How to Specify End Caps, Spacers and Supports in a Custom Multi-Pocket Filter

Replacing a non-standard pocket or bag-style filter with a custom multi-pocket version is rarely just a matter of matching width, height and depth. The small construction details around the frame, end caps, spacers and internal supports often determine whether the filter fits properly, seals well and keeps its shape in service.

For mechanical contractors and industrial buyers, getting those details right at the specification stage can prevent installation issues and avoid unnecessary back-and-forth when the old filter is no longer available in standard form.

Why the frame details matter more than the face size

A pocket filter can look straightforward from the front, but the filter housing only cares about the full assembly. If the frame profile, sealing edge or pocket suspension points differ from the original, the replacement may not sit correctly in the rail or holding frame.

When specifying a custom version, confirm the following before ordering:

• •Actual outside dimensions of the filter, not just the nominal face size
• •Frame material and thickness
• •Type of sealing edge or gasket, if used
• •Orientation of the pockets in the housing
• •Clearance around adjacent filters, access doors and guide channels
• •Any clips, tabs or locating features on the original unit

If you need help documenting the existing filter, our how to measure an air filter guide is a useful starting point.

Specifying end caps for a custom pocket filter

End caps are a key part of the pocket structure. They help hold the media in position and can affect how the pockets open under airflow and how the filter behaves when loaded with dust.

What to confirm about the end caps

When replacing a non-standard unit, ask what the original end cap is doing in the assembly. In some designs it forms part of the seal, while in others it mainly supports the pocket ends and protects the media.

Useful details to capture include:

• •End cap material, such as galvanised steel, aluminium or plastic
• •End cap depth and edge fold
• •Whether the cap is flat, reinforced or shaped to suit a particular header arrangement
• •How the media is attached to the cap
• •Whether the cap also acts as a spacer or support point

If the original filter had damaged or distorted end caps, it is worth checking whether the housing design contributed to the issue. A custom replacement can often be adjusted to better suit the installation rather than copying a problem exactly as it was.

When the end cap affects sealing

In some housings, the end cap sits close to the sealing surface or helps align the filter in the track. If that is the case, a small change in lip profile or thickness can alter the fit.

That is especially important where the filter sits in a bank of multiple modules, or where there is limited room for compression against the gasket or frame seal.

Spacers and pocket spacing: what keeps the bags open

Spacers are what keep the pockets separated so the media can load evenly and air can pass through the full filter area. If the pocket spacing is wrong, the bags may touch each other, restrict airflow or lose effective surface area.

Common spacer details to specify

Depending on the design, spacers may be internal strips, rigid separators, sewn-in supports or shaped retainers. The specification should cover:

• •Number of pockets
• •Spacing between pockets
• •Spacer material and stiffness
• •Whether spacers are continuous or segmented
• •How spacers are fixed to the frame or media
• •Whether the pockets need to hold their shape under variable airflow

This is particularly important in systems where the filter face is exposed to changing operating conditions, or where dust loading can deform lightweight pockets over time.

If the existing filter used non-standard spacing, ask for it to be measured rather than estimated. Even a small difference can affect how the pockets open in the housing.

Internal supports and how they affect performance

Internal supports are often overlooked until the replacement filter starts to sag, twist or rub against the housing. The support arrangement needs to suit both the media and the application.

Support styles to discuss with the manufacturer

Different pocket filters use different support methods. The right choice depends on the original construction and the space available in the system:

• •Sewn-in support tape or webbing
• •Plastic or metal support rods
• •Welded or bonded pocket separators
• •Reinforced header frames
• •Corner or edge bracing for larger filters

If the original filter was a non-standard size, the support pattern may need to be adjusted so the pockets do not collapse or overspread. This matters in deeper filters, where unsupported media can distort the airflow path.

Matching the support to the air handling unit

Support details also need to suit how the filter is installed. For example, a filter fitted in a tight access plenum may need a more robust frame edge, while a filter in a high-vibration application may need extra reinforcement at the headers or pocket seams.

The goal is not just to copy the old filter, but to produce a replacement that sits securely and works with the housing as it is actually built.

Other specification points that can affect fit

End caps, spacers and supports are central to the build, but they are not the only details that matter. When you are replacing a discontinued or unusual bag filter, it helps to confirm the whole assembly.

Also check the following

• •Pocket length and depth
• •Media type and weight
• •Number of pockets
• •Frame style, including header and side rails
• •Seal type, including gasket, flat frame or knife-edge arrangement
• •Direction of airflow and the intended installation orientation
• •Available access for removal and installation

If you are working to site requirements under AS 1668.2:2024, confirm the filter specification with the mechanical services engineer or building owner before ordering. Site conditions, equipment constraints and maintenance access all matter when selecting a replacement.

For commercial systems, our commercial filter servicing and filter servicing pages may also help when you are planning maintenance or replacement work.

When a custom replacement makes more sense than a close match

A close match is not always the best answer. If the original filter was built to a non-standard dimension or used a special end cap or support layout, forcing a standard product into the opening can create installation problems or poor sealing.

A custom multi-pocket filter is often the better option when:

• •The original filter is obsolete or no longer available
• •The housing has unusual clearances or locating features
• •The existing filter shows uneven wear, collapse or bypass at the edges
• •The pocket arrangement is tied to a specific frame geometry
• •The application needs a different media or support configuration

Some filters are designed to be washable - check the product instructions. If the existing unit was washable, do not assume the replacement should be cleaned the same way unless that is clearly stated in the product documentation.

If you are comparing options across different product types, our our products page gives a useful overview, and custom air filters is the best place to start when the size or construction is unusual.

Getting the specification right the first time

The best way to order a custom pocket filter is to treat it as a construction job, not just a size match. Measure the housing carefully, note how the old filter is held in place, and record the end cap, spacer and support details before the replacement is made.

At Total Air Filtration, a family-owned air filter manufacturer and supplier in Hillcrest, Brisbane QLD, established 2009, we can help you specify or make the right replacement for a non-standard pocket or bag-style filter. Call 07 3800 3448 or request a quote when you are ready to talk through the details. You can also contact us if you need help confirming measurements or construction features before ordering.