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AprilAire · humidifier · whole house bypass

AprilAire 400 replacement filter

Use part 45. Replace at least twice per heating season (manufacturer owner's manual).

The filter that fits

Maker lists this model

AprilAire Water Panel 45

AprilAire 45 Replacement Water Panel

Part number
45
How many
1 per unitpacks: 45 02 (2), 45 10 (10)
How often
Replace at least twice per heating season (manufacturer owner's manual)
Size
13 x 10 x 2 in13 x 10 x 2 in nominal (12.875 x 9.875 x 1.75 actual)
Buy Water Panel 45 on AmazonManufacturer page

Opens an Amazon search for AprilAire Water Panel 45. Check the listing says 45.

See the sources

Does your exact model share this filter?

AprilAire labels several versions of the 400. Check the label on your unit.

  • 400 takes 45
  • 400A same filter
  • 400M same filter
  • 400KZ same filter
  • 400MKZ same filter

Look-alike models that take a different filter

The look-alike 500 takes panel 10 and the 600 takes panel 35.

Other genuine options from AprilAire

Variants and kits the maker sells for this model. Same fit, different media or packaging.

  • AprilAire H445EZ1A EZKit (2-pack; 45 panel pre-assembled with 4403 scale control insert and 4407 distribution tray)
    H445EZ1A · Find on Amazon
  • AprilAire 4788 Maintenance Kit with 45 Water Panel
    4788 · Find on Amazon

Sources

High confidence: the manufacturer names this exact model on a live page. Each line is the page we read and the day we read it.

  • Which filter fits shop.aprilaire.com Maker's own page checked Oct 1, 2026
    AprilAire 45 Replacement Water Panel For 400, 400A, and 400M Whole-House Humidifiers ... Compatible Models: 400, 400A, 400M, 400KZ, 400MKZ
  • How many storage.googleapis.com Maker's own page checked Oct 1, 2026
    AprilAire owner's manual B2209190D (09.23), dealer-hosted copy, PDF p9 (Model 400 maintenance): 'Slide the used Water Panel (G) out ... Slide a new genuine AprilAire Water Panel 45 (G) into the scale control insert'; the second panel in the box is a spare
  • How often storage.googleapis.com Maker's own page checked Oct 1, 2026
    AprilAire owner's manual B2209190D (09.23), dealer-hosted copy, PDF p7: 'The Water Panel should be changed at least once a year with the exception of Models 400 and 400M which should be changed at least twice per heating season'
  • Model variants shop.aprilaire.com Maker's own page checked Oct 1, 2026
    Compatible Models: 400, 400A, 400M; 'AprilAire recommends replacing the water panel assembly twice annually'
  • Size shop.aprilaire.com Maker's own page checked Oct 1, 2026
    Materials: Paper - Wicking, Aluminum Nominal Measurements: 13H x 10W x 2D (inches) Actual Measurements: 12.875H x 9.875W x 1.75D (inches)
  • What's in it shop.aprilaire.com Maker's own page checked Oct 1, 2026
    The heavy-duty paper panel is reinforced with 2 layers of aluminum and coated with an anti-microbial agent
  • Packs shop.aprilaire.com Maker's own page checked Oct 1, 2026
    Package Size 2-pack, 10-pack (no single panel); SKUs '45 02', '45 10'
Show all 11 sources

Which filter fits

  1. shop.aprilaire.com Maker's own page checked Oct 1, 2026
    AprilAire 45 Replacement Water Panel For 400, 400A, and 400M Whole-House Humidifiers ... Compatible Models: 400, 400A, 400M, 400KZ, 400MKZ
  2. storage.googleapis.com Maker's own page checked Oct 1, 2026
    AprilAire owner's manual B2209190D (09.23), dealer-hosted copy, PDF p8 table: 'UNIT WATER PANEL PART NUMBER 300 35 400 45 500 10 600 35 700 35'
  3. shop.aprilaire.com Maker's own page checked Oct 1, 2026
    INCLUDED IN THE BOX: 400 Humidifier ... (2) Model 45 Water Panel
  4. storage.googleapis.com Maker's own page checked Oct 1, 2026
    AprilAire owner's manual B2209190D (09.23), dealer-hosted copy, PDF p3: 'With bypass Models 400, 500, and 600 Series, HVAC system air is moved from the supply duct through the humidifier into the return duct'
  5. shop.aprilaire.com Maker's own page checked Oct 1, 2026
    AprilAire 45 Replacement Water Panel

How many

  1. storage.googleapis.com Maker's own page checked Oct 1, 2026
    AprilAire owner's manual B2209190D (09.23), dealer-hosted copy, PDF p9 (Model 400 maintenance): 'Slide the used Water Panel (G) out ... Slide a new genuine AprilAire Water Panel 45 (G) into the scale control insert'; the second panel in the box is a spare

How often

  1. storage.googleapis.com Maker's own page checked Oct 1, 2026
    AprilAire owner's manual B2209190D (09.23), dealer-hosted copy, PDF p7: 'The Water Panel should be changed at least once a year with the exception of Models 400 and 400M which should be changed at least twice per heating season'

Model variants

  1. shop.aprilaire.com Maker's own page checked Oct 1, 2026
    Compatible Models: 400, 400A, 400M; 'AprilAire recommends replacing the water panel assembly twice annually'

Size

  1. shop.aprilaire.com Maker's own page checked Oct 1, 2026
    Materials: Paper - Wicking, Aluminum Nominal Measurements: 13H x 10W x 2D (inches) Actual Measurements: 12.875H x 9.875W x 1.75D (inches)

What's in it

  1. shop.aprilaire.com Maker's own page checked Oct 1, 2026
    The heavy-duty paper panel is reinforced with 2 layers of aluminum and coated with an anti-microbial agent

Packs

  1. shop.aprilaire.com Maker's own page checked Oct 1, 2026
    Package Size 2-pack, 10-pack (no single panel); SKUs '45 02', '45 10'