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U4 is one of the most common error codes on Daikin VRV and VRF systems. It indicates a communication fault between the indoor and outdoor units. The system can’t operate if the units can’t talk to each other. The good news: U4 is usually a wiring or connection issue rather than a catastrophic component failure. The bad news: on a multi-unit VRV system, tracking down a communication fault requires methodical diagnosis.

We see this issue regularly in Central London. A proactive VRV systems agreement is the best way to prevent this fault from recurring, and it means faster response times if you do need an emergency breakdown service.

What U4 Actually Means

Daikin VRV systems use a two-wire serial communication bus connecting every indoor unit back to the outdoor unit. This bus carries operational data in both directions — temperature readings, mode commands, compressor demands, error reports, and system status. When an indoor unit loses communication with the outdoor unit, the system raises a U4 error and shuts down the affected circuits as a protective measure. No communication means no coordinated operation, and running without coordination risks equipment damage.

The scope of the U4 error tells you a lot about the fault location. U4 on a single indoor unit suggests a problem with that unit’s wiring or PCB. U4 across multiple indoor units simultaneously points to a fault on the main communication trunk, at the outdoor unit’s comms board, or on the shared section of the bus.

Loose Wiring Terminations

This is the number one cause of U4 errors across all Daikin VRV installations. The communication bus connects via screw terminals at every indoor unit and at the outdoor unit. Over years of continuous operation, vibration — particularly from the outdoor unit’s compressor — gradually loosens these connections. A termination that was properly torqued during installation can be barely making contact after three or four years of service.

The fix is simple in principle: check and re-tighten every termination on the communication bus. Start at the unit displaying the error and work outward. The outdoor unit terminations deserve particular attention because they experience the most vibration. Use a torque screwdriver set to the manufacturer’s specification — overtightening damages the terminal block just as surely as undertightening causes intermittent connections.

Damaged Communication Cable

Recent building works are the biggest risk factor here. Ceiling tile replacement, electrical rewiring, cable tray modifications, partition installation, fire alarm upgrades — any work in the ceiling void or risers can damage the VRV communication cable. We’ve seen cables nicked by electricians routing new containment, crushed during ceiling grid reinstallation, and chafed through where they run over sharp bracket edges. Sometimes the damage is obvious. Sometimes it’s a partial nick in the insulation that causes intermittent faults only when the cable moves or the temperature changes.

If U4 appeared shortly after building work, inspect the cable route between the affected indoor unit and the outdoor unit. Look for visible damage, new cable ties that are compressing the comms cable, or areas where the cable has been disturbed. Damaged communication cable should be replaced in full sections — splicing VRV comms cable is unreliable and not recommended by Daikin.

Indoor Unit Address Settings

Every indoor unit on a Daikin VRV system has a unique address on the communication bus. On older systems, these addresses are set using DIP switches on the indoor unit’s PCB. On newer systems, addresses are assigned through commissioning software via the system controller. If an address gets corrupted — after a power surge, a PCB replacement, or an accidental DIP switch change during maintenance — the outdoor unit can’t identify that indoor unit on the bus. Communication fails, U4 appears.

If a unit’s address has been corrupted or duplicated, it needs resetting to its correct value. On DIP switch systems, this means physically accessing the indoor unit PCB and verifying the switch positions against the commissioning record. On software-addressed systems, the commissioning tool reconnects and reassigns the address. Either way, the system needs reinitialisation after an address correction to rebuild the communication map.

PCB Failure on Indoor or Outdoor Unit

The communication interface circuitry on either an indoor or outdoor unit PCB can fail independently of other board functions. A power surge, moisture ingress into the control enclosure, or simple component aging can damage the comms transceiver while leaving other circuits intact. A failed comms chip on an indoor unit’s PCB means that specific unit goes silent on the bus. A failed comms section on the outdoor unit’s main PCB can knock out communication with every indoor unit on the system.

Diagnosis involves systematically isolating units from the communication bus. Disconnect indoor units one at a time and check whether the remaining units can communicate. If disconnecting a specific indoor unit restores communication for everything else, that unit’s PCB is the culprit. If no combination of indoor unit disconnections restores communication, the outdoor unit’s comms board is likely at fault. PCB replacement resolves the issue — provided the correct board is available.

