AirSelect3D Blog
EN 13053 Casing Classes Explained: Thermal Transmittance, Bridging, Leakage
A casing datasheet full of class codes — T2, TB2, L1(M), F9 — tells a consulting engineer more about a unit's real-world performance than the nominal airflow ever will. EN 13053 defines exactly what each class means and how it is measured, and getting the classes wrong (or not stating them at all) is one of the fastest ways a technical data sheet gets bounced back by a reviewing engineer.
What EN 13053 actually classifies
EN 13053 is the European standard for air handling unit ratings and performance — it does not size components, it defines how casing quality is measured and reported so units from different manufacturers can be compared on equal terms. Four properties matter most for casing selection:
- Thermal transmittance (T class) — how much heat passes through the casing wall itself.
- Thermal bridging (TB class) — how much the casing's internal structure (frame, brackets, fasteners) locally short-circuits the insulation.
- Casing air leakage (L class) — how much air escapes through panel joints and door seals at a reference pressure.
- Filter bypass leakage (F class) — how much unfiltered air slips around the filter frame instead of through the filter media.
Each is measured and rated independently — a unit can be excellent on leakage and mediocre on thermal bridging, so quoting a single "quality grade" without the individual class letters hides real information.
Thermal transmittance and bridging: T and TB classes
T classes (T1 through T5, with T5 being a reference "no requirement" class) rate the mean thermal transmittance of the casing panels in W/(m²·K). TB classes (TB1 through TB5) separately rate the thermal bridging factor — the ratio between the actual local surface temperature drop at a structural bridge and the transmittance of the surrounding panel.
Why they are reported separately: a well-insulated 50 mm panel can still perform poorly overall if the internal frame studs are uninsulated steel running straight through the wall. TB1 (best) requires the bridging factor to stay below roughly 0.10; a poorly bridged casing can sit above 0.60, meaning condensation risk at the bridge points even when the panel field itself is comfortably above dew point. On cooling-duty units running low supply air temperatures in humid climates, TB class is often the more decisive number than T class for avoiding sweating cabinets.
Casing air leakage: L classes
L classes rate how much air leaks out of (positive pressure, supply side) or into (negative pressure, extract side) the casing at a specified test pressure, expressed in leakage rate per m² of casing surface.
| L class | Leakage limit (indicative, positive pressure) | Typical use |
|---|---|---|
| L1(M) | ≤ 0.15 l/(s·m²) | High-quality, energy-critical projects |
| L2(M) | ≤ 0.44 l/(s·m²) | Standard commercial specification |
| L3(M) | ≤ 1.32 l/(s·m²) | Minimum acceptable, budget projects |
The "(M)" suffix denotes the medium-pressure test regime most commercial AHUs are rated under; a high-pressure variant exists for units running higher static. The practical impact compounds with unit size: a leakage difference that looks trivial per m² becomes a measurable percentage of total airflow on a large casing, and it is airflow the fan already paid SFP to move — see our SFP guide for how wasted static translates into absorbed power. Leakage on the extract side is worse than it looks on paper: infiltrated air bypasses the filter bank entirely.
Filter bypass leakage: F classes
F classes rate leakage specifically around the filter frame — air that takes the path of least resistance around the media rather than through it. This is reported as a percentage of total airflow bypassing the filter at the class's reference pressure drop, and it directly undermines whatever ePM1/ePM2.5 filter class the specification calls for. A unit fitted with a premium ePM1 60% filter but a loose frame with 5% bypass is, in practice, delivering meaningfully worse air quality than the filter's own rating implies — the media never sees that fraction of the airstream.
Why sales quotes skip these numbers
Casing class testing requires a certified test rig and a physical prototype run — it is not something a curve-fit spreadsheet can produce, which is why generic selection tools frequently omit T/TB/L/F classes altogether or quote a single vague "premium casing" claim instead of the four class letters. A reviewing consultant checking a submittal against a project spec that calls for TB2/L1(M)/F9 has no way to verify compliance from marketing language alone.
Where the tooling matters
AirSelect3D pulls casing performance directly from the manufacturer's certified geometry and construction data behind each selection, so the T/TB/L/F classes on the generated technical data sheet trace back to the actual casing being quoted — not a generic claim applied across a product range. When a project spec calls out a minimum casing class, that constraint is visible in the same selection screen as airflow and face velocity, before the dossier goes to the consultant for review.
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