Common fan coil faults and how to fix them
Common fan coil faults and how to eliminate them – practical service guide
A fan coil is a relatively simple device: a fan, a heat exchanger, a condensate tray, a filter, and a control system. Despite this apparent simplicity, there is surprisingly a large number of things that can stop working – and most of them have a specific, identifiable cause. After years of experience with customer requests ranging from family homes to hotels and administrative buildings, I know that the vast majority of service calls are repeated. The same faults, the same causes, the same solutions. This article is an attempt to document everything important in one place – so you know what to fix yourself and when to call a technician.
If you are just getting to know fan coils, I recommend first reading our other articles in the Knowledge Center: How to choose a fan coil – performance, type of mounting and number of pipes and Installation of a fan coil – procedure, connection and commissioning. Understanding the basic principles will significantly simplify reading this diagnostic guide.
How a fan coil works – the basis for understanding faults
Before we get into the faults themselves, it is useful to have the principle of operation in mind. A fan coil is essentially an air-to-water heat exchanger with forced air circulation. The fan draws air from the room through the filter, the air passes through the heat exchanger (where it gives up or gains heat from/to the heat transfer medium – hot or cold water), and the returned air is discharged back into the room. During cooling, air humidity condenses on the heat exchanger – condensate, which drains from the condensate tray and into the sewer via a drain.
Each of these components can be a source of a fault. Faults can be divided into several basic groups: performance issues (the fan coil heats/cooling poorly), water issues (condensate overflows, dripping), noise issues (fan, motor), electronic and control issues, and finally issues with contamination and biological contamination. Let's look at them one by one.
1. Fan coil heats or cools poorly – diagnosis of performance issues
This is probably the most common reason customers report a problem. They come in January with a complaint "the fan coil does not heat the room" or in summer "it is 27 °C in the room, even though it is cooling at full capacity". There can be several causes.
Clogged air filter
By far the most common cause of reduced performance. The filter captures dust, hair, fibers. If it is not cleaned regularly, the air resistance increases so much that the fan practically stops working effectively – the air flow drops by 30–60 % compared to the nominal state. Result? The heat exchanger is working, but the air simply does not pass through it quickly enough. Performance drops dramatically, and yet the appliance consumes almost the same amount of energy.
The solution is simple: remove the filter, clean it (vacuum, lukewarm water, not hot), let it dry and put it back. The frequency of cleaning depends on the environment – in a regular apartment, once every 6–8 weeks is sufficient, in a restaurant or workshop it may be once every 2 weeks. If you operate IVAR.SLI fan coils, spare and additional parts such as intake set for IVAR.SLI DC 400 or intake grid for IVAR.SLI DC 600 are easily available – such accessories affect not only the aesthetics, but also the air flow dynamics into the unit.
Air block in the heat exchanger (air venting)
If the fan coil was not properly vented during commissioning, or if the system is drawing air during operation (for example, due to pressure drop in the system), an air block will block the circulation of the heat transfer medium in the heat exchanger. Symptom: the fan coil blows air, but it is only warm or completely cold/hot (depending on the season). It is necessary to check the vent valve on the heat exchanger and release the air – typically by a manual vent valve (nut with a small hole). The procedure is the same as with any radiator.
Dirty fins on the heat exchanger
Even if the filter is working, over time a fine dust, grease (in kitchen areas), or biological film settles on the fins of the heat exchanger. The fins are very fine and dense – their contamination significantly worsens heat transfer. Cleaning the fins is a delicate matter: special sprays for cleaning heat exchangers (alkaline or neutral) are used, never a high-pressure water jet directed perpendicular to the fins, because they will bend. The correct way is to spray parallel to the fins or to use a special brush.
Incorrect supply water temperature
Fancoil is designed for specific input parameters: typically 7/12 °C for cooling, 45/40 °C or 55/45 °C for heating (depending on the project). If the heat/cold source does not deliver the correct temperature, the fancoil performance correspondingly decreases. Diagnosis: measure the water temperature at the fancoil inlet and outlet. A large temperature difference (e.g. 8–10 K instead of the design 5 K) means low water flow; a small temperature difference (1–2 K) on the other hand means that the water is flowing too quickly and does not have time to transfer energy.