Power Supply Issues Causing Communication Dropout

The communication bus requires stable power to function reliably. If the supply voltage to an indoor unit drops below the operating threshold — due to a loose power connection, a degraded transformer, or an overloaded electrical circuit — the unit’s PCB may not have enough power to sustain the communication signal. The unit appears powered on — LEDs illuminated, display active — but it can’t maintain the bus communication protocol. The result is an intermittent U4 that comes and goes, often correlating with other electrical loads switching on and off in the building.

If your site uses Daikin units, our specialist engineers know these systems inside out. This problem often goes hand in hand with VRV system error codes, so it is worth investigating both if you are experiencing intermittent faults.

Check the incoming power supply voltage at every unit displaying U4. Compare the measured voltage to Daikin’s specified operating range for that model. Low voltage or voltage that sags under load points to a power supply fault rather than a communication fault — fixing the electrics fixes the U4.

Earth Fault on the Communication Line

An earth fault occurs when the communication cable’s insulation breaks down and a conductor contacts earthed metalwork — a cable tray, a pipe, a structural beam. This corrupts the communication signal across the entire bus, potentially causing U4 errors on multiple or all indoor units simultaneously. Earth faults are often intermittent: the cable touches metalwork when the building warms and expands, loses contact when it cools. A system that works fine on Monday morning but throws U4 every afternoon is a classic earth fault pattern.

An insulation resistance test on the communication cable — measured section by section between each pair of units — will locate an earth fault. The affected cable section needs replacing, and the route should be inspected to prevent the same damage recurring.

ADK and Daikin U4 Repairs

U4 is the Daikin VRV error code we see most often, and it’s typically resolved on the first visit. Our engineers carry replacement PCBs for common Daikin indoor and outdoor units, along with the manufacturer’s diagnostic interface to interrogate the bus, read detailed fault histories, and pinpoint exactly where communication has broken down. Most U4 repairs are completed within a couple of hours.

If your Daikin VRV system is flashing U4, call ADK on 0207 801 0808. We’ll find the fault, fix it, and get your system communicating again.

The post Daikin Error Code U4 — VRV Communication Fault Explained first appeared on ADK Kooling.

Hotel room AC complaints don’t just annoy guests. They damage your ratings, suppress bookings, and cost significantly more to fix reactively than they would have cost to prevent. Here’s what’s actually going wrong behind the grille and how to stop these complaints before they hit the internet.

We handle this type of problem frequently for businesses in West London and the surrounding area. If your system is showing these symptoms, our PPM team can usually diagnose the root cause on the first visit.

Dirty Filters — The Number One Culprit

Every fan coil unit in every guest room has an air filter. When that filter clogs with dust, the unit can’t move enough air. The room doesn’t cool properly, the fan runs louder trying to compensate, and the guest notices both problems immediately. In severe cases, the evaporator coil ices up behind the clogged filter, and the unit stops producing any meaningful cooling at all.

Housekeeping staff are in every room daily. They should be trained to visually check the fan coil unit and flag rooms where the filter looks dirty or the unit sounds abnormal. Maintenance should be changing filters on a fixed schedule — quarterly at minimum, monthly in high-occupancy rooms or during peak season. This single action prevents more guest complaints than every other item on this list combined.

Fan Coil Unit Valve Stuck

Hotel HVAC systems are typically either 2-pipe or 4-pipe configurations. In a 2-pipe system, the same pipework carries either chilled water for cooling or hot water for heating, switched seasonally. In a 4-pipe system, both are available year-round. Each fan coil unit has a motorised valve controlled by the room thermostat that opens and closes to regulate water flow through the coil.

These valves seize. Particularly in rooms that sit unoccupied for extended periods, the valve can stick in the closed position. The fan runs, air blows, but it’s room-temperature air because no chilled or hot water is flowing through the coil. The guest sets the thermostat to 18°C, waits twenty minutes, and nothing changes. They call reception, and now you have an unhappy guest and a maintenance call-out during service hours.

