How Often Should You Change Your Furnace Filter in Minnesota?

Check a standard 1-inch furnace filter every month while the heat or AC is running and change it whenever it looks dirty, and in any case at least every three months. That is ENERGY STAR's advice, and it fits most Minnesota houses. A deeper 4- or 5-inch media filter lasts much longer, usually 6 to 12 months depending on the maker, and pets, dust, smoke and a fan left running all shorten every one of those numbers.

The "every 90 days" printed on the package is a ceiling, not a schedule. The filter tells you when it's done if you look at it.

What the manufacturers and ENERGY STAR say

The EPA's page on MERV ratings adds the rule that sits over all of these: every filter needs periodic replacement, and you should follow the filter maker's recommendation. Goodman's installation instructions put the stakes bluntly, calling improper filter maintenance the most common cause of inadequate heating or cooling performance.

Why do furnace filters clog faster in Minnesota houses?

A Minnesota furnace runs from October into May, and in a house with central air the same blower and filter work through July and August. There are only a few weeks a year when the filter gets a break.

Things that shorten the interval:

• Pets. Dog and cat hair mats on the face of a 1-inch filter fast. With a shedding dog, a monthly change in winter is normal.
• Remodeling. Drywall sanding can load a filter in days. Change it when the work ends, and cover the return grilles while you sand.
• Wildfire smoke. The EPA's wildfire smoke guidance recommends a MERV 13 or higher filter during smoky periods and says to check it often and replace it when it looks dark brown or black. On a smoky summer we tell people to look weekly.
• Running the fan on "On." The EPA suggests switching the fan from Auto to On during smoke events so the filter cleans the air continuously. That helps, and it also means far more air goes through the filter. AprilAire says the same about its media filters: if the fan runs all the time, check it at six months.
• Older city houses. Many Minneapolis and Saint Paul furnaces sit on ductwork left from a gravity system conversion, often with too little return air. A slightly dirty filter starves those systems sooner than it would in a newer house.

If a filter turns black in a couple of weeks, it isn't dust. That's usually soot and deserves its own look; see why a furnace filter turns black.

How to check it

1. Turn the thermostat off or to a low setting so the blower isn't running while the filter is out.
2. Slide it out and look at the entering side, the side facing away from the furnace. Gray or fuzzy with visible dust means change it.
3. Hold it up to a light. If light barely comes through, it's done.
4. Look at the slot. Dust piled on the blower side, or a filter bowed in the middle, means air has been leaking around it or it's overdue.
5. Put the door or cover back firmly. On many furnaces the blower door presses a safety switch, and a loose door leaves the furnace off.

How do you find the right furnace filter size and direction?

• Size. Read the large numbers on the old filter's frame, such as 16x25x1. That's the nominal size you buy; the true size printed nearby is slightly smaller, and that's normal.
• Depth. A 1-inch slot takes a 1-inch filter. A media cabinet takes the specific media made for it, which a hardware store may not stock, so buy a spare.
• Arrow. The airflow arrow points toward the furnace, in the direction air travels from the return duct into the blower.
• Fit. Gaps around the edges let air skip the filter completely. Goodman's instructions say a replacement must be the same type and size as the one it replaces.
• Date it. Write the install date on the frame with a marker. It settles the "when did we last change this" question.

What MERV rating should a furnace filter be?

A higher MERV rating catches smaller particles, but it also resists airflow more, especially in a thin 1-inch panel. The EPA recommends at least MERV 13, or "as high a rating as your system fan and filter slot can accommodate," and says you may need an HVAC technician to decide what yours can handle. Our post on the best MERV rating for allergies goes through that choice.

The number a technician uses to decide is static pressure, the resistance the blower pushes against. A dense or dirty filter raises it, which cuts airflow, can trip the furnace's high limit in winter and can freeze the AC coil in summer. Our explainer on static pressure covers how it's measured. If your system is already near its limit, the fix is a deeper media cabinet or more return air, not a denser thin filter.

And if you're tempted to run without one while you wait for the right size, read what happens when you run a furnace without a filter first.

What we check at a tune-up

Every maintenance visit includes the filter: we note the size, check for bypass around the edges, and suggest a realistic change interval for your house. A furnace or AC tune-up is from $129, or $219 for both. If you're ready to stop buying 1-inch filters every month, we install media cabinets as part of our air purification service, and we can measure static pressure before recommending one.

When should you call a technician about the furnace filter?

Call if the filter is dirty again within two weeks, turns black, or is sucked out of shape, or if the furnace keeps shutting off on the high limit even with a clean filter. Those point to a duct, blower or combustion problem, not a filter schedule. We're open 7 days, 7am–9pm, including maintenance in Minneapolis. Call 612-429-3465.

Questions

Sources

• ENERGY STAR energystar.gov
• U.S. Environmental Protection Agency epa.gov