Incorrect hydraulic flow
Fancoil requires a specific water flow (stated in l/h in the technical documentation). The reasons for insufficient flow may be: clogged water filter (strainer) at the fancoil inlet, clogged or improperly adjusted control valves, air in the piping, or insufficient pump pressure. Solution: check and clean the inlet strainer, verify the setting of balancing valves, bleed the system.
2. Condensate – why it drips and how to stop it
This is the second most common problem we encounter in practice. Condensate forms during cooling operation – the heat exchanger has a surface temperature below the dew point of the ambient air, humidity condenses on the fins and drips into the tray below the heat exchanger. This is normal. The problem occurs when the condensate flows out of the tray.
Clogged condensate pipe
The most common cause of tray overflow and subsequent water leakage into the room or ceiling. The condensate pipe (usually ø16–20 mm PVC hose) gets clogged with algae, mold, and biofilm, especially in summer. Symptom: water remains in the tray, does not drain, and eventually overflows. Solution: clean the pipe by flushing with water, or use tablets or liquids to prevent biofilm formation. I recommend flushing the hose under pressure once a year (before the cooling season).
Incorrect fancoil slope
Ceiling or ducted fancoils must be installed with the correct slope of the condensation tray towards the drain – typically 1–2 % (i.e., at least 1 cm per meter of length). If the fancoil was installed "in the plane" or even with the opposite slope, condensate accumulates at the opposite end of the tray and drips from the wrong place. Diagnosis is simple: use a spirit level. Solution: adjust the mounting and slope of the unit.
Clogged condensation tray
The tray gets clogged over time with sediment, biofilm, and dead algae. This can cause the condensate to not drain to the drain, but to accumulate and eventually overflow the edge. Cleaning the tray is part of annual maintenance – disassemble, clean with a brush, and disinfect.
Condensation on non-insulated pipe
This is not a fault of the fancoil itself, but a system installation error. Cold water pipes (during cooling) must be wrapped with vapor-tight insulation of sufficient thickness – typically at least 13 mm elastomeric insulation (Armaflex or equivalent). If the insulation is missing or damaged, air humidity condenses on the cold pipe and this water drips and can be mistaken for condensate from the tray. Solution: add or replace the pipe insulation.
3. Noise and vibrations – diagnosis and solutions
Fancoil should be quiet. Modern units with proper settings have noise levels of 25–35 dB(A) at the lowest speed, which is barely perceptible. If you hear more from the fancoil, something is wrong.
Fan noise – types of noise
Squeaking or whining: Usually worn fan bearings. Fan motor bearings have a typical lifespan of 20 000–40 000 hours – in continuous operation this may be only 5–7 years. Solution: replace the motor or its bearings (if the motor is serviceable). In practice, it is cheaper to replace the entire motor assembly.
Vibrations and booming: Most often caused by loose mounting of the fancoil (loose threaded rods, missing damping washers), or rigid contact between the fancoil body and the ceiling/wall structure. Solution: tighten the suspension rods, check the presence of rubber or silicone vibration dampers. During installation, it is important that there is no rigid connection between the fancoil and the ceiling structure – only through anti-vibration pads.
Flow noise – hissing or blowing: Can be caused by too high air speed due to an unsuitable outlet or grille size. For example, if the original grille was replaced with a decorative one with a smaller passage area, the air has to flow faster and noisier. Check whether standard outlets and grilles corresponding to the specific model are used.
Vibrations from the pipe
If the pipe is rigidly attached to the fancoil without flexible connections, motor vibrations are transferred to the pipe and further into the building structure. Solution: always install flexible hoses (rubber or metal-plastic compensators) between the pipe and the fancoil inlet/outlet. The length of the flexible connection should be at least 150–200 mm.
Noise from the condensate pipe
With a large amount of condensate or incorrect slope, air can bubble in the condensate pipe. Solution: correct pipe slope and possibly bleed the condensate system.
4. Fan motor and control faults
The fan motor is the heart of the fancoil. With DC INVERTER motors (the current standard for modern IVAR.SLI units), control is smooth and the motor is less stressed than with older AC motors with fixed speeds. Despite this, faults do occur.