During annual maintenance, every valve on every fan coil unit should be cycled through its full range of travel. This keeps the mechanism free and catches valves that are starting to stick before they seize completely. Once a valve has fully seized, it needs replacing — a straightforward job requiring isolation of the water circuit serving that unit.

Thermostat Problems

Hotel room thermostats endure more abuse than most HVAC components. Guests prod every button, adjust settings they don’t understand, and occasionally attempt creative solutions to perceived temperature problems. Battery-powered thermostats go flat. Wired thermostats get knocked off the wall during luggage handling. Guests switch units off at the isolator behind the curtain and don’t mention it at checkout.

Dead batteries are the single most common thermostat-related complaint. The display goes blank, the unit doesn’t respond, and the guest reports that the AC is broken. Maintenance should replace thermostat batteries on a fixed schedule — every six months. It takes 30 seconds per room and eliminates an entire category of complaints.

Guest confusion is the other major factor. Some guests switch the unit to “Fan Only” without realising they’ve disabled the cooling function. Others can’t distinguish between heating and cooling modes. Clear, simple operating instructions positioned near the thermostat help considerably. Better still, install tamper-resistant controllers that limit adjustable parameters to temperature setpoint only.

The Musty Smell — Condensate Tray Overflow

That damp, musty odour guests complain about almost always originates from the condensate tray inside the fan coil unit. When the unit cools air, it pulls moisture out of the room atmosphere. This condensation collects in a tray beneath the coil and drains away through a small pipe. If the drain blocks — or the tray doesn’t empty properly — stagnant water sits there growing mould, bacteria, and biofilm. Every time the fan runs, it pushes air across that contaminated water and straight into the guest room.

Condensate trays should be cleaned and flushed at every service visit. The drain line needs checking for blockages. Condensate tray tablets — slow-release biocide blocks dropped into the tray — inhibit microbial growth between service visits. They cost pennies per room and prevent the musty smell that generates one-star reviews. Most hotels don’t bother with them until the complaints become impossible to ignore.

Seasonal Changeover Failures on 2-Pipe Systems

Hotels running 2-pipe fan coil systems face a specific vulnerability each spring and autumn: the seasonal changeover. When the central plant switches from heating mode to cooling mode (or vice versa), every individual fan coil unit needs to be verified. Valves checked, controls confirmed, air purged from the pipework, and thermostats set to the correct mode.

We have written a separate guide on AC thermostats reading the wrong temperature which covers a related failure mode worth checking. Similarly, our article on air conditioning leaking water explains another common cause of the same underlying issue.

We’ve attended hotels in June where half the rooms have no cooling because the changeover in March wasn’t completed properly. Some rooms were missed. Others had air locks in the pipework. A few had valves that seized during the changeover. Each room needs individual verification — there’s no shortcut. Doing it properly in spring prevents a summer full of complaints.

Guest Operating Errors

Not every complaint represents a genuine fault. Many guests don’t understand hotel AC controls, especially international visitors accustomed to different systems. Common misunderstandings include expecting instant cooling from a fan coil unit that takes several minutes to reach temperature, not realising the unit only operates when the key card is in the slot, or setting the thermostat below the system’s minimum cooling setpoint and wondering why the room won’t go any colder.

Clear, multilingual operating instructions in the room reduce these complaints measurably. A laminated card near the thermostat explaining the basics — insert key card, set desired temperature, allow five minutes — saves your maintenance team dozens of unnecessary call-outs per month during peak season.

ADK’s Hotel PPM Contracts

We run planned preventive maintenance contracts for hotels across London. Quarterly filter changes, annual deep cleans of every fan coil unit, valve exercising, condensate tray treatment, thermostat battery replacement, and full seasonal changeover support. Our hotel clients see guest AC complaint rates drop by up to 90% within the first year of a PPM contract. That’s not a sales figure — it’s what actually happens when the maintenance is done properly.

If your guests are complaining about the AC, the problem isn’t the guests. Call ADK on 0207 801 0808 and let’s fix it properly.

