Buyer guide
Cleanroom HVAC design: AHU, recirculation and FFU options
Choose the air system from the cleanliness class, heat load and number of rooms: a central air-handling unit (AHU) with ducted terminal filters, an AHU with recirculation, or ceiling fan filter units (FFUs) fed by a make-up air unit. The class does not fix the air volume: FDA's 2004 guidance gives 20 air changes per hour only as a typical floor for ISO 8 supporting rooms and says significantly higher rates are normal for ISO 7 and ISO 5.
By ZY Cleanroom · Updated · Sources at the end of the page
The three layouts
These are design trade-offs from practice, not code rules.
| Layout | How it works | Suits | Watch |
|---|---|---|---|
| Central AHU, ducted | One AHU treats all supply air. Terminal filters sit in the ceiling. Return or extract is ducted. | Hospital rooms, mixed pressure regimes, many small rooms | Duct and plant space, fan power at high air volumes |
| AHU with recirculation | Most room air returns through filters. The AHU treats make-up air and room loads. | High air volume with low fresh air demand | Avoid recirculating air that carries hazardous process material |
| Ceiling FFUs with make-up air | Each FFU holds a fan and filter in the ceiling grid. Air returns by wall risers or the plenum. A separate AHU supplies make-up air. | Modular rooms, retrofits, zones of different class | Noise, ceiling access for service, plenum leakage |
What drives each choice
Air speed sets the volume in unidirectional zones. EU GMP Annex 1 gives 0.36 to 0.54 m/s as a guidance value at the working position (para 4.30). FDA's 2004 guidance gives 0.45 m/s with a range of plus or minus 20 percent.
Heat and humidity need coils. An FFU moves and filters air but does not cool or dehumidify, so the make-up unit carries those loads.
Pressure control differs. A central AHU sets cascades by balancing supply and extract. An FFU room relies on make-up air and a tight envelope.
Sizing arithmetic
The last line shows why designers use a small unidirectional zone inside a larger ISO 7 or ISO 8 room. The FFU data sheet, not this arithmetic, sets real airflow.
| Item | Value | Basis |
|---|---|---|
| Unidirectional air speed, EU | 0.36 to 0.54 m/s | EU GMP Annex 1, 2022, para 4.30 |
| Unidirectional air speed, FDA | 0.45 m/s ± 20% | FDA guidance, 2004, footnote 5 |
| One 1,200 × 600 mm FFU face at 0.45 m/s | 1,166 m³/h | Calculated: 0.72 m² × 0.45 m/s × 3,600 |
| Room of 100 m², 3 m high, at 20 changes per hour | 6,000 m³/h | Calculated |
| Same 100 m² ceiling fully covered at 0.45 m/s | 162,000 m³/h, or 540 changes per hour | Calculated |
Filters and casing
Filter grades follow EN 1822. HTM 03-01 Part A, Table 10 lists EPA E10 at 85% to E11 at 95% for ultra-clean theatre terminals, E12 at 99.5% to HEPA H13 at 99.95% for immunosuppressed patient rooms, and H14 at 99.995% for pharmacy aseptic supply. All are at the most penetrating particle size.
Ask for the AHU casing leakage class. HTM 03-01 Table 8 sets BS EN 1886 class L2, tested at +700 Pa and −400 Pa, as the minimum for hospital AHUs.
Who designs what
The owner writes the user requirements: class, process heat, people, pressure, temperature and humidity. The HVAC designer sets air volumes, plant and controls. The cleanroom builder makes the envelope tight enough for those figures.
Commissioning follows a design brief. HTM 03-01 gives design airflow +10% and −0% as the usual tolerance and calls for independent validation on the client's behalf (paras 11.29 and 11.16).
A regulator approves the owner's facility, not the contractor. To have a room list reviewed, send it to ZY Cleanroom, a cleanroom design-and-build contractor with its own panel factory in Tianjin, China.
Questions
Common questions
Can FFUs replace an AHU?
No. FFUs filter and move room air. Make-up air, cooling, heating and humidity control still need an air-handling unit.
How many air changes does ISO 7 need?
ISO 14644-1 sets none. FDA's 2004 guidance says significantly higher than 20 per hour is normally needed for ISO 7 areas. Design from loads and recovery tests.
What air speed does an ISO 5 unidirectional zone need?
EU GMP Annex 1 gives 0.36 to 0.54 m/s as a guidance value. FDA's 2004 guidance gives 0.45 m/s plus or minus 20 percent.
What filter grade goes in an FFU?
That follows your class and risk. EN 1822 grade H13 is 99.95% and H14 is 99.995% at the most penetrating particle size (HTM 03-01, Table 10).
Related guides
- HEPA filter integrity and leak testing
- Class 100, 1,000, 10,000 and 100,000 Cleanrooms Explained
- Cosmetics factory rooms: what ISO 22716 and regulators say about premises
- Cleanroom pressure alarm: how to find the cause
- Turnkey cleanroom contractor
- Room list builder
Sources
- FDA, Guidance for Industry: Sterile Drug Products Produced by Aseptic Processing, Current Good Manufacturing Practice (September 2004, nonbinding; checked 31 October 2026)
- EudraLex Volume 4, Annex 1: Manufacture of Sterile Medicinal Products (published 25 August 2022, applied from 25 August 2023; checked 31 October 2026)
- NHS England, Health Technical Memorandum 03-01, Specialised ventilation for healthcare premises, Part A (published 22 June 2021; checked 31 October 2026)
- ISO 14644-1:2015, Cleanrooms and associated controlled environments, Part 1: Classification of air cleanliness by particle concentration (published 9 December 2015; checked 31 October 2026)
Project enquiry
Send what you have
A room list, a floor plan or only the floor area is enough to start. After you submit, send drawings by email or WhatsApp.
We reply by email within one working day. We can sign an NDA before you send drawings.
WhatsApp +44 7962 561184 is our UK number for export enquiries. The factory is in Tianjin, China.
Email: [email protected]
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