Fan coil fan is not rotating – complete failure
First check: power supply. Measure the voltage at the motor terminals with a multimeter. If the voltage is present, the motor receives a signal but does not rotate, it is likely blocked (mechanically – foreign object, ice under extreme conditions) or the winding is burned out. If there is no voltage, look for a fault in the regulation or the fuse. A separate category is DC INVERTER motors – these receive a controlled signal from the electronics and diagnosis requires knowledge of the specific communication protocol.
Fan coil fan rotates only at certain speeds
With older fan coils with AC motors and tapped windings (3–4 speeds), the problem may be a broken winding or a faulty regulating resistor/capacitor. Symptom: the fan coil runs at 1st and 3rd speed, but at 2nd speed the motor neither stops nor starts. Diagnosis: measuring the resistance of individual windings with an ohmmeter. If it is a 4-speed 230V regulator for IVAR fan coils, the problem may be in the regulator itself – a faulty switch or its electronic equivalent.
Motor overheating
The motor is equipped with thermal protection (thermal cut-out), which disconnects it when the maximum temperature is exceeded. Reasons for overheating: dirty filter (the motor operates under higher resistance and higher current), mechanical friction (worn bearings), or insufficient cooling (blocked air duct). Solution: after cooling down and removing the cause, the motor usually resets itself. If it repeats, the motor must be replaced.
5. Control and thermostat faults
The fan coil control is the brain of the whole system. Its failure is manifested by the fact that the fan coil either does not work at all or works uncontrollably (for example, it heats continuously or does not start at all). The topic of control is discussed in detail in a separate article Fan coil control – from a simple switch to smooth DC INVERTER control in our Knowledge Center. Here we summarize only the most common faults.
Thermostat is not measuring temperature correctly
If the thermostat (room sensor) is placed in an inappropriate location – near the fan coil outlet, in direct sunlight, or near another heat source – it measures distorted values and regulates incorrectly. The fan coil may, for example, stop too early because the thermostat "thinks" it is sufficiently warm, although it is still cold in another part of the room. Solution: relocate the sensor.
Non-functional two-way valve
Most fan coils are controlled by a combination of fan speed regulator and a two-way (or three-way) motor valve on the water circuit. If the valve freezes in the closed position, water will not flow and the fan coil will not work. If it freezes in the open position, the fan coil heats or cools unnecessarily – uncontrollably. Diagnosis: with a closed valve, there should be no temperature difference at the fan coil inlet and outlet. Solution: replace the valve or its actuator.
Bus communication faults (Modbus, BACnet)
In professional installations with central control (BMS), fan coils are connected via communication protocols. A communication fault is manifested by the fact that the fan coils ignore commands from the central unit. Diagnosis requires special tools and knowledge – this is exclusively the domain of service technicians.
6. Biological contamination – mold, bacteria and odor
From a biological perspective, a fan coil is a demanding device. The condensate tray, wet heat exchanger, and filter are an ideal environment for mold and bacteria growth. This is a problem that customers often ignore or underestimate, but it has real health consequences.
Odor when starting up
Classic symptom: after starting the fan coil (especially after a long period of inactivity), an unpleasant musty or sour odor is noticeable. Cause: biofilm on the heat exchanger fins and in the tray. Solution: thorough chemical cleaning of the heat exchanger and tray with disinfectants, cleaning the condensate pipe.
UVC disinfection – modern solution
A professional and long-term solution to biological contamination is the installation of a germicidal UVC lamp in the heat exchanger area. UVC radiation (wavelength 253.7 nm) deactivates the DNA of microorganisms and prevents their growth on the fins and in the tray. For IVAR.SL and SLI fan coils, the UVC lamp for IVAR.SL, SLI 400 with a length of 475 mm is available – this is a solution specifically designed for these units, so no improvisation is needed. The lamp usually operates during fan coil operation and significantly extends the time between chemical cleanings.
It is important to remember that UVC lamps have a lifespan – typically 8,000–12,000 hours of operation, after which their effectiveness decreases even though they still glow. Replacement should be part of the annual service.
