Common Mistakes When Installing Bag Filters in Existing Metal Frames

When bag filters are installed into existing metal frames, the frame often looks serviceable at first glance but the fit tells a different story. Small issues such as a twisted frame, worn sealing surfaces or misaligned clips can lead to bypass, awkward installation and avoidable callbacks.

For mechanical contractors and facility managers, the challenge is not just choosing the right filter. It is making sure the frame, gasket, clips and access arrangement all work together as intended in an older installation.

Why older metal frames cause problems

Existing metal frames have usually seen years of service, maintenance access, vibration and occasional rough handling. Over time, that can change the shape and finish of the frame enough to affect how the new bag filter seats.

Common age-related issues include:

• •Bent or bowed frame rails
• •Corrosion on the seating surface
• •Damaged or compressed gasket lands
• •Loose retaining hardware
• •Uneven door pressure or latch tension

A filter that appears to fit can still leave small gaps if the frame is no longer true. Those gaps matter because bag filters rely on a consistent seal around the perimeter, not just the filter media itself.

If the installation is being altered to suit a different filter size or depth, check the dimensions carefully before work starts. If you are not sure what is currently installed, how to measure an air filter is a useful starting point.

Poor seating of the filter is a common retrofit error

One of the most frequent mistakes is assuming the filter is seated correctly simply because it has been pushed into place. In older frames, a small twist or obstruction can stop the filter from sitting flat across the full face.

This can happen when:

• •The frame has a lip or weld bead that interferes with the filter header
• •Dust build-up prevents the filter from closing fully against the seal surface
• •The pocket or bag section is folded or pinched during insertion
• •The access door is closed before the filter is fully aligned

A proper fit should feel even and controlled, with no visible distortion in the filter frame or header. If the filter requires force to install, something is usually out of alignment.

It helps to inspect the frame before unpacking multiple filters. If one frame opening is tight, inconsistent or damaged, resolving that first is usually faster than trying to force a new filter into place.

Frame distortion is often the hidden issue

A metal frame that looks square from a distance may still be out of plane. Even minor distortion can create a diagonal gap, uneven compression or a rock-and-wobble fit that is hard to detect until the system is running.

Things to check include:

Signs the frame is no longer square

• •One corner sits proud of the others
• •The filter rocks when pressed lightly at opposite corners
• •The gasket compresses unevenly around the perimeter
• •The retaining clips engage on one side but not the other

What distortion can do

• •Reduce the quality of the seal
• •Make clips difficult to latch
• •Cause the filter to sit under tension
• •Increase the chance of repeated leakage around the frame edge

If a frame is badly distorted, replacing or straightening it may be the better option than trying to compensate with extra force or thicker seal material. A patchwork fix can create more problems later, particularly in plant rooms where access is limited and maintenance time is tight.

Missing or damaged seals should not be overlooked

Older installations often have missing, flattened or brittle seals. In some cases, the original seal has been removed during previous maintenance and not replaced. In others, the seal is still there but no longer performs its job properly.

Before installing a new bag filter, check for:

• •Missing gasket strips
• •Cracked or hardened seal material
• •Gaps at corners and joins
• •Sealant residue that prevents full contact
• •Old adhesive that has lifted from the frame

Do not assume the filter itself will make up for a poor sealing surface. Bag filters are designed to work with the frame and its sealing system, not against a damaged one.

If the system uses a specific gasket profile or seal arrangement, match it carefully rather than improvising with whatever material is on hand. In retrofit work, “close enough” often becomes a recurring maintenance issue.

Incorrect clip alignment can compromise the fit

Clip alignment is another common point of failure in older metal frames. Clips that were once neat and consistent may be bent, loose or positioned slightly off centre after years of use.

Typical problems include:

• •Clips that do not engage evenly on both sides
• •Retainers that apply pressure at the wrong point on the filter header
• •Clips that are too loose to hold the filter firmly
• •Clips that need excessive force to close
• •Misalignment caused by previous repairs or modified frames

If the clips are not aligned correctly, the filter may appear secure but still move under airflow or vibration. That movement can disturb the seal and make future removal difficult.

A good rule in retrofit work is to inspect the clips before fitting the new filter, not after. If the hardware is worn or bent, replace or adjust it before the filter goes in.

Practical checks before commissioning

Once the filter is installed, a quick visual check is not enough. A deliberate handover inspection can catch problems before the system is returned to service.

Useful checks include:

• •Confirm the filter sits evenly in the frame
• •Check all corners for uniform compression
• •Make sure no part of the bag or media is trapped in the frame
• •Verify all clips are fully engaged and correctly positioned
• •Inspect the seal line for obvious gaps or light leaks where visible
• •Ensure access doors close without forcing the frame out of shape

If pressure gauges are fitted, monitor pressure drop as part of normal maintenance and compare readings with the system’s expected condition. Inspection frequency depends on the system, site conditions and dust load, and you should follow the filter and equipment manufacturer instructions.

For projects involving building ventilation requirements, site requirements should be confirmed with the mechanical services engineer or building owner. If you are working to AS 1668.2:2024 related expectations, the details should still be checked against the actual installation and design intent.

Choosing the right replacement approach

Not every older frame is worth reusing as-is. Some frames can be cleaned, realigned and resealed. Others are too distorted or corroded to give a dependable result without modification.

A sensible retrofit plan may include:

• •Measuring the opening accurately before ordering replacements
• •Checking whether the existing frame can accept the intended filter depth
• •Verifying that the seal method suits the frame condition
• •Replacing worn clips, latches or retainers
• •Choosing a filter built for the application rather than forcing a near match

If the application needs a specific product type, our products page is a good place to review available options. For sites with unusual dimensions or legacy housings, custom air filters may be the cleaner solution.

Some filters are designed to be washable - check the product instructions before attempting any cleaning or reuse.

Get the fit right the first time

Retrofit installations often fail because small imperfections in the old frame are ignored. Poor seating, frame distortion, missing seals and incorrect clip alignment can all undermine a new bag filter long before the filter itself is the problem.

Total Air Filtration is a family-owned air filter manufacturer and supplier based in Hillcrest, Brisbane QLD. We can help supply or make the right filters for older installations, including retrofit situations that need careful measurement and a practical fit. Call 07 3800 3448 or request a quote. If you need support with ongoing maintenance, see filter servicing or commercial filter servicing.