When a filtration stage underperforms, the instinct is to specify better media. In a surprising number of cases the media is fine and air is going around it. Bypass at the frame is common, easy to miss, and it undoes the work of the stage entirely.
Why gaps matter disproportionately
Air takes the path of least resistance, and an open gap around a panel offers far less resistance than the panel itself. The flow through a gap does not scale with its area – it takes a larger share than you would expect, because it is competing against a much more restrictive path.
A gap that looks like a couple of percent of the face area can pass a great deal more than two percent of the air. And everything that passes the gap is unfiltered, regardless of what the panel caught.
Where the gaps come from
Panels made to the dimensions of the filter they replace rather than the opening. Frames that have bowed or racked over time. Holding channels designed for a compressible disposable panel, which sealed by squashing and cannot seal a rigid one. And panels that have gone slightly wavy from heat or poor storage, so they no longer sit flat against the face.
The last one is worth checking specifically. Hold the panel on a flat surface and look for rocking. A panel that has bowed will bypass along the bow even if its dimensions are correct.
Fixing it
Three options in order of effort. Make the panel to the actual aperture rather than the nominal size. Add a compressible sealing edge or flange that bridges an uneven frame face. Or add gasket material to the frame itself, which is often the cheapest and the most durable.
Where the frame is badly distorted, no panel will seal, and the honest answer is to repair or replace the frame. It is a bigger job and it is the only one that works.
Checking after installation
Run the system and feel around the frame edges by hand, or with a thin strip of paper or a smoke pencil if you have one. Air escaping at the edge is immediately obvious to the hand. This takes a minute and it is the check that catches most bypass before it becomes a performance complaint.
Most of it traces back to measuring the opening correctly in the first place.
Sealing options, compared
Make the panel to the aperture. Cheapest and best where the frame face is flat and the opening is square. Fails where either is not true.
Compressible edge or flange on the panel. Bridges small irregularities in the frame face and tolerates a slightly uneven opening. Slightly more expensive and the most reliable option in older plant.
Gasket on the frame. Cheap, durable, and the right answer where several panels sit in a common frame. It does need the frame face to be roughly flat, and the gasket has to be replaced eventually.
When the frame is the problem
If a frame has racked badly or its face is significantly bowed, no panel will seal and no gasket will fully compensate. The honest answer is to repair or replace the frame, which is a bigger job than ordering a panel and is the only one that works. It is worth saying so rather than shipping panels that cannot seal.
What we can do
We supply cut-to-size filter panels made to the actual opening, with a compressible sealing edge where the frame face needs it. Send us the aperture and a photograph of the empty frame and we will tell you what the panel needs to seal properly.