How to Change a Furnace Filter

Start at the furnace or air handler. Most of them sit in a basement, an attic, a garage or a utility closet. Look at the point where the big return duct meets the cabinet, because the filter always lives on the return side, ahead of the blower.

On this Trane the slot is a narrow gap low on the left, and the cardboard edge of the filter pokes out of it. Other units hold the filter in a slide-out tray on top or underneath. A flashlight earns its keep in a dark closet.

The size is printed along the cardboard edge of the filter that is already in the slot. This one reads 16x25x1, which is height by width by thickness in inches. Buy that exact size. A filter half an inch small leaves a gap, and dirty air goes around it instead of through it.

No old filter to read? Measure the opening and the depth of the slot with a tape measure, then match it to a stock size. Common ones are 16x25x1, 20x25x1 and 16x20x1, and some cabinets take a 4 inch deep filter.

Kill the power before anything comes out of the slot. On the thermostat, open System Mode and set it to Off. Heating and cooling both stop, so the blower cannot start while the slot is empty.

Two other routes work. Many furnaces have a light switch mounted right on the side of the cabinet, and that switch does the same job. If yours has no switch, go to the panel and flip the breaker labeled furnace or air handler.

Take hold of the cardboard edge sticking out of the slot and draw the filter straight toward you. Keep it level. Months of dust sit on the return face, and tipping the frame dumps that dust on the closet floor.

Go slow and stop when it is about halfway out. That gives you a clear look at the frame while the filter is still sitting the way it was installed, which is what the next step depends on.

Look along the cardboard edge for a small printed arrow marked air flow direction. Every pleated filter carries one. Note which way it points while the old filter is still half in the slot, because that is the direction the new one has to go.

The arrow points toward the furnace and the blower, away from the return duct. Air leaves your rooms, comes through the return, passes through the filter in the direction of the arrow, then reaches the blower.

Before the new filter goes anywhere near the slot, write the date on its cardboard edge with a permanent marker. Put it on the face you can see from the front, so you can read it later without pulling the filter back out.

One inch pleated filters run one to three months. Reid changes his every two. Pets, a gravel road or anyone in the house with allergies moves you toward the short end of that range.

Turn the new filter over and compare the two faces. One side is open pleated media. The other has a wire grid stretched across the frame, holding the pleats apart under suction.

That mesh side goes toward the blower, the same way the arrow points. It is a backstop: if the media ever starts to break down, the grid keeps it out of the blower motor instead of letting it clog the wheel.

Hold the filter with the arrow aimed at the furnace, line the frame up with the slot and push it in until it stops. It should go with light pressure. If it binds, pull it back out and check the size rather than forcing it.

Look at the frame once it is seated. Nothing bowed, no gap around the edges. Turn the power back on at the thermostat, the switch or the breaker, and you are set for the next couple of months.