TL;DR Quick Answers

A 24x30 return air filter grille is a return vent cover that holds a 24x30x1 air filter in a rack behind a hinged, louvered door. The 24x30 is the duct opening it covers, and suppliers also call it a filter-back grille or filter grille door. 

ENERGY STAR, the EPA's efficiency program, notes that with an attic unit, the filter may sit just inside a screen on a ceiling or wall of a top-floor room or hallway.

A screwed-on grille can still hide a filter, so look before assuming the filter sits at the air handler. Check the furnace too. In one Lawrence Berkeley National Laboratory test home with filters in both places, the ceiling grille filter loaded up while the furnace filter did not.

Plan on seven steps, a stepladder, and ideally a helper.

What To Have Ready Before You Climb

• A stepladder that reaches the grille below its highest marked step.
• Flat-head and Phillips screwdrivers, since the Building America guide notes most return grilles have screws or levers.
• 1-inch masking tape to seal a loose filter.
• Safety glasses and a helper, our additions.

 The Seven Steps 

1. Turn the system off. Switch the thermostat and fan off, per ENERGY STAR.
2. Prep the filter on the ground. Match the printed size, remove the wrap, and date the frame.
3. Set up the ladder. Open it fully, spreaders locked, on level floor.
4. Support the door. Hold it while releasing the latches, then lower it.
5. Remove the old filter. Hand it to your helper.
6. Seat the new filter. Point the arrow up and set it flat in the rack.
7. Close and restart. Latch the door flush, climb down, and turn the system on.

If the door won't close flat, check for a bowed frame or a filter thicker than the rack.

The arrow points up, into the ceiling, because room air travels up into the return duct. The Building America Solution Center, a U.S. Department of Energy resource run by Pacific Northwest National Laboratory, says in its return grille filter guide that arrows should point into the duct.

"Toward the blower" is hard to picture from a ladder, so we read it as "away from your face." If the old filter pointed down at the room, it was backward. Any wire support grid faces up, and the guide warns that this wire can be very sharp.

The National Institute for Occupational Safety and Health (NIOSH), part of the CDC, notes that many ladder injuries happen at home and lists overreaching and carrying objects as improper use. A 24x30 grille is also larger than the Building America guide's roughly 20 by 25 inch example.

• Stay at or below the highest standing level on the label and off the top cap, per the stepladder use and care guidance from the American Ladder Institute, a nonprofit safety group.
• Set the ladder beside the grille so the door swings clear.
• Move the ladder instead of leaning for a far latch.

 Yes, with the arrow pointing into the wall, and many wall grilles are reachable from the floor. The real difference is gravity: a ceiling filter hangs on the door, so we recheck its fit. 

A 24x30 grille with a 1-inch rack takes a 24x30x1 filter. The size on the old frame is the nominal size, and the filter runs slightly smaller so it can slide in and seal. With no label, measure the rack inside the door to the nearest eighth of an inch, not the wider outer face.

Filterbuy is an American air filter manufacturer that cuts, pleats, and frames HVAC filters to order in its own U.S. plants. Filterbuy lists its 24x30x1 at 23.88 by 29.88 by 0.75 inches and makes it in MERV 8, MERV 11, MERV 13, and Odor Eliminator on its 24x30x1 product page, which also notes that cut sizes vary between brands and advises checking your HVAC manual for the right rating.

A 24x30x2 air filter is twice the nominal depth. If your rack is 2 inches deep, the Building America guide favors the thickest filter it accepts.

Most homes change a 1-inch filter every 60 to 90 days, the range the EPA's guide to air cleaners in the home says manufacturers typically recommend, or sooner if filters come out heavily soiled.

Intervals vary: Building America says 3 to 6 months, and Consumer Reports says three months for 1-inch filters. We let the calendar set the outer limit and a monthly look, as ENERGY STAR recommends, set the real schedule. Tie it to a monthly home maintenance routine and lean short for a 1-inch ceiling filter.

•  Gaps around the filter. The EPA advises a snug fit, and the Building America guide suggests sealing gaps with masking tape.

• Whistling after an upgrade. Air may be forced around a denser filter. Berkeley Lab stopped MERV 16 testing in two of 10 homes over it, so recheck the fit and ask a technician.

• A filter that falls or shifts. Keep a hand under it as the door lowers, and confirm it sits flat at each monthly check.

• A latch that won't hold. Repair it rather than propping the door shut.