Air Conditioning Troubleshooting Guide for Building Managers

What You Need to Know

Commercial air conditioning systems fail for predictable reasons. As a building manager, you do not need to be an HVAC technician, but you do need to diagnose problems quickly enough to know whether to call a contractor, call an engineer, or fix it yourself. The difference between a $200 filter change and a $50,000 system replacement often comes down to catching problems early.

The 6 most common complaints in commercial buildings are: spaces too hot, spaces too cold, excessive noise, unpleasant odours, condensation or water leaks, and high energy bills. Each has a distinct set of causes and a logical diagnostic sequence. Most can be narrowed down to 2 or 3 likely causes within minutes using basic observations and no specialist tools.

This guide walks through each problem category with specific diagnostic steps, likely causes ranked by probability, and clear guidance on when the issue is a maintenance item (call your HVAC contractor) versus a design deficiency (call a building services engineer). It references AS 1668.2:2024 for ventilation requirements and AS/NZS 3823 for equipment performance standards where relevant.

• Space too hot (cooling not working). Check the thermostat setting first. Then check whether the outdoor unit compressor is running. If the fan spins but the compressor does not, it has tripped on a safety. Common causes: dirty condenser coils (restricted airflow causes high head pressure), low refrigerant from a leak, failed compressor capacitor, or tripped circuit breaker. If the compressor runs but cooling is weak, measure the supply air temperature at the diffuser. It should be 12 to 16 degrees C for a properly functioning system. Above 18 degrees indicates low charge or a blocked filter. (AS/NZS 3823.1.1 defines rated cooling capacity conditions)
• Space too cold (overcooling). Usually a controls issue rather than an equipment fault. Check whether the thermostat is set too low or is located in a warm spot (near a window or above equipment) that causes it to call for more cooling than the space needs. In VAV systems, a stuck-open damper on one zone overcools that zone while undercooling others. In VRF systems, check that the indoor unit is set to the correct mode and has not been overridden by an occupant. (BMS trend logs will show the control sequence)
• Excessive noise. Identify the noise type. A rattling or vibrating sound is usually a loose panel, failed anti-vibration mount, or debris in the ductwork. A high-pitched whine suggests a bearing failure in a fan motor or compressor. A rumbling or pulsing sound indicates duct resonance or fan surge. Noise that only occurs at certain times may be linked to an equipment scheduling issue. The NCC 2025 references AS/NZS 2107 for acceptable noise levels: 40 dB(A) for open plan offices and 35 dB(A) for meeting rooms. (AS/NZS 2107:2016 sets noise criteria by room type)
• Unpleasant odours. Musty or mouldy smells indicate microbial growth in the ductwork, drain pan, or filter. This is a health hazard under AS 1668.2:2024 and must be addressed immediately. Check that the condensate drain is flowing freely and the drain pan is not holding stagnant water. Chemical or burnt smells suggest an electrical fault (overheating wiring or motor) and require immediate shutdown and inspection. Exhaust odours entering the supply air indicate a problem with outdoor air intake location or separation distance from exhaust outlets. (AS 1668.2:2024, Clause 4.3 covers outdoor air quality)
• Condensation and water leaks. Condensation on diffusers or ductwork means the surface temperature is below the dew point of the room air. Common causes: missing or damaged duct insulation, failed vapour barrier, or supply air temperature set too low. Water dripping from ceiling panels is usually a blocked condensate drain on a fan coil unit or AHU. In humid climates, inadequate condensation management during design leads to chronic issues that maintenance alone cannot fix. (NCC 2025, Part F6 covers condensation management)
• High energy bills. If energy consumption has increased by more than 15% to 20% without a change in occupancy or operating hours, investigate. The most common causes are dirty filters (increasing fan energy), refrigerant leaks (reducing efficiency), failed economiser dampers (not using free cooling when outdoor conditions allow), and simultaneous heating and cooling (a controls fault where the system heats and cools the same zone). Review BMS trend logs for supply air temperature, return air temperature, and outdoor air damper position. A well-maintained system should achieve a coefficient of performance (COP) of 3.0 to 4.5 for cooling. (Section J of NCC 2025 sets HVAC energy efficiency requirements)
• Poor ventilation or stuffiness. Occupants complaining of stuffiness, headaches, or fatigue may indicate insufficient outdoor air. AS 1668.2:2024 requires 10 L/s per person of outdoor air for standard offices. If the outdoor air damper is closed, stuck, or the economiser is faulty, the system recirculates stale air. CO2 levels above 1,000 ppm confirm inadequate ventilation. A handheld CO2 meter costs $50 to $150 and provides instant diagnosis. (AS 1668.2:2024, Table 4.1 specifies minimum outdoor air rates)

Step 1: Confirm the complaint. Go to the affected space and verify the issue yourself. Use a handheld thermometer to measure the air temperature at the thermostat location and at the supply diffuser. Record the time, date, outdoor temperature, and occupancy level. Many complaints are subjective or intermittent, and having measured data eliminates guesswork when you brief the contractor.

Step 2: Check the controls. Look at the thermostat or BMS screen for the affected zone. Confirm the setpoint (typically 22 to 24 degrees C for cooling in offices), the current room temperature, and the operating mode (heating, cooling, auto, off). If the system has a BMS, check the trend logs for the last 24 to 48 hours. Sudden changes in performance suggest a component failure. Gradual degradation suggests a maintenance issue (dirty filters, fouled coils).

Step 3: Inspect the equipment. Walk to the plant room, rooftop, or ceiling void where the equipment serving that zone is located. Check for obvious issues: tripped breakers on the electrical panel, error codes on the unit controller, water pooling under the unit, ice on refrigerant pipes (indicating low charge or restricted airflow), and unusual sounds. For split systems and VRF, check both the indoor and outdoor units. The outdoor unit is often on the roof and forgotten during inspections.

Step 4: Check the filters. Dirty filters are the single most common cause of HVAC performance problems. A filter that is 50% blocked increases fan energy by 15% to 25% and reduces airflow, which drops cooling capacity. Most commercial systems use G4 or M5 panel filters that should be inspected monthly and replaced every 3 to 6 months depending on the environment. If the filter is visibly grey or black, it is overdue for replacement.

Step 5: Decide who to call. If the issue is a component failure (tripped compressor, blown fuse, failed motor, refrigerant leak, blocked drain), call your HVAC maintenance contractor. They fix equipment. If the issue persists after the equipment is confirmed to be working correctly, or if the building use has changed since the system was designed, call a building services engineer. Engineers assess whether the system design is adequate for the current conditions and recommend modifications. Common scenarios requiring an engineer include: tenant fitout changes that alter heat loads, converting open plan space to enclosed rooms (changing the air distribution requirements), adding IT server rooms or commercial kitchens, and persistent comfort complaints that contractors cannot resolve.

Preventive maintenance schedule. The most cost-effective approach is preventing failures rather than diagnosing them. AIRAH recommends quarterly preventive maintenance for commercial systems. The minimum schedule includes: monthly filter inspections, quarterly condenser coil cleaning (especially in dusty or coastal environments), quarterly belt and bearing checks on AHU fans, six-monthly refrigerant circuit checks (superheat and subcooling measurements), and annual comprehensive inspections covering electrical connections, controls calibration, and drainage systems. Well-maintained systems last 15 to 20 years. Neglected systems fail in 8 to 12 years.