The post Hotel Room AC Guest Complaints — Common Causes and Fixes first appeared on ADK Kooling.

This is one of the most common autumn call-outs we handle across London. Heat pumps are designed to do both jobs — heating and cooling — using the same refrigerant circuit. But the changeover between modes relies on several components working in sequence, and when any one of them fails, you get exactly this: a system stuck blowing cold air when you need heat.

This is one of the most common callouts we attend across North London. If your system has not been serviced recently, booking a PPM visit can often resolve the underlying cause before a full breakdown occurs.

How Heat Pump Reversal Works

Understanding the mechanics helps narrow down the fault. In cooling mode, the system absorbs heat from inside the building and dumps it outside through the outdoor coil. In heating mode, it does the opposite — pulls heat from the outdoor air and delivers it inside. Same compressor, same refrigerant, same pipework. The only thing that changes is the direction of flow.

The component that makes this possible is the four-way reversing valve, mounted inside the outdoor unit. This valve redirects refrigerant flow so the indoor coil switches from evaporator to condenser and vice versa. When the reversing valve or anything controlling it fails, the system stays locked in whichever mode it was last running. Since most systems spend the summer in cooling, that’s where they get stuck.

Failed Four-Way Reversing Valve

The most common mechanical failure behind this symptom. The four-way reversing valve uses a sliding mechanism inside a brass body to redirect refrigerant. Over time — especially after months of continuous operation in one mode — the slider can seize. Corrosion, particulate contamination in the refrigerant, or simple mechanical wear can all prevent it from shifting position when the system calls for a mode change.

The telltale sign is that the system runs and pressures look reasonable, but the outdoor coil is acting as a condenser when it should be an evaporator. A thermal imaging camera makes this immediately obvious — the outdoor coil reads hot when it should be cold in heating mode. Replacing a reversing valve means recovering the refrigerant, brazing out the old valve, brazing in the new one, pressure testing, evacuating, and recharging. It’s a significant repair but a definitive one.

Reversing Valve Solenoid Coil Failure

The solenoid coil is the electrical pilot that triggers the reversing valve to change position. These coils sit on top of the valve body, exposed to weather on outdoor units. They burn out, suffer water ingress through cracked insulation, or lose their electrical connection at the terminal block. When the coil fails, the valve never receives the signal to shift.

This is actually the best-case scenario. Solenoid coils are cheap, readily available, and can be replaced without touching the refrigerant circuit. An engineer tests the coil with a multimeter — no continuity means the coil is dead. Swap it out, power up, command the system into heating mode, and the valve should shift. Ten-minute fix once diagnosed.

Thermostat or Controller Not Set to Heat Mode

We include this because it accounts for more call-outs than anyone wants to admit. The wall controller is still set to “Cool Only” from summer. The BMS hasn’t been switched from summer to winter schedule. The thermostat is in manual cooling mode rather than auto changeover. Some systems require a deliberate manual changeover at the controller or through the building management system — they don’t switch automatically.

Before assuming a mechanical fault, check every controller on the system. Check the BMS setpoints and schedules. Verify whether the system is configured for auto changeover or manual. This is a five-minute fix that saves an engineer visit and a diagnostic charge.

Outdoor Unit PCB Fault

The outdoor unit’s main printed circuit board controls the reversing valve output. If the relay or triac that drives the solenoid coil has failed on the PCB, the board runs the compressor perfectly but never sends the electrical signal to energise the reversing valve. The system cools fine because the default valve position is cooling. But heating requires the PCB to actively switch the valve — and if that output circuit is dead, nothing happens.

Diagnosis involves measuring for voltage at the reversing valve solenoid terminals when the system is commanded into heating mode. No voltage present with a confirmed good solenoid coil points directly at the PCB. Replacement boards for most commercial heat pumps are available, though lead times vary by manufacturer and model.

If you are also experiencing AC thermostats reading the wrong temperature, the two faults may share a common cause. Our engineers frequently find that addressing one resolves the other. You might also want to read our guide on an AC system not cooling the building for additional troubleshooting steps.

