HVAC equipment · Casing classes, filter bypass, leakage and certification
An AHU takes outdoor or return air, filters, heats, cools and sometimes humidifies it, and fans it into the ducts. Its box matters as much as its components: a leaky casing bypasses the filters, a poorly insulated one wastes energy and sweats. Specify the casing classes and ask how they were tested.
Modular, packaged and cleanroom air handling units for commercial, industrial and controlled environments, including DX and chilled-water units. Fan coil units, fans sold alone and electrostatic precipitators are outside this page.
CNFX editorial research · Sources checked October 12, 2026 · 4 sources
Swegon explains that EN 1886:2025 classifies AHU casings by mechanical strength, air leakage, filter bypass leakage, thermal transmittance, thermal bridging and acoustic insulation. For strength, deflection at ±1000 Pa below 4 mm/m is class D1, up to 10 mm/m D2, above that D3. [1]
Thermal transmittance is now graded U1 to U3 (it was a T-class before the 2025 update): U1 allows up to 1 W/m²K, U3 more than 1.8 W/m²K. Thermal bridging is graded TB1 (best) to TB4; Swegon notes that at TB2 the risk of condensation on a unit in a normal plantroom is low even in winter, while TB3 or TB4 raise the risk of condensation or frost on the casing. [1]
Datasheets may still quote T classes; ask which edition the classes refer to.
Give the supplier the airflow and external static pressure, the coil duties, the filter grades, humidification, the installation (indoor plantroom or outdoor), climate, and the space and access available.
02
Protect the filtration with low leakage and bypass
Swegon describes three casing leakage classes, L1 (lowest) to L3: L3 for low to medium filtration, L2 for medium to high filtration, L1 for very high or HEPA filtration. FläktGroup gives the L1 limit as 0.22 l/s per m² at +700 Pa. [1][2]
Filter frame bypass is graded by leakage as a share of nominal airflow; for example, an F9 frame class must leak below 0.54%, while an F7 frame may leak up to 2%. Where filters sit upstream of the fan under negative pressure, casing leakage can also pull dirty air in downstream of the filter, so both must suit the filter grade. [1]
03
Ask how the classes were tested
Swegon explains that Eurovent certification measures casing classes in an independent laboratory on a model box of prescribed size built with the unit's casing design, so that different units can be compared. FläktGroup adds that EN 13053 covers the rating of components and thermal efficiency in non-residential ventilation units (over 250 m³/h) and EN 16798 the system the AHU serves. [1][2]
Swegon notes that actual units are also tested and results can differ, because they have more doors and penetrations for ducts, cables and pipes; Eurovent publishes both. It also points to Eurovent's quality criteria (6/18-2022), which recommend insulation flammability classes (A1 best to F) and casing corrosivity classes (C1 clean indoor to C5 and CX severe). Ask whether quoted classes are for the model box or the delivered unit. [1]
04
Marine, cleanroom, DX inverter and desiccant units on offer
Ecoair lists modular, horizontal, ceiling, jet-flow and marine AHUs; Airwoods lists cleanroom, DX and industrial AHUs; Holtop lists DC inverter DX AHUs and heat-recovery ventilators; Suzhou Shengtong Jiuhe lists cleanroom AHUs; Ant lists AHUs and desiccant dehumidifying AHUs; Jiema lists heat-exchanger AHUs; Vrcoolertech lists AHUs with coils and evaporators. Ask each for casing classes and how they were tested. [3][4]
05
Casing, filter, coil and fan sections and what to fix for each
Send the duty, filtration and installation data together
Paste it into your enquiry below or into an email to a maker, then replace every bracket.
Duty: [supply and exhaust airflow; external static pressure; heating and cooling loads; humidity control]
Air quality: [filter grades; cleanroom class if any; fresh-air fraction]
Casing: [required D, L, U (or T) and TB classes; insertion loss]
Energy: [heat recovery type; fan efficiency; controls]
Installation: [indoor or outdoor; climate; space; access; sections for transport]
Documents: [selection printout; casing class certificate or test report (model box or real unit); drawings]
Seven makers below show air handling units on their own sites, from Ecoair's marine and jet-flow units to Holtop's DC inverter DX units and Ant's desiccant dehumidifying AHUs.
Ningbo Ecoair Environmental Technology Co., Ltd.
Manufacturing site to be confirmed for the order
Lists modular, horizontal, ceiling, jet-flow and marine air handling units.
Explore company information and manufacturing capabilities, then discuss the scope of your project with a relevant supplier.
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Compiled and source-reviewed by the CNFX editorial team.
Each manufacturer above links to the page on that company's own website where the product is listed, with the date we checked it. How we source and review
· Page last updated 13 October 2026
Buyer questions
What does L1, L2 or L3 mean for an AHU?
Casing air-leakage classes under EN 1886; L1 leaks least. Swegon matches L3 to low to medium filtration and L2 to medium to high; L1 suits very high or HEPA filtration. [1]
Why does AHU thermal bridging matter?
Poor thermal bridging classes (TB3, TB4) raise the risk of condensation or frost on the casing, leading to corrosion and hygiene problems. [1]
Are AHU casing classes tested on the real unit?
Certification tests use a model box built with the same casing design; results for the actual unit can differ because of extra doors and penetrations. [1]
Sources and what still needs a direct check
Sources checked 2026-10-12. Swegon and FläktGroup are AHU manufacturers; their articles explain EN 1886, EN 13053 and EN 16798 and Eurovent certification. The standards themselves were not read for this page. Supplier pages describe their own offers.
