Filters, summer smoke and winter inversion

Two seasons, two different problems

The Treasure Valley has two distinct air quality seasons, and they are not the same problem wearing different hats.

Late summer into early autumn brings wildfire smoke, sometimes from fires hundreds of miles away. Fine particulate, arriving from outside, for days at a time.

Late autumn into winter brings inversion: cold dense air settles on the valley floor with warmer air above it acting like a lid, and everything emitted underneath that lid stays there. Particulate again, but generated locally and trapped rather than blown in.

For your heating system, both come down to the same question, which is what your filter is actually doing.

What the filter is for

There is a common misconception worth clearing up: the filter in your furnace was originally put there to protect the equipment, not you. Its first job is keeping the blower wheel and the heat exchanger clean.

Filters that meaningfully clean the air you breathe are a later idea, and they work, but they work by being denser. Denser means more resistance, and more resistance means less airflow, and less airflow causes exactly the faults covered elsewhere on this site: overheating furnaces, frozen coils, short cycling.

So it is a trade-off rather than a straight upgrade.

Choosing a filter without causing a problem

• A higher-rated filter genuinely filters better. That part of the marketing is true.
• A filter one inch thick has a small surface area, so a dense one restricts a lot. This is where people get into trouble.
• A thicker filter cabinet, four or five inches, has far more surface area, so it can be dense and still not restrict much. If you want serious filtration, that is the change that makes it work.
• Change it more often during smoke season. A filter loaded with smoke particulate is restricting air whatever it says on the box.
• If you fit a denser filter and the system starts misbehaving, the filter is the first thing to undo.

What a filter cannot do

• It does nothing about air coming in through gaps. During a smoke event, a leaky house is a bigger factor than the filter. Closing windows and keeping doors shut matters more than an upgrade.
• It does not remove gases or smells. Particulate filters catch particles. Smoke odor is partly gaseous and it passes straight through.
• It does not work when the system is off. In September the heating is off and the cooling may be off, so the filter is only doing something if the fan is running.
• It is not a substitute for a purifier in a room where somebody is genuinely struggling.

Burn restrictions

Open burning and residential wood burning in this valley are restricted according to air quality, with thresholds that tighten as conditions worsen, and the rules differ between incorporated cities and the unincorporated county.

We have not reproduced the thresholds here because we have not verified the current figures from the authority that sets them, and an out-of-date number would be worse than none. If you heat with wood, check the current status with the county rather than relying on a website.

What to do during a smoke event

Related

Weak airflow covers what happens when a filter restricts too much. Before you call has the filter check in context with the other three things worth doing first.