on the side of that filter tells you what particle sizes it catches. Most builders ship homes with a MERV 8, which is fine for protecting the furnace blower but does almost nothing for wildfire smoke.

• MERV 8: catches dust, pollen, pet dander — the basics
• MERV 11: pulls out finer particles, including a lot of wildfire smoke
• MERV 13: catches most of what an N95 catches, including most smoke and many viral droplets

You cannot just drop a MERV 13 into a system designed for a MERV 8 and walk away — the higher filter restricts airflow, and a system that cannot pull enough air through it will short-cycle and eventually damage the blower. The fix is a thicker filter cabinet (a 4-inch or 5-inch media cabinet) that gives the filter more surface area, so the system breathes normally even with a higher MERV rating. This is a one-time upgrade and runs a few hundred dollars installed.

What humidity does to a Puget Sound home

In summer, the goal is to keep indoor relative humidity below 60% so the house feels cool at higher thermostat setpoints. In winter, the goal is to keep it above 30% so floors do not gap and your sinuses do not crack. The wide middle band — 35% to 50% — is also where dust mites and mold are least likely to thrive.

The challenge in this climate is the shoulder seasons. October and April are wet, mild months where the AC will not run and the furnace barely runs. Without the HVAC cycling, you lose your main path for moisture removal. A whole-home dehumidifier ducted into the return is the cleanest fix, and it pairs with your existing thermostat so you set it once and forget it.

Wildfire smoke days

When the AQI climbs above 100, the right move is to seal the house up tight and circulate air through the highest-MERV filter you can run. Here is the practical protocol we walk our maintenance customers through:

• Close all windows and doors before smoke arrives, not after
• Turn the thermostat fan to "on" instead of "auto" so air keeps cycling through the filter
• Verify the fresh-air intake (if you have an HRV or ERV) is in recirculation mode
• Run a portable HEPA in the bedroom you sleep in

None of this requires new equipment if your system was sized and installed properly. If it does require a new piece of equipment — say, the filter cabinet upgrade or a sealed combustion furnace replacing an atmospheric unit — we will tell you why on the quote, not bury it in line items.

The annual tune-up matters here more than most places

Puget Sound HVAC equipment runs in saltwater-adjacent air and tends to accumulate green corrosion on outdoor coils faster than systems in dry climates. A spring tune-up that includes a coil wash, a refrigerant pressure check, and a static-pressure test on the duct system catches the things that quietly cut efficiency long before they cause a breakdown.

If you want a technician to walk through your specific setup and tell you what is and is not worth changing, give the office a call. The number is at the top of every page.