Whole-house humidifier pad sizes (water panels)

A humidifier water panel (evaporator pad) is not a furnace air filter. Order by the pad part number or humidifier model on the unit sticker — AprilAire panels 10 / 12 / 35 / 45 and Honeywell Home pads in the HC22 / HC26 families are the common retail lines. Nominal face size is smaller than a return grille filter (often ~10×10 or ~10×13), and it is almost never the same SKU as your furnace filter size.

Pad vs furnace air filter

• Furnace / AC return filter — cardboard-framed panel in a wall/ceiling grille or media cabinet (e.g. 16x25x1, AprilAire 210 / 213, Honeywell FC100A1037 FC100A1029). Captures dust from circulating air.
• Humidifier water panel / pad — evaporative insert inside a whole-house humidifier on or near the furnace. Water coats a mesh or wicking pad; warm air picks up moisture. It clogs with mineral scale, not the same dust load as a return filter.

Do not substitute a 1-inch furnace filter for a humidifier pad, and do not assume “10” on an AprilAire water panel means a 10-inch square return filter. Wrong part = poor humidification or a physical no-fit.

If your home also has ductless wall cassettes, those indoor filters are a third category — small mesh behind the front panel, not a water panel or return filter: mini-split filters.

This page is equipment and fit care, not medical advice or health cures. We do not invent humidity health outcomes.

How to identify the pad from the sticker

1. Find the humidifier cabinet — usually a metal box on the furnace / air-handler plenum or a bypass duct connection. It is separate from the return-filter grille.
2. Read the unit model on the nameplate (examples: AprilAire 600, 700, 400; Honeywell Home HE220, HE260).
3. Match the water-panel / pad part from the manufacturer chart (tables below), or from the printed number on the old pad frame if it is still readable.
4. If the pad print is gone — use the unit model first. Measuring alone is a last resort; many pads share similar faces (35 vs 45 both list a 13×10×2 nominal) but are different materials and not interchangeable.
5. Turn humidification off and shut the water supply before opening the cover. Follow the carton / owner’s manual for your exact series (AprilAire notes unique steps by model).

AprilAire’s own change guide lists water panels 10, 12, 35, or 45 for the classic evaporative lineup. Newer HydroCore / HYDROL cartridges are a different product family — if your box says HydroCore, do not assume a classic 10/35 pad fits.

AprilAire water panels (verified shop specs)

Dimensions below are from AprilAire shop product pages (accessed 14 Sep 2026). Height × width × depth as AprilAire prints them.

35 vs 45: same nominal 13×10×2 label family, different published actual and materials (35 aluminum / synthetic fiber; 45 paper-wicking with aluminum). Order by part number for the 400 series — do not “eyeball swap” a 35 into a 400.

Not verified as AprilAire water-panel part numbers: standalone “50 / 51 / 55 / 60” panels did not appear on AprilAire’s water-panel collection or the classic 10/12/35/45 shop SKUs we fetched (14 Sep 2026). If a merchant lists those digits, confirm against the unit model sticker before buying. Assembly kits such as H510EZ1A (500 series), H635EZ1A (600), H735EZ1A (700), and H445EZ1A (400) are full tray/pad assemblies that include a classic panel — not separate “model 50” pads.

Honeywell Home humidifier pads (verified)

Honeywell Home / Resideo sell evaporative pads mainly in two size families. Specs below are from Honeywell Home product pages (accessed 14 Sep 2026). Exact SKU letters (A / E / P) mark standard vs antimicrobial coatings — match the pad your manual names.

Cross-brand listings mean a Honeywell-branded pad may physically match some AprilAire cabinets in the same size class — but AprilAire still publishes its own 10 / 35 part numbers for those models. Prefer the pad stamped on your unit or the OEM chart for that brand. We did not invent third-party “universal” SKUs.

Honeywell change cadence (manufacturer support): replace the humidification pad annually at the start of the heating season; with hard water, Honeywell notes you may need a mid-season replacement. Separate bypass-humidifier FAQ wording: replace pads once every season.

Change intervals (manufacturer-style only)

• AprilAire 10, 12, 35 — once per heating season (AprilAire water-panel change guide).
• AprilAire 45 — twice per heating season (same guide; 400-series wicking panel).
• Honeywell Home pads — annually at heating-season start; hard water may shorten that (Honeywell Home support, updated 2 Sep 2025).
• Visual check — heavy white scale, crumbling coating, or a pad that no longer wets evenly is a replace-now signal even if the calendar says otherwise.

Season changes are a useful reminder to open both the return air filter and the humidifier cover — they are different maintenance items. Furnace-filter calendar: seasonal change reminder · thickness bands: how often.

What NOT to do

• Do not order by furnace filter size (16x25x1, 20x25x4, etc.) when you need a humidifier pad.
• Do not swap AprilAire 35 for 45 (or the reverse) because the nominal print looks similar.
• Do not confuse AprilAire media air filters 210 / 213 with water panels 10 / 35 — different products: 210 vs 213.
• Do not confuse Honeywell FC100A1037 media cartridges with HC22 / HC26 humidifier pads: FC100A1037 FC100A1029.
• Do not invent dimensions or prices from marketplace titles alone when the OEM shop lists actuals.

Where to look next (no pack prices)

This guide does not list dollar prices — humidifier pad packs move often. Use the model tables above, then open manufacturer charts or merchant results for the exact part number. Price-free external references we fetched:

• AprilAire OEM — Water panels collection · Panel 10 · Panel 12 · Panel 35 · Panel 45 · How to change / cadence
• Honeywell Home OEM — HC22 pad page (HE220 / HE225) · HC26 standard pad page · When to replace pad
• Related FilterBySize pages — AprilAire 210 / 213 air filters · Honeywell FC100A1037 · How often (furnace filters) · Seasonal change · Move-in checklist · Mini-split filters Return grille · How to measure · MERV · All furnace filter sizes

Full policy: disclosure.