Quick answer
Commercial HVAC usually stops cooling properly because of control settings, blocked filters, dirty or obstructed outdoor coils, failed fans, a refrigerant leak, or a fault in central plant such as a chiller. Facilities teams can safely check modes, setpoints, schedules, fault codes and whether outdoor units are clear. Anything involving panels, electrics or refrigerant needs an engineer.
Start with what is affected
The pattern of the problem points to its cause:
| Pattern | Likely area to investigate |
|---|---|
| One room or one indoor unit | That unit’s controller, filter, fan, valve or sensor |
| One floor or zone | The VRF outdoor unit, branch controller, AHU or zone controls |
| The whole building | Central plant (chiller, pumps), BMS schedules or electrical supply |
| Only on hot afternoons | Capacity limit, dirty condensers, refrigerant charge |
| After a fit-out or re-plan | Zoning, sensor location, balancing and controller settings |
Common causes
Control settings
Before anything else, check the mode, setpoint and time schedule. Units left in heating or fan-only mode, setpoints changed by occupants, central controllers locking out local controls, and BMS schedules that no longer match occupancy are all frequent causes.
Blocked filters and coils
Blocked filters reduce airflow, so the space gets less cooling even when the refrigerant system is working. Dirty indoor coils have a similar effect. Dirty outdoor coils reduce the system’s ability to reject heat and can cause high-pressure trips on hot days.
Fans and electrical faults
Failed indoor fan motors, outdoor fan motors or capacitors stop heat transfer. Tripped protective devices may stop units altogether.
Refrigerant leaks
Gradual loss of cooling over weeks, iced pipework or coils, oily residue at joints or hissing all suggest a leak. Leaks must be located and repaired by F-gas certified engineers before recharging.
Central plant problems
In chilled water systems, a chiller fault, failed pump, closed valve or air in the system can starve AHUs and fan coils of chilled water. Look for alarms on the chiller and BMS.
Safe checks for facilities teams
- Confirm the controller mode and setpoint, and whether a central controller has locked them
- Note any fault codes on controllers or alarms on the BMS
- Check whether other areas served by the same system are affected
- Check outdoor units are not obstructed by stored items, debris or vegetation
- Check that grilles and diffusers are not blocked by furniture or partitions
- Record when the problem started and whether it is getting worse
Do not open panels, reset breakers repeatedly or touch refrigerant pipework. Inside HVAC units there are live electrical parts, rotating fans and pressurised refrigerant.
When to call an engineer
Call when the simple checks do not explain the problem, when fault codes are present, when there is water leaking or ice forming, or when cooling has failed in a critical space. For server rooms or other critical areas, use our 24/7 contact line on 020 7998 9005. See emergency HVAC support.
Preventing repeat failures
Most cooling failures are preventable. Pre-summer servicing, regular filter checks and keeping condensers clean make a big difference. See commercial air conditioning services and HVAC repairs.
Frequently asked questions
Why is the air conditioning running but blowing warm air?
Common reasons are a unit in fan-only or heating mode, a VRF system with indoor units demanding heating and cooling at once, a failed outdoor unit or compressor, low refrigerant charge from a leak, or dirty outdoor coils causing high-pressure trips.
Can low refrigerant just be topped up?
Low refrigerant almost always means a leak. Under the F-gas rules the leak should be found and repaired before recharging, and only F-gas certified engineers can do this work.
Why does cooling fail on the hottest days?
Systems close to their capacity limit lose cooling first in heatwaves. Dirty condensers, restricted airflow round outdoor units and refrigerant problems make it worse. Pre-summer servicing reduces the risk.
UK guidance and references
Primary and industry sources relevant to this subject. Always check the current edition and how it applies to your building.
- F gas in refrigeration and air conditioning GOV.UK
- Workplace temperatures Health and Safety Executive
This guide is general information for commercial building operators. It does not replace an assessment of your building by a competent engineer. If there is an immediate risk to people, contact the emergency services first.