Inversion season and your furnace filter

HVAC tips for Utah homes

Most winters, the Salt Lake Valley gets a few stretches where cold air settles on the valley floor and a layer of warmer air sits on top of it like a lid. That's an inversion. Car exhaust, wood smoke and other pollution get trapped under the lid, and the air turns hazy and gray until a storm comes through and clears it out.

You can't do much about the air outside. You can do a fair amount about the air inside. The furnace filter is the first place to start, because every bit of air your furnace moves goes through it.

What the filter actually does

Your furnace pulls air from the house through return grilles, runs it past the filter, heats it and pushes it back out. The filter was originally there to protect the blower and heat exchanger from dust. A better filter can also catch the fine particles that make inversion air so unpleasant.

Outdoor air gets into every house through small gaps, doors opening and exhaust fans running. On bad-air days, that air carries fine particles with it. A decent filter, run often enough, pulls some of those back out.

MERV ratings in plain terms

Filters are rated on a MERV scale. Higher numbers catch smaller particles. Roughly:

• MERV 1–4: the thin fiberglass filters. They protect the equipment from big stuff and not much else.
• MERV 5–8: pleated filters that catch dust, pollen and pet hair. A reasonable baseline.
• MERV 11–13: catch a much larger share of the fine particles that come with smoke and smog.

The EPA's general guidance for homes is to use a MERV 13 filter, or the highest rating your system can handle. That last part matters.

Don't choke the furnace

A higher-rated filter is denser. A dense filter in a thin, one-inch slot can restrict airflow. When a furnace can't move enough air, it runs hot. That can trip its safety limit and make it shut off and restart over and over. Over time it's hard on the equipment.

A thicker media filter, four or five inches deep, has much more surface area. It can hold a high MERV rating without starving the furnace for air. If you want better filtration and your furnace has a one-inch slot, a media cabinet is often the right fix. We can tell you whether your system has room for one.

How often to change it

The number printed on the package assumes average conditions. Inversion season is not average. Here's a practical rule for Salt Lake homes:

• One-inch pleated filters: check monthly through the heating season. Change it when it looks gray, or every one to three months at most.
• Four- to five-inch media filters: check every few months. Many last six months or more, but pets, remodeling or a long run of bad-air days will shorten that.
• After a smoky spell or a long inversion: pull the filter and look. If it's dark, swap it.

Write the date on the edge of the filter when you put it in. It saves guessing later. If you're not sure which size you need, our filter finder walks you through it.

Other things that help on bad-air days

Let the fan run

The filter only cleans air when air is moving through it. In mild weather the furnace may barely run. Setting the thermostat fan to "On" or to a circulate mode keeps air moving through the filter. It uses more electricity, so many people only do it on red-air days.

Cut down on outside air where it makes sense

Keep windows closed on bad days. Seal obvious drafts around doors. But don't stop using your kitchen range hood when you cook, especially on a gas stove. Cooking adds its own particles, and the hood is there for a reason.

Add a portable air cleaner

A good portable HEPA air cleaner in the bedroom is a cheap, effective addition. Match its size to the room. Run it with the door closed at night.

Check the air forecast

The Utah Division of Air Quality posts daily forecasts and action days at air.utah.gov. It's a simple way to know when to turn the fan on and keep the windows shut.

Whole-home options

If you want more than a filter can do, there are whole-home air cleaners, UV lights and ventilation systems that bring in fresh air through a filter. They aren't right for every house. It depends on your ductwork, your furnace and what you're trying to fix. Our indoor air quality page covers what we install.

When to call a pro

• Your furnace shuts off and restarts often, especially after you switched to a higher-MERV filter.
• The filter is visibly bent or sucked into the slot, or there's dust building up around the filter door.
• Some rooms get far less air than others, or the house stays dusty no matter what filter you use. That can point to duct leaks.
• You want a media cabinet or a whole-home air cleaner added.
• The furnace makes new noises or smells. Start with furnace repair.

A yearly tune-up is also a good time to size up your filter setup. You can keep track of it with our maintenance checklist, or let a maintenance plan handle the reminders.