Defrost Cycle Stuck

Heat pumps in heating mode periodically run a defrost cycle to clear ice from the outdoor coil. During defrost, the system temporarily reverses back to cooling mode — sending hot refrigerant gas through the outdoor coil to melt the frost. Once the coil is clear, the system switches back to heating. If the defrost cycle fails to terminate — due to a faulty defrost sensor, a stuck timer, or a controller logic error — the system stays in cooling mode permanently. It thinks it’s still defrosting when it isn’t.

Check the defrost thermostat or sensor on the outdoor coil. Verify the defrost termination temperature setting. On many systems, you can manually force a defrost exit through the service menu or by cycling power to the outdoor unit. If the sensor has failed, replace it and the defrost cycle should terminate normally.

Low Refrigerant Affecting Heating Capacity

A heat pump with a slow leak might cool adequately during summer when ambient temperatures are higher and the cooling demand is moderate. But heating mode is a harder job — the system has to extract heat from cold outdoor air, which requires the full refrigerant charge to work efficiently. A system that’s 15-20% low on refrigerant may manage cooling but produce barely noticeable heat output. Suction pressure drops too low, the compressor runs inefficiently, and the supply air stays lukewarm.

If the system heated fine last winter but struggles this year, a refrigerant leak with gradual charge loss is a strong possibility. An engineer checks operating pressures and compares them to the manufacturer’s expected values for the current ambient conditions. If the charge is low, the leak has to be found and repaired before topping up — otherwise you’re back in the same position next autumn.

What ADK Does About It

We carry reversing valve solenoid coils, common PCBs, and refrigerant on every van. For a solenoid or controller fault, most heat pump changeover issues are resolved on the first visit. For a seized reversing valve, we diagnose on-site and schedule the replacement with minimal downtime. If your heat pump is blowing cold when it should be heating, call ADK on 0207 801 0808. We’ll get your system switched over properly.

The post Heat Pump Not Switching from Cooling to Heating — What to Check first appeared on ADK Kooling.

The AC system is only as good as the temperature reading it’s working from. If the thermostat or sensor is wrong, the system targets the wrong temperature and the room is either too warm or too cold regardless of how well the AC unit itself is performing. Here are the most likely reasons the reading is off.

We see this issue regularly in Mayfair. A proactive PPM agreement is the best way to prevent this fault from recurring, and it means faster response times if you do need an commercial AC installation.

Thermostat in Direct Sunlight or Near a Heat Source

This is the most common cause and the easiest to overlook. A thermostat mounted on a wall that gets afternoon sun will read several degrees higher than the actual room temperature. The same happens when a thermostat is positioned near a radiator, above a piece of electrical equipment, next to a hot water pipe running through the wall, or beside a window with solar gain.

The result: the AC system overcools because the thermostat thinks the room is warmer than it is. Or in winter, the heating never kicks in because the thermostat is being warmed by an external heat source and thinks the room has already reached setpoint. Either way, the occupants are uncomfortable and the energy bills are wrong.

The fix is relocation. The thermostat needs to be on an interior wall, approximately 1.5 metres above floor level, away from direct sunlight, heat sources, draughts, and doorways. It sounds basic, but we regularly find thermostats installed in the worst possible locations — above photocopiers, next to south-facing windows, or directly beneath downlighters that pump heat onto the sensor.

Failed NTC Sensor

Most commercial HVAC thermostats use an NTC (Negative Temperature Coefficient) thermistor to measure temperature. These are generally reliable, but they do drift over time and they can fail. A failed NTC sensor might read a fixed offset — consistently 3°C low, for example — or it might give wildly erratic readings that bounce around with no relation to the actual room temperature.

An offset reading is particularly insidious because the system appears to work normally — it heats and cools in response to demand — but it’s always targeting the wrong temperature. A quick comparison with a calibrated handheld thermometer placed next to the thermostat will reveal any offset. If the NTC has failed, the sensor or the entire thermostat unit needs replacing.