Legionella – underestimated risk of condensate
In commercial and healthcare operations, the risk of legionellosis with fan coils is real – especially if the condensate tray is not regularly cleaned and disinfected. The temperature of the condensate (typically 15–25 °C during cooling operation) is ideal for the growth of the Legionella bacteria. Preventive measures: regular cleaning of the tray (at least twice a year), disinfection, sufficient slope for drainage. In operations regulated according to hygiene standards, a service protocol is mandatory.
7. Installation errors as a source of faults – what happens in the first year of operation
A large part of faults does not originate in the fan coil itself, but in installation errors. From practice, we know that the first year of operation reveals most of them.
Incorrect hydraulic connection
Guaranteed problems include, for example, swapping the inlet and outlet pipes (the fan coil works, but performance is reduced), omitting balancing valves (uneven flow distribution with multiple fan coils), or missing air vents on the heat exchanger. Details on correct connection can be found in the article Two-pipe and four-pipe fan coil connections – differences and when to use which.
Service accessibility
Fan coils built into special installation boxes or into drywall partitions must have an inspection opening preserved. If it was covered with plaster or cladding, every service visit becomes a demolition operation. For recessed and built-in fan coils, there are dedicated solutions: installation box for air conditioners and fan coils is an example of a solution that facilitates hidden installation while maintaining accessibility for regular service.
Poor flow regulation during commissioning
If during the first system start-up (commissioning) the flows through individual fan coils are not correctly set using balancing valves, some will be over-dimensioned (noisy, with a small temperature difference), others under-dimensioned (weak performance). This is an error that is difficult to correct later without a complete system balancing.
8. Seasonal switching – problems when switching summer/winter
Two-pipe fan coil systems are switched seasonally – in winter hot water flows in, in summer cold water. During this switching, characteristic problems occur, to which you should be prepared.
When starting cooling for the first time in spring/summer, it is very important to: check the condensate pipe (it may have clogged during winter), bleed the system, check the pipe insulation (it may have been damaged during winter operation), and test the function of the control valve (it may have jammed in the position set during winter).
When switching to winter heating, on the other hand, check: the tray should be clean and dry, the filter should be cleaned after the summer season, and the fan motor should be tested at all speeds.
9. How to correctly diagnose a fan coil unit failure – step by step
If you want to solve the problem yourself (or prepare materials for a technician), we recommend the following procedure:
- Step 1 – Visual inspection: Check the filter (remove and visually inspect), the external condition of the fan coil, visible cracks or water leaks.
- Step 2 – Functional test: Turn on the fan coil at each fan speed. Listen for sounds. Place your hand in front of the outlet – do you feel air? Is it warm or cold (according to the setting)?
- Step 3 – Water inspection: Check whether water is flowing (feel the temperature of the pipe at the fan coil inlet). During heating, the inlet pipe should be clearly warm.
- Step 4 – Condensate inspection: Look into the drip pan (if accessible) – is there water in it? Is it draining? Is the pipe clear?
- Step 5 – Control inspection: Check the thermostat or control panel. Is the setting correct? Is it not in an error state?
- Step 6 – Measurement: If you have a multimeter, measure the power supply at the motor terminals. If you have a thermometer, measure the temperature of the inlet and outlet water.
The results of these steps usually clearly show in which part of the system the problem lies and significantly simplify communication with the service technician.
10. When to call a professional and when you can do it yourself
Cleaning the filter, visual inspection, cleaning the condensate pipe by rinsing with water, and purging the heat exchanger are common tasks that any technically skilled user can handle. Replacing the fan motor is a bit more complex, but manageable for electricians or skilled DIYers (provided safety rules for working under voltage are followed).
On the other hand, leave the following to a professional: work on the hydraulic circuit (valve replacement, leak repair), work on control electronics and BMS, diagnosis of communication failures, reprogramming of controllers, and any interventions in the electrical installation beyond simple voltage measurement.
More about regular maintenance can be found in the article Cleaning and maintenance of fan coils – filters, UVC lamps and regular service, where the cleaning procedures for individual components are described in detail.
