4-inch or 5-inch?
Start with the part.
Different products can use different size labels while being sold as replacements for the same original part. Those labels do not make all filters interchangeable.
Two numbers doing different jobs
The product's size label. It is a buying reference, not a measurement of every edge.
Filterbuy's published measurements for its specific FC100A1029 replacement, in inches.
A concrete example: FC100A1029
Resideo's replacement guide labels the original FC100A1029 as 16 × 25 × 4 inches. Filterbuy labels its replacement 16 × 25 × 5 inches and lists an actual size of 15.75 × 24.75 × 4.38 inches. Koch's MERV 11 OEM-2 replacement uses 16 × 25 × 4.4 inches, with actual dimensions of 15.80 × 24.75 × 4.375 inches.
Each aftermarket supplier explicitly names FC100A1029 in its reference. That cross-reference is more informative than the shared 16 × 25 label. The measurements still belong to the individual products; they are not interchangeable specifications for the original filter.
Compare the FC100A1029 options →A reliable order of checks
- Read the cabinet's full model number.
- Find its manufacturer-listed replacement part.
- Check that the exact product you plan to buy explicitly names that part or cabinet.
- Compare the product's actual dimensions and confirm the equipment requirements.
Do not buy a generic 4- or 5-inch filter solely because the width and height label match. If the cross-reference or dimensions conflict, ask the supplier to confirm before purchasing.
Sources
- Resideo Filter Replacement Guide. Checked 2026-10-09.
- Filterbuy Honeywell FC100A1029 replacement product listing. Checked 2026-10-09.
- Koch Air Bear OEM-2 technical bulletin. Checked 2026-10-09.