How to Measure AHU and HEPA Air Filters for Replacement

AI-generated cover illustration of filter-frame measurement; tape markings are not ordering specifications.

To measure an AHU or HEPA replacement filter, record the outside frame width, height and depth in millimetres. Describe the gasket, flange or gel channel separately. Bag filters also require the pocket length and number of pockets. Matching the frame size alone does not establish performance compatibility.

Nominal and actual filter dimensions

A nominal size is a size designation; actual dimensions describe the manufactured filter. They can differ. Do not derive an order size from the designation alone or subtract an arbitrary clearance from every holding frame. Compare the old filter, the drawing and the installed frame, especially if the old frame is bent or the previous filter did not fit properly.

How to measure the existing filter

1. Record before removal: photograph the label, equipment identifier, airflow direction and mounting arrangement. Access and removal should follow the facility's shutdown and maintenance procedure and be performed by assigned personnel.
2. Measure width and height: measure between the outside edges of the frame, not the media opening. Mark the viewing direction and dimensions on a photo.
3. Measure depth: record frame depth along the airflow direction. Describe projecting flanges and seals separately. Avoid touching or deforming the filter media with measuring tools.
4. Record the seal: note gasket location, number of gasketed faces, clamp arrangement or gel-channel interface. Photograph the mating components.
5. Check the installation: compare the housing opening, tracks and available rear clearance. Resolve differences from the drawing before placing the order.

For a bag filter, distinguish pocket length from header thickness. Record pocket count and clearance behind the filter. See how to select F7, F8 and F9 filters and the V-PAK F8 product data.

For HEPA filters, specify gasket or gel sealing. The Ultracel II-GEL page illustrates a gel-seal product. Neither appearance nor size identifies an H13 or H14 efficiency class; check the label and technical documentation. Read the H13 and H14 comparison for selection context.

Information to send for a quotation

Mark unknown information as unconfirmed rather than guessing. Frame material, media, sealing, quantity and testing requirements affect the quotation; equal dimensions do not guarantee equal price or interchangeability.

Frequently asked questions

In what order should filter dimensions be recorded?

Record width × height × depth in millimetres, state the viewing direction and mark each dimension on a photo or drawing to avoid supplier convention differences.

Do HEPA dimensions include the gasket?

Check the drawing for the exact model. Record the frame dimensions and uncompressed gasket thickness separately, with a photo of the sealing face.

Can I request a replacement without the old label?

Send photos, dimensions and the application for initial advice. Confirm the required efficiency, airflow and pressure drop from design records or the responsible engineer before ordering.

Technical reference: Camfil Hi-Flo data lists width, height, depth, airflow, initial pressure drop and pocket count separately. Confirm VAF orders against documentation for the exact VAF model.