When a commercial air conditioning system stops, the facilities manager usually hears about it from the people sitting under it, and the priority is to get a qualified engineer on site. Knowing why commercial systems stop, and what information speeds up a repair, makes that call more effective.
This guide explains the common causes of commercial air conditioning failure, what the symptoms usually point to, and where the legal line sits between a building user and a certified engineer.
Why has our commercial air conditioning stopped working?
Failures can be grouped broadly into five types, and the symptom usually points to one.
| Cause | Typical symptom |
|---|---|
| Power or controls | Dead controller, blank display, or the whole system off after a power cut or fire alarm test |
| Airflow | Weak air from the grilles, uneven temperatures, ice on the indoor unit |
| Condensate drainage | Water dripping from a cassette or the unit shutting down with a drain fault code |
| Refrigerant circuit | Running but not cooling or heating, ice on pipework, fault codes for pressure or temperature |
| Component failure | Tripping breakers, loud or unusual noise, a fan or compressor that will not start |
Some causes, such as a schedule change or a fire alarm interlock, turn out to be simple. Others involve the refrigerant circuit or electrical components. Telling them apart reliably is a job for a qualified engineer who can read the system’s fault history.
What can stop a commercial system without a fault in the equipment?
Not every stoppage is a breakdown. Several causes sit outside the equipment itself:
- Controller settings. A system left in heat mode after winter, or in fan-only mode, runs without cooling and can look like a breakdown.
- Schedules and central control. Time clocks, central controllers and building management systems switch plant on and off. Clock changes, bank holiday settings and changes to building occupancy can leave a system off at the wrong times.
- Power interruptions. A tripped breaker, or an isolator left off after other work, stops the outdoor unit. A breaker that trips repeatedly points to an electrical fault.
- Fire alarm interlocks. Many systems shut down on a fire signal, including during alarm tests, and need resetting afterwards.
- Blocked airflow. Dirty filters, or leaves, litter, stored materials or heavy frost around the outdoor unit, reduce output and can trip protection devices.
On some systems, switching off and on repeatedly clears the fault history that an engineer uses to diagnose the problem, which is one reason engineers prefer a system left as found.
What do the fault codes on the controller mean?
A fault code identifies which unit has stopped and which protection has operated. It is the single most useful thing to pass to an engineer.
Codes are specific to each manufacturer and often to each model range, so the same letters mean different things on different systems. Engineers usually ask for a photograph of the display, including any unit or address number shown. On VRF and VRV systems, the central controller will often show which indoor unit raised the fault.
The manufacturer's operating manual lists what each code means, but a code identifies the affected circuit rather than the underlying cause.
Why is the system running but not cooling or heating?
If the fans are running and the air is at room temperature, the fault usually lies in one of four places.
- Restricted airflow. Blocked filters or coils reduce output and can cause the indoor coil to ice over.
- Outdoor unit not running. The indoor fan runs, but nothing is moving heat. This points to a power, control or component fault outdoors.
- Low refrigerant charge. Usually the result of a leak. Output falls gradually over weeks before the system stops.
- Defrost in heating mode. In cold weather a reverse-cycle system pauses heating for several minutes to clear ice from the outdoor coil. This is normal. Frequent or prolonged defrosts are not.
Changes in load matter too. More staff, more equipment or a new partition layout can leave a healthy system unable to hold temperature.
Why is the unit leaking water or smelling?
Water from an indoor unit is almost always condensate that cannot drain away. In cooling mode the indoor coil removes moisture from the air, and that water leaves through a drain line or a small pump. A blocked line, a failed pump or a tripped float switch sends it through the ceiling instead.
While the unit runs in cooling mode, it keeps producing condensate, so water continues to form until the unit is switched off. A blocked drain is a servicing issue, not a refrigerant one.
Musty smells usually come from the same place: standing water and dirt on the coil or in the drain tray. A burning or electrical smell is different: it points to an electrical fault and is treated as urgent, with the unit isolated until an engineer has inspected it.
When is a breakdown an emergency?
A breakdown is an emergency when the loss of cooling or heating threatens equipment, stock, safety or the ability to trade.
- Server and comms rooms. Temperatures can rise quickly and equipment may shut down or fail. IT continuity plans often include temporary cooling for this reason.
- Spaces holding temperature-sensitive stock or processes. Laboratories, pharmacies and some manufacturing areas.
- Occupied workplaces. The HSE says employers must provide a reasonable indoor temperature, with a minimum that should normally be at least 16°C, or 13°C for rigorous physical work. There is no legal maximum, but heat must be assessed as a risk. See HSE guidance on workplace temperature.
