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HEPA Filter Cassette: Sizes, Seals & Replacement Guide
Replacing a HEPA filter cassette looks simple until the new filter arrives and does not fit the terminal housing, creates excessive pressure drop, or fails an integrity test. For UK cleanrooms, procurement teams should match actual frame dimensions, depth, airflow, seal interface, efficiency class, and certification—not just order another “H14 filter” with the same nominal face size.
This guide explains what engineers and maintenance teams should check before specifying or replacing terminal filters.
HEPA Filter Cassette Sizes, Frames and Airflow Selection
Common Cassette Dimensions for Ceiling Terminal Housings and FFU Systems
Common HEPA filter cassette sizes include 305 × 305 mm, 305 × 610 mm, 457 × 457 mm, and 610 × 610 mm, although rectangular and project-specific dimensions are also widely used.
Depth varies much more than many buyers expect. Cleanroom mini-pleat panels may be approximately 66–100 mm deep, while higher-capacity box filters commonly use depths around 292 mm. Camfil, for example, lists cleanroom HEPA/ULPA panels with 53, 70, and 100 mm pack depths, while Freudenberg offers 292 mm H14 units in several face sizes.
Example size | Typical application | Example rated airflow* |
305 × 305 mm | Small terminal / equipment | 120–375 m³/h |
305 × 610 mm | Terminal housing | 300–1,100 m³/h |
610 × 610 mm | Ceiling terminal / FFU | 1,200–2,400+ m³/h |
610 × 610 × 292 mm | High-capacity HVAC HEPA | Up to several thousand m³/h |
*Actual airflow depends on media area, depth, filter design, and acceptable pressure drop. Manufacturer data must govern final selection.
Matching Face Size, Filter Depth and Rated Airflow to the Existing Housing
A 610 × 610 mm filter is not automatically interchangeable with every other 610 × 610 mm filter. Frame depth, flange position, gel-channel geometry, gasket location, protective grids, and housing clearances can differ.
Pressure drop also matters. One Freudenberg 610 × 610 × 292 mm H14 design is rated at 2,400 m³/h with an initial pressure drop of 230 Pa, while other mini-pleat designs operate at very different airflow and resistance points.
From the projects we’ve completed, airflow problems after filter changes are often traced to replacement filters selected by face dimension and H14 classification alone. The fan or FFU then has to operate at a different duty point to maintain the original room airflow.
Why Nominal Size Alone Is Not Enough When Ordering a Replacement Filter
Before ordering, physically verify:
- Exact width, length, and depth.
- Frame and media construction.
- Seal position and type.
- Rated airflow and initial pressure drop.
- Filter efficiency.
- Housing knife-edge or gasket mating surface.
- Upstream/downstream orientation.
A photograph of the existing label and housing interface can prevent a costly mismatch.
Gel Seal, Gasket Seal and Leak-Tight HEPA Filter Installation
Gel Seal HEPA Filter vs Gasket Seal: Which Design Is Better?
A gel seal HEPA filter uses a fluid gel channel that mates with a knife-edge on the housing. This arrangement is common in room-side replaceable terminal housings because the knife-edge penetrates the gel and creates a continuous sealing path.
Camfil describes gel systems as particularly suitable for bottom-loading and frequently replaced cleanroom filters, while gasket systems depend on controlled compression between the filter flange and mating surface.
Seal | Advantages | Limitations |
Gel | Strong bypass control; accommodates minor surface irregularities | Gel can be damaged during handling |
Gasket | Simple, familiar, lower complexity | Depends heavily on even compression |
Knife-edge + gel | Excellent for terminal housings | Housing and filter geometry must match |
Neither design compensates for a bent frame, damaged housing, or incorrect installation.
Knife-Edge Interfaces, Clamping Pressure and Common Bypass-Leakage Risks
With a gel seal HEPA filter, the knife-edge must enter the gel uniformly without bottoming out or cutting through the channel.
For gasket systems, insufficient clamping can leave leakage paths; excessive or uneven compression may distort the gasket or frame. Camfil terminal housings use controlled clamping arrangements specifically to maintain a reliable seal.
In our experience, filter-media efficiency receives more procurement attention than the seal, even though bypass leakage around the frame can defeat the purpose of an H14 filter.
EN 1822 / ISO 29463 Efficiency Classes, Test Reports and Supplier Documentation
H14 filters are commonly specified at ≥99.995% efficiency at the most penetrating particle size, or MPPS. Commercial H14 products may correspond to ISO 45 H under ISO 29463 classification.
ISO 29463-1:2024 covers classification, testing, marking, and documentation for high-efficiency filters, while ISO 29463-5:2022 defines filter-element efficiency testing at MPPS.
Ask suppliers for individual filter test documentation where required rather than accepting only a generic product brochure.
HEPA Filter Replacement and Validation Checklist for UK Cleanrooms
When to Replace a HEPA Filter Based on Pressure Drop, Damage and Test Results
There is no universal rule requiring filters to be replaced every 12 or 24 months.
A HEPA filter replacement should be driven by condition and validated performance: excessive pressure drop, damaged media, contaminated filter surfaces, failed integrity testing, damaged seals, or inability to maintain required airflow.
Some manufacturers recommend replacement when resistance reaches approximately twice the initial pressure drop, although the cleanroom’s approved operating limits and HVAC capability should control the final decision.
Post-Replacement Integrity Testing, Airflow Verification and Room Requalification
Changing the filter is not the end of the job.
ISO 14644-3:2019 provides cleanroom performance test methods, while EU GMP Annex 1 requires periodic requalification that includes final-filter integrity testing and airflow-volume measurement.
For sterile pharmaceutical areas, Annex 1 specifies maximum requalification intervals of six months for Grades A and B and 12 months for Grades C and D. It also identifies final-filter replacement as maintenance that can require appropriate requalification through change control.
After HEPA filter replacement, a typical controlled process therefore includes filter integrity testing, airflow measurement, pressure-differential verification, and any additional qualification required by the facility’s risk assessment and contamination-control strategy.
Cost and Procurement Risks to Check Before Ordering
Standard UK H14 filters show how widely prices can vary. Current retail examples for 610 × 610 × 292 mm H14 filters range from about £140 to £268 excluding VAT, depending on frame construction, while specialist units can exceed £600 excluding VAT.
Those prices should not be treated as cleanroom terminal-filter quotations. Gel seals, individual scan certificates, special dimensions, ePTFE media, stainless construction, pharmaceutical documentation, and low-pressure-drop designs can materially increase cost.
For critical facilities, buying the cheapest filter can therefore cost more once validation failure, downtime, and repeat testing are included.
What to Include in an RFQ: Size, Seal Type, Efficiency, Pressure Drop and Certificates
For accurate procurement, specify:
- Exact W × H × D dimensions
- Frame material and construction
- Required H13/H14 or ULPA efficiency
- EN 1822 / ISO 29463 test requirement
- Rated airflow in m³/h
- Initial pressure drop at rated airflow
- Gel, gasket, or knife-edge interface
- Gel-channel dimensions where applicable
- Protective face screens
- Individual test certificate requirement
- Installation orientation
- Existing housing manufacturer/model
- Required chemical or decontamination resistance
These details make HEPA filter cassette sizes only one part of the replacement specification rather than the entire purchasing decision.
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We'll call you
In order to serve you better, if you could kindly send an e-mail to nick@biogreenness.com for questions and details.
