Quick answer
Air handling unit faults usually start in a few places: clogged filters, worn fan belts or failing fan motors, blocked condensate drains, seized dampers and actuators, dirty or frost-damaged coils, and drifting sensors. Planned AHU maintenance checks each section in turn, records filter pressure drops and airflow, keeps drain trays clean and dry, and tests frost protection and controls before they are needed.
What an AHU does
An air handling unit (AHU) draws in outside air, mixes it with return air where the design allows, filters it, heats or cools it, sometimes humidifies it, and supplies it to the building through ductwork. Many units also recover heat from the extract air. Because so much of a building’s fresh air passes through one or two AHUs, a fault in one section can affect comfort and air quality across whole floors.
Faults section by section
| Section | Common faults | What you may notice |
|---|---|---|
| Fresh air and mixing dampers | Seized blades, failed actuators, linkages slipped | Stuffy spaces, frost trips, high energy use |
| Filters | Clogged media, poorly sealed frames, air bypassing filters | Falling airflow, dirty ductwork, rising fan energy |
| Heating coil | Stuck valve, airlocks, frost damage | Cold supply air, frost alarms, water leaks |
| Cooling coil | Dirty fins, valve faults, blocked drain | Poor cooling, water carry-over, damp ceiling tiles |
| Condensate tray and trap | Standing water, dry or wrongly sized trap | Leaks, odours, hygiene risk |
| Humidifier | Scale, failed cylinders or nozzles, water treatment issues | Low humidity, alarms, hygiene risk |
| Fan and motor | Worn belts, bearing wear, imbalance, failed EC fan or inverter | Noise, vibration, low airflow, trips |
| Heat recovery | Wheel drive belt failure, fouled plates, bypass damper faults | Higher heating and cooling demand |
| Sensors and controls | Drifting temperature or pressure sensors, overridden points | Hunting temperatures, units running out of hours |
What planned AHU maintenance should cover
A good AHU maintenance visit works through the unit in airflow order and records readings, not just ticks:
- Isolate safely and lock off before opening access panels
- Record the pressure drop across each filter bank and replace filters when they reach their final value
- Check filter frames and seals so air cannot bypass the filters
- Inspect coils for fouling, damage and leaks and clean them where needed
- Clean condensate trays, check they drain fully and check traps are primed and correctly sized
- Check fan belts for wear and tension, or check EC fans and inverters for faults and alarms
- Listen and feel for bearing noise and vibration
- Stroke dampers and valves through their full range and confirm actuators respond
- Test the frost protection sequence
- Compare sensor readings with a calibrated instrument
- Check heat recovery operation and bypass
- Note casing and door seal condition and any air leakage
- Record findings and recommended remedial work
Frequencies are usually set from a recognised schedule such as SFG20 and adjusted for the unit’s duty and condition. See AHU services.
Hygiene: drain trays, humidifiers and legionella
Water in an AHU is a hygiene risk. Drain trays that hold water, blocked condensate lines and poorly maintained humidifiers can all support bacterial growth.
HSE’s approved code of practice L8 requires the duty holder to assess and control legionella risk from water systems. For AHUs this usually means:
- trays that slope to a drain and are kept clean and dry
- traps that are correctly sized for the pressure in that section so they do not empty or hold water back
- humidifiers maintained and cleaned to the manufacturer’s instructions and included in the water risk assessment
Ductwork cleanliness is a separate consideration, usually assessed against the industry specification BESA TR19 Air.
Frost protection
A heating coil full of water can freeze and split in hours if very cold air passes over it while the coil is not heated. Most AHUs have a frost sequence that opens the heating valve, starts pumps and, if the supply temperature keeps falling, stops the fan and closes the fresh air damper.
Before winter, check that:
- the frost thermostat or sensor works and is in the right place
- heating valves open fully when called
- fresh air dampers close tightly when the unit stops
- heating plant is available whenever the frost sequence may need it
See preparing commercial HVAC for winter.
Controls and energy
AHUs are among the largest energy users in many commercial buildings. Common savings come from:
- matching time schedules to actual occupancy
- resetting supply temperature setpoints that have been overridden
- making sure heating and cooling coils are not fighting each other
- fixing failed heat recovery
- using variable speed fans on demand control, for example from CO2 sensors, where the design allows
See HVAC controls and BMS and our energy efficiency guide.
Repair or upgrade?
When an AHU is old, common upgrades include replacing belt drives with EC fans, adding or repairing heat recovery, replacing coils and upgrading controls. When the casing, insulation and internal condition are poor, full replacement may make more sense. A system assessment can compare the options.
For AHU servicing, faults or upgrades in London, contact us or call 020 7998 9005.
Frequently asked questions
How often should AHU filters be changed?
There is no single interval. Filters should be changed when their pressure drop reaches the final value set by the manufacturer or designer, or at a fixed maximum interval if that comes first. A differential pressure gauge across each filter bank makes this objective rather than guesswork.
Do AHUs need legionella checks?
AHUs with humidifiers, especially spray or evaporative types, and any unit where water can stand in drain trays should be covered by the building's legionella risk assessment under HSE's ACOP L8. Keeping trays clean and draining freely is a basic control.
Why does our AHU keep tripping on frost?
Common causes are a heating coil valve that is stuck or slow to open, no hot water available from the boiler or heat pump plant, a fresh air damper that does not close when the unit stops, or a frost thermostat that is badly located or set. Each needs investigating rather than repeatedly resetting.
What is a normal AHU service frequency?
Many AHUs receive a planned visit every three or six months, with some tasks annually. The right frequency depends on the unit, its duty and the environment. Schedules such as SFG20 are commonly used to set them.
Are belt drives or EC fans better?
Direct-drive EC fans remove belt wear and adjustment and are usually more efficient at part load, which is why many older belt-driven AHUs are upgraded. Whether it is worthwhile depends on the unit's condition, running hours and remaining life.
UK guidance and references
Primary and industry sources relevant to this subject. Always check the current edition and how it applies to your building.
- Legionnaires' disease Health and Safety Executive
- Ventilation in the workplace Health and Safety Executive
- SFG20 maintenance standard BESA / SFG20
This guide is general information for commercial building operators. It does not replace an assessment of your building by a competent engineer. If there is an immediate risk to people, contact the emergency services first.