Thermostat Needs Recalibration

Some commercial controllers and BMS (Building Management System) sensors allow field calibration — an offset value that compensates for sensor drift or installation position. If this calibration has been set incorrectly, accidentally changed, or never done in the first place, the reading will be off by whatever the offset error is.

Check the controller settings. Look for a calibration or offset parameter. Compare the displayed temperature with a known-accurate thermometer and adjust the offset accordingly. On BMS-connected sensors, this is usually a parameter in the controller software rather than a physical adjustment on the sensor.

Wireless Thermostat with Dead Batteries

Wireless thermostats are increasingly common in commercial fit-outs because they avoid the cost of running cable back to the controller. But they run on batteries, and when those batteries die or run low, the thermostat behaviour becomes erratic. It might display a temperature reading that’s frozen at the last value before the battery dropped, send intermittent signals to the controller, or stop communicating entirely.

The AC system may default to a fallback mode — either running continuously or shutting down — depending on how it’s programmed to handle a lost sensor. Check the battery level. Replace with fresh batteries. If the thermostat has been running on dying batteries for weeks, it may need to be re-paired with the controller.

Thermostat in a Different Zone

In larger offices, it’s not unusual to find that the thermostat controlling a particular AC unit is physically located in a different area than the space the unit serves. The thermostat might be in a corridor, a server room, or a partitioned section of open plan that was created after the original HVAC installation.

The thermostat reads the correct temperature — for where it is. But that bears no relationship to the temperature in the space people are actually trying to cool. The fix is either relocating the thermostat into the served space or adding a remote sensor in the correct zone and configuring the controller to read from that sensor instead.

We have written a separate guide on an AC system not cooling the building which covers a related failure mode worth checking. Similarly, our article on AC short cycling causes and fixes explains another common cause of the same underlying issue.

Dirty or Blocked Return Air Sensor on a Ducted System

Ducted AC systems often measure room temperature via a sensor in the return air path rather than a wall-mounted thermostat. If that return air grille is blocked — by furniture, boxes, or a notice board someone has stuck over it — the sensor reads stagnant air that doesn’t represent the room conditions. Dust build-up on the sensor itself can also insulate it from the airstream and cause inaccurate readings.

Check the return air path. Make sure the grille is clear and unobstructed. Clean the sensor if accessible. On concealed ducted units, the sensor may be inside the return air plenum — an engineer will need to access the ceiling void to inspect and clean it.

Optimal Thermostat Placement — Getting It Right

For anyone planning a new installation or relocating a thermostat, the rules are straightforward. Mount it on an interior wall at 1.5 metres above floor level. Keep it away from windows, exterior walls, direct sunlight, heat-generating equipment, supply air diffusers, and doorways. It should be in the occupied zone of the space it controls — not in a cupboard, corridor, or adjacent room.

In open-plan offices, a central position works best. In rooms with multiple AC units, each unit ideally needs its own sensor in the zone it serves. Getting placement right during installation avoids years of comfort complaints and wasted energy.

ADK Can Fix It

Temperature control problems in commercial buildings are rarely about the AC unit itself — they’re about what the system is being told. A thermostat reading wrong by even 2°C can make a space permanently uncomfortable and waste significant energy. ADK engineers diagnose and correct thermostat and sensor faults across London — recalibration, relocation, replacement, BMS reprogramming, whatever it takes to get the reading right and the room comfortable. Call us.

The post AC Thermostat Reading Wrong Temperature — Causes and Fixes first appeared on ADK Kooling.

This is one of the most common call-outs we get on multi-split and VRV/VRF systems in London offices. The good news: because the other units still work, the outdoor unit is almost certainly fine. The problem is isolated to that one indoor unit or its connection back to the outdoor board. That narrows things down considerably.

This is one of the most common callouts we attend across West London. If your system has not been serviced recently, booking a commercial AC installation visit can often resolve the underlying cause before a full breakdown occurs.

Communication Fault Between Indoor and Outdoor PCB

This is the most common cause by a wide margin. Multi-split systems rely on serial communication between each indoor unit and the outdoor controller. If that communication link drops — due to a loose wire, corroded terminal, or a failed PCB — the outdoor unit simply stops sending refrigerant to that indoor unit.

