In a hospital, an air handling unit is not just a comfort device. In operating rooms, intensive care units, sterile storage, and isolation rooms, the unit is part of infection control. Water, dust, or biofilm that builds up inside the unit can turn a system meant to distribute clean air into a source of contamination.
What is a hygienic air handling unit?
A hygienic air handling unit is an air handling unit designed with interior surfaces that do not hold dirt or water, that is easy to clean and sealed, and that is equipped with staged filtration. It is used in areas with high hygiene requirements such as hospitals, pharmaceutical plants, laboratories, and food production.
In Europe, guidelines and standards such as VDI 6022 and EN 13053 are used for hygienic unit design and ventilation hygiene, while DIN 1946-4, widely referenced in Germany, is used for hospital ventilation. In Türkiye as well, projects refer to these sources and to the relevant regulations of the Ministry of Health. The maintenance and test plan should be set up according to the standard the project was designed to.
What should the casing and interior surfaces be like?
The casing of a hygienic unit should be designed so that no point is left where dirt and moisture can cling. A surface that cannot be cleaned becomes a source of microorganisms over time, no matter how good the filters are.
- Smooth interior surfaces: Panel interior surfaces should be flat, corrosion-resistant, and made of material suitable for disinfectants. Exposed insulation material or screw heads trap dirt.
- Corners and joints: Interior corners should be rounded and sealed wherever possible, and there should be no gaps at the joints.
- Airtightness: Air leakage from the casing both lowers efficiency and lets unfiltered air into the system.
- Inspection windows and interior lighting: The fan, coil, and filter sections should be visually inspectable without opening the door.
- Removable components: Components such as the droplet eliminator, coil, and fan should be removable or accessible for cleaning.
How are filtration stages set up?
In hospital air handling units, filtration is not provided by a single filter but by a staged arrangement running from coarse filters to HEPA. Each stage protects the next and extends the life of the terminal filter.
In common practice, the first fine filter stage is at the unit's fresh air inlet, and a second, higher-class fine filter stage comes after the fan and coils. In the most critical areas such as operating rooms, a terminal HEPA filter is used in the room ceiling, in a laminar flow ceiling, or at the supply outlet. Filter classes used to be defined by EN 779; today they are defined by ISO 16890 for fine filters and EN 1822 for HEPA filters.
In a HEPA filter, it is not only the filter class that determines efficiency. A small leak in the filter frame can make the whole stage ineffective as air bypasses the filter. For this reason, a leak test should be done after installation and periodically.
For filter classes and replacement criteria, see our article when to replace air handling unit filters: G4, F7, and HEPA.
How are pressure differential, temperature, and humidity controlled?
The basic principle in hospital ventilation is that air flows from the cleaner area toward the less clean area. This flow is achieved by creating a controlled pressure differential between rooms.
- Operating rooms: They are kept at positive pressure relative to corridors and adjacent spaces, so air flows outward.
- Infectious isolation rooms: Negative pressure is applied for patients with contagious diseases; air does not escape from the room and is discharged through the exhaust.
- Protective isolation rooms: Positive pressure is preferred for immunosuppressed patients.
Temperature and humidity must also be controlled together. In operating rooms, temperature is generally kept within a range the surgical team can adjust, for example roughly 20-24 °C. Relative humidity is targeted at roughly 30-60%; very low humidity raises the risk of static electricity and dryness, while very high humidity raises the risk of microorganism growth. Exact values are determined according to the standard applied in the project and the hospital's needs.
To monitor these values continuously, pressure differential indicators and temperature and humidity sensors should be connected to the automation system, and an alarm should be generated when limits are exceeded.
How are fresh air and return air managed?
In critical hospital areas, the fresh air ratio and the use of return air are design decisions based on infection risk. The general rule is that as risk increases, return air is filtered more tightly or not used at all.
Air extracted from isolation rooms and infectious areas is generally not returned to other spaces; it is exhausted outdoors with appropriate filtration. In operating rooms, depending on the standard the project is based on, a high proportion of fresh air or a controlled return air that has passed through staged filtration may be used. This decision should be made at the design stage, not during operation.
