Why One Room is Cold While Another is Warm: Multi-Split Aircon Refrigerant Migration

In Singapore, multi-split air conditioning systems are extremely common in HDB, condominium, and landed residential configurations. Known colloquially as "System 2", "System 3", or "System 4", these structures couple multiple indoor fancoil units (FCU) to a single, high-capacity outdoor condenser unit. This design saves valuable ledge space and maximizes power usage. However, multi-split setups can sometimes display an unusual symptom: one bedroom experiences exceptionally cold air, while another bedroom cooled by the same system feels warm, humid, and sluggish. When this occurs, the issue is often related to refrigerant migration, a thermodynamic process where refrigerant distribution becomes unbalanced across the indoor units. Let us explore the mechanics of refrigerant migration, how modern inverter compressors manage multi-split distribution, and what steps you can take to resolve uneven cooling. --- ## 1. What Exactly is Multi-Split Refrigerant Migration? In a multi-split system, the outdoor condenser containing the [inverter compressor](/blog/how-inverter-compressors-control-humidity-dew-point-science) distributes liquid refrigerant to all active indoor units through a branching network of copper pipes. ### Liquid and Gas Redistribution: Refrigerant naturally migrates toward the coldest parts of the system. Additionally, physical piping lengths, elevation splits, and gravity affect how liquid is distributed: * **The Active vs. Inactive Overlap:** When you turn on Fancoil A but keep Fancoil B switched off, refrigerant should theoretically flow only through Fancoil A's path. However, due to pressure differences and bypass pathways within the system, a small amount of liquid refrigerant can bleed into Fancoil B's circuit. * **The "Cold Room" Mystery:** This slight bleed of refrigerant can cause Fancoil B's evaporator coil to drop in temperature, even though the unit is powered off. Condensed moisture can collect on the cold coils, occasionally causing the classic "dripping water from inactive units" symptom. To learn more about water and condensation issues, read our guide on [aircon water dripping vs condensation](/blog/aircon-water-dripping-vs-condensation). --- ## 2. The Mechanics of the Electronic Expansion Valve (EEV) To regulate refrigerant flow to each fancoil, modern multi-split systems utilize a micro-motorized component called an Electronic Expansion Valve (EEV). ### How EEVs Maintain Temperature Balance: Every fancoil circuit features its own dedicated EEV inside the outdoor unit or a separate joint manifold. Based on temperature readings from your room's air sensors, the [indoor PCB motherboard](/blog/aircon-pcb-motherboard-singapore-guide) sends electrical commands to open or compress the EEV orifice. * **Micro-Staged Control:** The EEV opening adjusts in micro-steps. If a bedroom requires heavy cooling, the valve core opens wider, allowing more refrigerant into that fancoil. * **When EEVs Malfunction:** If an EEV's stepper motor burns out or if its internal orifice becomes partially blocked by oil or soot, the valve can get stuck. If it gets stuck in a closed position, that specific room will blow warm air. If it gets stuck wide open, that room's coil receives an excessive volume of refrigerant, freezing the indoor unit. To explore why compressors experience electrical shut downs or cycle problems, check out our report on [why your aircon compressor short-cycles and thermistor faults](/blog/why-aircon-compressor-short-cycles-causes-solutions). --- ## 3. Critical Signs of Improper Refrigerant Distribution How can you identify if your multi-split cooling issue is caused by refrigerant migration or valve imbalances? Look for these common physical symptoms: ### Drooping Evaporator Temperature: If you feel a freezing-cold draft from an inactive fancoil unit, refrigerant is migrating and cooling the idle evaporator. This issue often leads to condensation on the outer plastic body of the quiet fancoil. ### One Room Fails to Cool Fully: When multiple indoor units are turned on simultaneously, one fancoil may struggle to cool. This can occur if the outdoor compressor is working on a skewed EEV setting, directing the majority of the cooling power to just one zone while leaving the other rooms warm. ### Whistling or Whizzing Sounds: Refrigerant flow through restricted valve orifices can create distinct sounds. If you hear high-frequency whistling or bubbling noises behind your bedroom walls, it often indicates pressurized refrigerant moving through an uncalibrated EEV opening. --- ## 4. Multi-Split Cooling Diagnostics Review this quick troubleshooting guide to understand the potential causes of uneven cooling in a multi-split setup: | Visual Symptom | Potential Source of Fault | Professional Diagnostic Step | | :--- | :--- | :--- | | **One room is icy cold, while the adjacent room remains warm** | Stuck Electronic Expansion Valve (EEV) or drifted room sensor | Diagnostic manifold pressure check and thermistor resistance test | | **Hissing or bubbling noises coming from a single fancoil unit** | High-pressure liquid refrigerant bypassing closed EEV channels | Replacement of the stepper motor assembly or solenoid coils | | **Water dripping from an inactive, powered-off indoor unit** | Refrigerant migration cooling the idle coils below dew point | Inspection of the joint bypass seals and expansion valve calibrator | | **Weak airflow and warm air from only one active room** | Partially clogged internal blower wheel or dusty filter mesh | Comprehensive fan wheel cleaning and air filter check | --- ## 5. Non-Refrigerant Reasons for Uneven Room Temperatures Before concluding that your system has a complex refrigerant issue, check for simpler, physical causes: * **Clogged Air Filters:** A heavy buildup of dust on your fancoil's air filters can block room air from crossing the evaporator coils, leaving the room warm. Fortunately, this is easy to resolve. To clean them yourself safely, read our step-by-step guide on [how to clean your aircon filters safely](/blog/how-to-clean-aircon-filters-safely). * **Dusty Blower Wheels:** If the cylindrical fan wheel inside your fancoil is coated in dirt, it cannot spin or blow air effectively. This results in weak airflow. For more details, explore our article on [aircon fan speed and slow weak airflow troubleshooting](/blog/aircon-fan-speed-slow-weak-airflow-singapore). --- ## 6. Why You Must Hire Professional HVAC Engineers Refrigerant distribution issues involve pressurized piping and delicate electrical circuits. Diagnosing and resolving these faults safely requires specialized training and equipment: * **High-Purity Gas Testing:** Certified technicians use digital manifold gauges to monitor liquid and gas pressures across the copper loops, identifying where restrictions or blockages are located. * **EEV and Motherboard Testing:** Technicians use specialized multimeters to test EEV coils and motherboard relays to ensure proper control communication. * **System Recharge:** If a refrigerant leak is contributing to migration issues, a professional gas top-up must be completed to restore pressure levels to manufacturer specifications. This ensures correct operation and prevents your unit from feeling not cold. **Is your System 2, 3, or 4 multi-split setup cooling unevenly, or are you hearing unusual whistling noises? Let our specialist team restore balance to your system with reliable aircon repair and servicing. Reach out to Sky Blue Aircon on WhatsApp at [+65 9248 7291](https://wa.me/6592487291) or call our hotlines at 6556 4042 to schedule a professional diagnostics appointment today!**