What is air handling unit maintenance, and why should it be done regularly?
Air handling unit maintenance is the set of procedures in which the unit's filters, fan, coils, humidifier, dampers and control elements are checked, cleaned and measured at set intervals. A unit whose maintenance lapses first loses efficiency, then degrades indoor air quality.
A dirty filter strains the fan and lowers airflow. A dirty coil reduces capacity, and more energy is spent on the chilled water or hot water side. A drain pan that holds water creates a suitable environment for bacteria and mold. All of these problems are caught early with a regular checklist.
The list below summarizes the order of checks we use in the field. The frequencies are general recommendations; they should be made more frequent according to running hours, the dust in the environment, the building's purpose and the manufacturer's instructions.
Monthly checks: filters and general condition
The focus of the monthly check is the filters, the differential pressure gauges and the unit's general operating condition. These checks are quick but give early warning of most faults.
- Filter differential pressure: Read and record the differential pressure of each filter stage. If the value approaches the final pressure drop given by the manufacturer, a replacement should be planned.
- Filter visual check: A torn or displaced filter, or one with a gap between its frame and the housing, lets dirty air reach the coil.
- Noise and vibration: A new sound, rubbing or increased vibration from the fan section should be noted.
- Door gaskets: Do the doors close fully, and are the gaskets in place? Air leakage means both energy loss and unfiltered air.
- Alarm history: The latest alarms on the automation screen and freeze thermostat warnings should be reviewed.
Writing the differential pressure values into the same table every month shows how fast the filter gets dirty. This trend lets you determine the replacement time by measurement rather than guesswork.
Quarterly checks: fan, belt and drainage
The quarterly check inspects the fan section, the drive system and the drain line. Some of this work requires the unit to be stopped and electrically secured.
- Belt tension and wear: A loose belt slips, and an over-tight belt wears the bearings. A cracked or glazed belt should be replaced, and on multi-belt drives the belts should be renewed as a set.
- Pulley alignment: Having the pulleys on the same axis directly affects belt life.
- Bearings: Noise, temperature and, if needed, lubrication are checked. Grease is not added to lifetime-lubricated bearings.
- Fan motor current: Current is measured against the phase-to-phase difference and the nameplate value. This measurement must be done by a qualified technician.
- Drain pan and trap: The pan must be sloped and no water should collect in it. The trap must be filled and at the correct height; otherwise the suction effect of the fan pulls the water back and the pan overflows.
- Vibration isolators and flexible connections: A torn flexible connection causes air leakage and noise.
Units with EC fans have no belt or pulley, but the cleaning of the fan module, the cable connections and the drive alarm records should be checked in the same period.
Six-month and annual checks: coils, humidifier and automation
Annual maintenance is a comprehensive service in which the unit's inner surfaces, coils and control system are overhauled in detail. It is recommended to do it before the transition to the cooling and heating seasons.
| Component | Check to perform | Recommended frequency |
|---|---|---|
| Cooling and heating coils | Fin cleaning, straightening bent fins, measuring the water inlet-outlet temperature difference | 1-2 times a year |
| Freeze thermostat | Set value and activation test | Before winter |
| Humidifier | Scale removal, cylinder or nozzle check, water quality | Every 6 months |
| Dampers and actuators | Full open-close test, mechanism lubrication | Every 6 months |
| Heat recovery exchanger | Cleaning, rotor belt and gaskets, sealing on plate types | Once a year |
| Sensors and automation | Calibration check of temperature, humidity and pressure sensors | Once a year |
| Inner surfaces and insulation | Check for dirt, corrosion, mold and insulation damage | Once a year |
Low-pressure water and chemicals approved by the manufacturer should be used for coil cleaning. High-pressure washing crushes the fins and increases air-side resistance. If the difference between the inlet and outlet temperatures of the chilled water coil has dropped clearly below the design value, the water flow or coil fouling should be investigated separately.
A maintenance plan for your air handling unit
Let us inspect your air handling units on site and prepare a measurement-based maintenance schedule and checklist.
Which measurements should be recorded?
The value of maintenance comes from recording the measured values. A one-off measurement describes the situation, while records over the years show the trend.
- Differential pressure of each filter stage (Pa)
- Fan motor current and speed or drive frequency
- Supply and return air temperature and humidity values
- Water inlet-outlet temperatures at the coils
- If possible, airflow measurement or flow estimation from the fan differential pressure
- Changes made: filter class and date, belt type, replaced part
These records make every job done on the unit traceable. In public buildings and hospitals, maintenance forms are also requested during inspections. For facilities with high hygiene requirements, you can also see our article hygienic air handling units in hospitals.
What can facility staff safely do?
Facility staff can safely take on the observation and recording tasks that can be done without opening the unit. These make the maintenance team's job easier and give early warning of faults.
- Reading the differential pressure gauges and entering them in the record table
- Checking for water, oil or unusual noise around the unit
- Noting the alarms on the automation screen
- Reporting complaints of reduced airflow at the air vents
Entering the fan section, belt adjustment, motor current measurement, and any work on the electrical panel and refrigerant circuits must be done by a qualified technician, with the unit de-energized and locked out. Rotating fans and pulleys carry a serious risk of injury.
For filter classes and replacement timing, our article when is the AHU filter replaced gives more detail.
Turning the checklist into a maintenance plan
A checklist becomes a maintenance plan once a schedule and responsibilities are added. Which job will be done in which month, by whom and on which form should be clear from the start.
At YOİ Mühendislik, in our air handling unit service we adapt this list to each unit's type and record the measurements on the same form at every visit. For facilities with several units, we tie the work into a single schedule under an annual maintenance contract. The scope and cost of maintenance are determined at the site survey according to the number and type of units.
Frequently asked questions
How often is air handling unit maintenance done?
Filters and a general check are done monthly; the fan, belt and drainage every three months; coil cleaning and an automation test at least once a year. In dusty environments and for units that run continuously, the intervals are shortened.
When is the AHU belt replaced?
It is replaced when cracks, glazing or wear appear on the belt, or when it cannot be kept tensioned. On multi-belt drives, belts should be replaced as a set.
Can facility staff do air handling unit maintenance?
Visual checks, differential pressure readings and record keeping can be done by facility staff. Work on the electrical panel, the fan motor and refrigerant circuits must be done by a qualified technician.
How much energy does an unmaintained AHU waste?
The exact rate varies by unit, but a dirty filter and coil make the fan draw more power and reduce capacity. If differential pressure and fan current are recorded regularly, the loss becomes measurable.
Related calculators
Detailed guide: Air handling unit maintenance: intervals, checklist, and maintenance form
A4 form with checklist, measurement table, and signature field.Download PDFView the form