Quick answer
Fan coil units usually need professional attention when they leak water, become noisy, fail to heat or cool their zone, cycle erratically or show controller faults. The most common causes are blocked condensate drains or failed pumps, dirty filters, worn fans, stuck valves or actuators, and faulty sensors. Regular servicing prevents most of these problems.
How fan coil units work
A fan coil unit (FCU) draws room air through a filter, passes it over a heating or cooling coil fed with hot or chilled water, and blows it back into the space. A controller opens valves and sets fan speed to hold the room at its setpoint. Units in ceiling voids often have condensate pumps to remove water from the cooling coil.
Warning signs and what they usually mean
| Sign | Likely causes |
|---|---|
| Water marks on ceiling tiles or drips | Blocked drain, failed condensate pump or float switch, cracked tray, leaking valve |
| Room not cooling or heating | Stuck valve, failed actuator, blocked filter, failed fan, controller fault, no water flow |
| Noisy or rattling unit | Worn fan bearings, loose panels, debris in fan, failing motor |
| Whistling | Restricted airflow, gaps around filters or panels |
| Musty smell | Standing water in drain tray, dirty coil or filter |
| Unit cycles on and off | Sensor fault, controller settings, short-cycling valves |
| Controller blank or showing errors | Power, communications or controller failure |
Why condensate is the most common problem
In cooling mode, water condenses on the coil and must drain away. Dust and biological growth build up in trays and drain lines, and condensate pumps fail. When the water cannot get away it overflows into the ceiling. Regular cleaning of trays and testing of pumps and float switches is the main defence.
Standing water is a hygiene risk. Drain trays that hold water can support microbial growth. Keep them clean and free-draining.
What servicing includes
- filter clean or replacement
- drain tray and line cleaning, pump and float switch tests
- fan and motor checks
- valve and actuator operation tests
- coil inspection
- controller, sensor and setpoint checks
When to replace rather than repair
Consider replacement when units are very old, parts are unavailable, fans use inefficient motors, or the layout has changed so much that units no longer serve the right spaces. EC fan upgrades and better controls are sometimes a middle route.
Next steps
See fan coil unit services. For room-by-room temperature complaints, read why some office rooms are too hot while others are too cold.
Frequently asked questions
Why is my fan coil unit leaking?
Most leaks come from a blocked condensate drain, a failed condensate pump or float switch, or a cracked drain tray. Less often, a valve or pipe joint leaks. Turn the unit off if water is near electrics and arrange a visit.
What is the difference between two-pipe and four-pipe fan coils?
Two-pipe units have one water circuit that supplies either heating or cooling depending on the season. Four-pipe units have separate heating and cooling circuits, so each unit can heat or cool at any time.
UK guidance and references
Primary and industry sources relevant to this subject. Always check the current edition and how it applies to your building.
- SFG20 standard maintenance specification BESA / SFG20
- Legionnaires' disease (ACOP L8) Health and Safety Executive
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.