PadMatch

PadMatch / Every humidifier water panel and furnace filter size

Part numberAprilAire 45
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Humidifier models
AprilAire 400 / 400M
Correct pad
AprilAire 45
Pad dimensions
9.875 in W x 12.875 in H x 1.75 in D (nominal 10 x 13 x 2)
Replacement interval
once per heating season

The correct replacement pad for the AprilAire 400 / 400M, and the sizes that look right but are not.

The AprilAire 400 / 400M takes the AprilAire 45 pad. Buy that part; a different-size pad lets water bypass into the plenum.
Fits these humidifier models
Fitment notes

The 400 is the drainless evaporative unit and is the only Aprilaire that takes the paper-based 45 wick. None of the mesh panels fit it.

The 400-series is an evaporative bypass humidifier with no fan and no water waste, so its wick is a different, paper-based part from the 35 and installs in either orientation with no paint dot.

The pad to buy

Find your model

Read the number stamped on the humidifier cabinet or printed on the old pad, then match it below. Confirm the frame size before buying a generic.

AprilAire 400400M
Related

Sources: AprilAire Water Panel 45 product page