The fresh air itself must also be clean. The unit's fresh air intake should be located away from exhaust outlets, generator stacks, ambulance entrances, and waste collection areas. The intake grille and coarse filter should be checked regularly for bird, leaf, and dust buildup. If a heat recovery exchanger is used, it should be verified that the exchanger type and its airtightness prevent exhaust air from mixing with fresh air.
On-site inspection for your hospital air handling units
Let us inspect and report on the hygiene, filtration, and pressure stages of your air handling units on site.
Drainage, humidifier, and coil: anywhere there is water is a risk
Every section of an air handling unit where water is present is the point that needs the most attention from a hygiene standpoint. Standing water, biofilm, and bacteria such as Legionella find a suitable environment to multiply there.
- Drain pan: The drain pan should be stainless steel, sloped in every direction, and fully draining. No water should remain in the pan.
- Trap: It should be of a height suited to the fan pressure and should not be allowed to dry out. A dried-out trap draws odor and dirty air in from the drain line.
- Humidifier: In hygienic applications, a steam humidifier is generally preferred. Water quality, scale, and the absence of condensation after the steam distributor should be checked.
- Cooling coil and droplet eliminator: The droplet eliminator should be sturdy and clean so that condensate does not mix into the air.
How should maintenance be planned for hospital air handling units?
Maintenance of hospital air handling units is a process planned together with the infection control committee, documented, and including hygiene inspections at set intervals. The purpose of maintenance is not only to keep the unit running but to document air quality.
- Continuous monitoring of filter differential pressures and recording of changes
- Regular cleaning and disinfection of the drain pan, trap, humidifier, and droplet eliminator
- Periodic leak testing of HEPA filters
- Verification of pressure differential, temperature, and humidity values between rooms
- Sensor calibrations and alarm tests
- Hygiene inspection and, if necessary, microbiological sampling from surfaces
During maintenance, operating rooms and critical areas need to be taken out of service in a planned way. For this reason, the maintenance schedule should be prepared together with hospital operations. For general inspection steps, our air handling unit maintenance checklist article can be a starting point.
Any intervention on an operating room air handling unit, including filter replacement, must be carried out by a qualified and trained team with personal protective equipment and contamination precautions.
Air handling unit maintenance in public hospitals
Air handling unit maintenance in public hospitals is mostly carried out through tenders and tied to a technical specification. The specification should clearly state the unit list, filter classes, test items, and reporting format.
With its TSE service qualification certificate, ISO certificates, and OHS-trained team, YOİ Mühendislik carries out air handling unit maintenance and public tender maintenance work in hospitals and public buildings. The scope, number of units, and test requirements are determined during the site survey.
Frequently asked questions
What sets a hygienic air handling unit apart from a standard unit?
Design features such as smooth, cleanable interior surfaces, a sloped stainless steel drain pan, sealed filter frames, removable components, and inspection windows with interior lighting. The goal is to prevent dirt, water, and microorganisms from accumulating inside the unit.
Which filters are used in operating rooms?
The common practice is two stages of fine filters in the air handling unit and a terminal HEPA filter in the room. The exact classes are determined during the design phase according to the type of operating room and the standard applied.
How often should hospital air handling unit maintenance be done?
Filter differential pressures should be monitored continuously, and general checks should be done monthly. Hygiene inspection, drainage and humidifier checks, and HEPA filter leak tests are planned periodically in line with the hospital's infection control program.
Why is an operating room kept at positive pressure?
Positive pressure makes air flow from the operating room toward the corridor. This prevents particles and microorganisms from dirtier areas from entering the sterile space.
Can an existing unit be converted to a hygienic type?
Some steps are possible, such as adding a filter stage, renewing the drain pan, and improving the interior surfaces. However, if the casing structure and interior surfaces are not suitable, replacing the unit may be the better choice; this decision is made after an on-site inspection.
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Detailed guide: Air handling unit maintenance: intervals, checklist, and maintenance form