On Daikin, Mitsubishi, and most other VRV/VRF systems, the outdoor unit keeps an error log that identifies exactly which indoor unit has faulted. That’s always the first thing we check. The error code will usually tell us whether it’s a communication error (often an E or U code on Daikin, or an A-series code on Mitsubishi) or something else entirely.

The fix might be as simple as re-terminating a loose connection on the indoor unit’s PCB. Or the PCB itself might need replacing. Either way, the diagnostic is straightforward once you’re reading the error codes properly.

Failed or Stuck Electronic Expansion Valve

Each indoor unit on a multi-split system has its own electronic expansion valve (EEV) that controls how much refrigerant flows through its heat exchanger. If that valve fails — stuck closed, stuck open, or responding erratically — the unit won’t cool properly even though everything else appears normal.

A stuck-closed valve means zero refrigerant flow. The unit blows air but there’s no cooling at all. A stuck-open valve can cause the coil to flood with liquid refrigerant, which may trigger a protection shutdown or produce wildly inconsistent temperatures. Either scenario means the valve or its coil driver needs replacing.

Wiring Termination Fault on That Specific Unit

Multi-split installations can run tens of metres of communication and power cable from the outdoor unit to each indoor unit, often through ceiling voids and risers. If a terminal has come loose, a cable has been damaged during other building works, or a junction wasn’t made properly during the original installation, that one unit loses its connection.

We see this particularly in offices that have had refurbishments. A ceiling contractor moves a cable, doesn’t realise what it is, and puts it back slightly wrong. Three weeks later, a unit stops working and nobody connects the two events. A methodical cable trace from the indoor unit back to the outdoor unit usually finds the problem.

Blocked Indoor Filter Restricting Airflow

A heavily blocked filter won’t normally stop the unit dead, but it can reduce airflow to the point where the evaporator coil ices up. Once ice forms on the coil, cooling capacity drops to almost nothing. The unit might cycle on and off, or just blow weakly warm air.

Pull the filters. If they’re clogged with dust and debris, clean them and check the coil behind. If there’s ice on the coil, switch the unit to fan-only mode and let it defrost before restarting in cooling mode. If the unit has been running with blocked filters for months, the coil itself may need a deep clean.

Failed Indoor Fan Motor

If the fan motor has failed or is running at a fraction of its normal speed, the unit may still be receiving refrigerant but can’t distribute the cooling. You’ll notice the unit is unusually quiet — or completely silent. The coil may ice up as a secondary effect because there’s not enough air moving across it.

This problem often goes hand in hand with split system error codes. If you are noticing both symptoms, it is worth getting an engineer out before the system fails completely. We have also covered an AC system not cooling the building in a separate guide.

Fan motors on indoor units do wear out, particularly in dusty environments or units that run continuously. The motor and fan assembly can be replaced without touching the refrigerant circuit, so it’s a relatively contained repair.

Refrigerant Distribution Issue in Branch Piping

Multi-split and VRF systems use branch distribution boxes (refnet joints or similar) to split the refrigerant flow from one set of outdoor pipes to multiple indoor units. If there’s a restriction in the branch piping serving that one unit — a kink, a brazed joint that’s partially blocked, or a faulty distribution header — that unit gets starved of refrigerant.

This is less common than communication or valve faults, but it does happen, especially on systems where the piping installation was rushed. Pressure and temperature measurements at the indoor unit versus the branch box will reveal the restriction.

What to Do Next

Start with the error code. On any VRV/VRF system, the outdoor unit’s error log is the fastest path to diagnosis. If you don’t have access to the controller or can’t read the codes, that’s where we come in.

ADK engineers work on multi-split and VRF systems across London every week — Daikin, Mitsubishi, Toshiba, Samsung, the lot. We carry PCB testers, EEV drivers, and the manufacturer diagnostic tools needed to pinpoint the fault on the first visit. One unit down doesn’t have to mean a full system replacement. Call us and we’ll get it sorted.

The post Multi-Split AC System — One Indoor Unit Not Working first appeared on ADK Kooling.