- Customer-facing premises. Gyms, restaurants and shops where conditions directly affect trade.
- Any electrical burning smell, repeated breaker trips or water near electrics.
Maintenance contracts often set agreed response times for emergency commercial air conditioning repairs.
What must be left to an F gas qualified engineer?
Work on the refrigerant circuit must be done by an individually qualified engineer. Most commercial systems contain fluorinated gases, and GOV.UK sets out the qualifications required to work with F gas.
In practice that covers leak testing, leak repair, recovering or adding refrigerant, and replacing any component in the sealed circuit. Electrical fault-finding inside the units also belongs with the engineer.
The building operator has duties too. GOV.UK guidance on checking F gas equipment for leaks sets minimum intervals for systems holding 5 tonnes of CO2 equivalent or more. Smaller systems fall outside these mandatory checks, so whether a particular system is in scope depends on its refrigerant charge.
| F gas in the system (CO2 equivalent) | Minimum leak check interval |
|---|---|
| 5 to 50 tonnes | Every 12 months |
| 50 to 500 tonnes | Every 6 months |
| 500 tonnes or more | Every 3 months; automatic leak detection is mandatory at this size |
A leak that is found must be repaired as soon as possible and re-tested within a month. Intervals can double where automatic leak detection is fitted. Missing checks or leaving leaks unrepaired can lead to a civil penalty.
This matters during a breakdown. A system that has lost charge has a leak, so the engineer's job is to find and repair it rather than only recharge the system.
What information helps an engineer fix it first time?
An engineer who arrives knowing the system and the symptom is more likely to carry the right parts. Engineers usually ask for:
- Manufacturer, model and approximate age, from the unit data plate or the asset register
- System type: split, multi-split, cassette, ducted or VRF/VRV
- Which areas and how many indoor units are affected
- The fault code and a photograph of the controller display
- What happened and when, including any power cut, alarm test or building work
- The date of the last service and last F gas leak check
- Access details: roof access, permits, keys, parking and site induction requirements
- How critical the space is, so the call can be prioritised
How do you stop it happening again?
Many breakdowns involve conditions that routine maintenance is designed to catch: dirty filters and coils, blocked drains, slow refrigerant loss and worn electrical components.
Three habits reduce repeat failures.
- Keep a fault log. Record every breakdown by unit, date, code and cause. A unit that fails twice in a season is telling you something.
- Service to a schedule. Regular commercial air conditioning servicing covers the items above and keeps F gas leak checks in date. This facilities manager's guide to air conditioning maintenance sets out what a visit should include.
- Review the pattern each year. Rising call-outs on older equipment change the balance between repairing and replacing. The cost comparison is covered in planned maintenance vs reactive repairs.
A breakdown handled well is short: a clear report and the right engineer on site. The work that prevents the next one happens between call-outs.
Frequently asked questions
Protection devices stop the system when they detect conditions outside safe limits, such as high or low refrigerant pressure, overheating, a blocked condensate drain or an electrical fault. The fault code shows which protection operated. Repeated lockouts point to an underlying fault that a reset does not fix, and on some systems repeated resets clear the fault history an engineer needs.
No. Running a unit in cooling mode keeps producing condensate, and water near light fittings, sockets or IT equipment is a safety risk as well as a damage risk.
Not on its own. A system that has lost refrigerant has a leak. GOV.UK guidance requires leaks to be repaired as soon as possible and re-tested within a month, and the work must be done by a qualified engineer.
That is usually a defrost cycle. Reverse-cycle systems pause heating briefly to clear ice from the outdoor coil, then resume. Defrosts that are frequent or last a long time are worth reporting.
On multi-split and VRF/VRV systems, a single indoor unit can stop on its own fault, most often a blocked drain, a failed fan or a local controller setting. The unit's address or location and its fault code help the engineer, as the rest of the system may be unaffected.
No. The HSE states there is no maximum workplace temperature, but employers must provide a reasonable indoor temperature and assess heat as a risk. The minimum should normally be at least 16°C, or 13°C where the work involves rigorous physical effort.
It depends on the system, how hard it works and the manufacturer's requirements, so there is no single figure. F gas leak checks have their own legal minimum intervals, from every 12 months to every 3 months for systems holding 5 tonnes of CO2 equivalent or more.
The system make and type, the fault code, which areas are affected, what happened beforehand, and how to get access to the plant. A photograph of the controller display and the unit data plate covers most of it.
