Goodman Furnace Filters: Size, Where It Goes, and How Often to Change It

Your Goodman furnace filter size is printed right on the cardboard edge of the filter that’s already in there, something like 16x20x1. The filter itself sits in one of a few places depending on how your system was installed: a large return air grille, a slot next to the furnace, or inside the blower compartment. Changing it is genuinely homeowner work, and it’s the single cheapest thing you can do to keep your system running well. Here’s how to handle all of it.

How to find your filter size

Don’t guess and don’t measure first. Pull the old filter and look at the cardboard frame around the edge. The size is printed right on it, written as width by height by thickness in inches, like 16x20x1 or 20x25x1. That last number is the thickness, and it matters as much as the other two.

If the print has worn off, measure the old filter yourself with a tape and round to the nearest standard size. Common sizes on Goodman systems include 16x20x1, 20x25x1, and 14x20x1, but the right one is whatever fits your slot. Buy the size that matches snug. A filter that’s too small lets air slip around the edges unfiltered, which defeats the point.

Where the filter actually sits

Here’s the thing that trips people up: Goodman doesn’t put the filter in one fixed spot. The installer decides where it goes based on the house, so two Goodman furnaces can have the filter in completely different places. There are three common spots.

A return air grille. A lot of systems put the filter behind a big louvered grille on a hallway wall or ceiling. Look for two tabs, knobs, or a hinged door that lets the grille swing open. The filter slides in right behind it.

A slot near the furnace. Some setups have a filter slot built into the return duct right where it meets the furnace cabinet, often with a small cover you slide off. The filter slides into that slot.

Inside the blower compartment. On some Goodman units the filter lives inside, behind the lower front access panel, sitting at the bottom near the blower. Pop the panel and you’ll see it.

If you can’t spot any of these, follow the big return duct back toward the furnace and look along the way for a grille or a slot. That return side is always where the filter lives, because the filter’s job is to clean air before it reaches the equipment.

Which direction it goes

Every filter has an airflow arrow printed on the frame. The arrow points toward the furnace, in the direction the air is moving, which is from the room into the equipment. Get this backward and the filter won’t work the way it should and can collapse over time. Look for the arrow before you slide the new one in.

What MERV to buy

MERV is the rating for how much the filter catches. Higher numbers catch finer particles, but they also push back harder against airflow.

For most homes, MERV 8 to 11 is the practical range. It handles dust, pollen, and pet dander without strangling your blower. You can go up to MERV 13 for finer filtration, but only if your system was set up to handle the extra resistance. This is the mistake I see most: someone buys the highest-MERV filter on the shelf thinking more is better, and they end up with weak airflow and a blower working overtime. A too-restrictive filter causes the same problems a dirty one does.

How often to change it

Swap a standard 1-inch filter every 1 to 3 months. If you’ve got pets, allergies, or you run the system hard, lean toward the shorter end. Thicker 4-inch media filters have more surface area and can usually go several months to a full season.

The easy habit is to check it once a month. Hold it up to a light. If you can’t see light through it and it looks gray and packed, it’s done. You don’t have to wait for a schedule if it’s clearly loaded.

Signs your filter is clogged

A dirty filter announces itself if you know what to listen for:

• Airflow from the vents drops off and the house won’t hold temperature.
• The system runs longer and your energy use creeps up.
• In bad cases the coil freezes over or the furnace overheats and shuts down on its own.
• Dust builds up faster around the house because air is slipping past a packed filter.

Most of these clear up the moment you put in a fresh filter.

When to call us

Changing the filter is yours to do, and it solves a real share of comfort complaints on its own. But if you’ve put in a clean, correctly sized filter and you’re still getting weak airflow, short cycling, a frozen coil, or a furnace that keeps shutting off, the filter wasn’t the whole story. At that point something else needs looking at, whether it’s the blower, the coil, or the ductwork.

We work across the East Bay and Tri-Valley, usually same or next-day when we can. Call (925) 999-4095 or book at bayareahvacservice.com. I stand behind the work, and I’d always rather tell you a five-dollar filter fixed it.

Key takeaways

• The filter size is printed on the cardboard edge of the old filter. If that's faded, measure it and round to the nearest standard size.
• Goodman doesn't put the filter in one fixed place. Look at a return grille, a slot near the furnace cabinet, or inside the blower compartment behind the lower panel.
• MERV 8 to 11 is the practical range for most homes. Higher MERV catches more but restricts airflow, so don't jump to the highest number without reason.
• Change a 1-inch filter every 1 to 3 months. Thicker 4-inch filters can go longer. If airflow problems stick around after a fresh filter, that's a service call.

Related questions

Written by Andrew Kuznetsov. Andrew is the founder and owner of Bay Area HVAC Service (ADRIUM Service Solutions). He holds a California Contractor License (CSLB #1136642), EPA 608 certification, and completed factory training at the Daikin/Goodman plant in Houston in 2025. He writes from direct field experience, not marketing copy.

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