11. Preventive maintenance – how to avoid failures
Prevention is always cheaper than repair. Experience from hundreds of cases shows that fan coils that receive regular care work for 15–20 years without major problems. Those neglected start causing problems after 3–5 years.
Monthly: Visual inspection of the filter, possibly its cleaning (depends on the environment).
Before each cooling season (spring): Thorough cleaning of the filter, cleaning of the heat exchanger fins, inspection of the condensate pipe, purging, testing functionality at all speeds, inspection of the UVC lamp.
Before each heating season (autumn): Cleaning of the drip pan, inspection of pipe insulation, testing of control valves, inspection of the fan coil housing seal.
Once every 2–3 years: Professional inspection of the electrical part, measurement of motor current draw, inspection of bearing condition, inspection and possible replacement of seals on the water circuit.
Most frequently asked questions (FAQ)
Why is the fan coil blowing air, but the room is not heating or cooling?
The most common causes are: a dirty filter (air is passing, but the flow is insufficient), an air lock in the heat exchanger (water is not flowing), or a closed water control valve (water is not entering the fan coil at all). Check the filter, purge the heat exchanger, and verify whether the control valve is open. If the inlet pipe of the fan coil is cold to the touch during heating, water is not flowing into it – the cause is likely in the control or valves.
The fan coil is dripping water on the floor – what should I do?
It is likely that the condensate pipe or the drip pan is clogged. Check whether water is standing in the drip pan – if so, try to flush the drain hose with water or blow it through. If water is dripping from a location outside the drip pan, the problem may be in non-insulated piping (condensation on cold piping) or in the incorrect slope of the fan coil. During heating operation, condensation should not occur at all – if you see water during heating, it may be a leak from the hydraulic circuit, not condensation.
The fan coil emits an unpleasant odor. What causes it?
It is most likely biological growth (mold, bacteria, biofilm) on the fins of the heat exchanger, in the drip pan, or in the condensate pipe. Solution: thorough cleaning with a disinfectant. A long-term solution is the installation of a UVC lamp, which permanently prevents biological growth. The odor typically occurs during the first start-up after a long period of inactivity or after the summer season.
The fan coil is much louder than at the time of installation. Is that normal?
No, increasing noise is always a sign of a problem. The most common causes: worn fan bearings (squeaking, humming), a dirty filter (the motor is working under higher load and is louder), a loose fan coil mounting (vibrations, thumping), or a foreign object caught in the fan circuit. Each of these problems has a specific solution – the most important thing is not to delay diagnosis, as worn bearings can cause motor failure after prolonged operation.
The controller does not display the correct room temperature. How to calibrate it?
First, check whether the thermostat is not placed in an unsuitable location (in direct sunlight, near the fan coil outlet, near an uninsulated external wall). Most modern controllers allow for a calibration correction (offset) in the range of ±5 °C in the settings menu. If the controller does not have such a function, or if the deviation is greater than 3–4 °C, it is likely a temperature sensor failure and the controller needs to be replaced.
Can I repair an embedded fan coil myself if it is covered with drywall?
It depends on whether a maintenance access panel (hatch) was made during installation. If yes, the filter and some other components are accessible. If not, any internal repair will require cutting into the drywall construction. Therefore, we strongly recommend insisting on the implementation of a sufficiently large maintenance hatch – at least 400 × 400 mm at each accessible location (filter, valves, condensate pipe) during any hidden installation of a fan coil.
Conclusion – a fan coil you take care of will serve for decades
Fan coils are reliable devices if treated properly. The vast majority of problems we encounter in practice have a simple cause and a relatively simple solution – cleaning the filter, purging, cleaning the condensate drain. Problems arise when these minor symptoms are ignored, because a small fault can develop into a serious failure (ceiling flooding, burnt motor, biological contamination).
Investing in regular maintenance – twice a year, before the season – will multiply return in terms of extended equipment lifespan and avoiding costly repairs. If you are considering expanding the system with additional components, also read our article Dimensions and installation requirements of IVAR SL, SLI and SLS fan coils – proper sizing and installation are the best prevention of future problems.
Do you have a question about this topic?
Not sure or dealing with a specific situation in your home? Write to us – we are happy to help.