Commercial air conditioning not cooling? Ten causes to check first
Updated 23 October 2026 · SEO Dons Editorial
What are the ten causes, in the order to check them?
The order runs from what staff can safely look at to what only a qualified engineer should touch. Manufacturers draw the same line: Daikin's manual says never to inspect or service the unit yourself, but that the end user may clean the air filter, suction grille, air outlet and outside panels.
| # | Cause | Site team can check | Needs a specialist |
|---|---|---|---|
| 1 | Mode and setpoint | Controller mode, setpoint and schedule; restart delays | Controls contractor if settings keep reverting |
| 2 | Zone controller conflicts | Which controller is master; any central override | Installer or controls specialist |
| 3 | Dirty filters | Clean filters and grilles with the unit off | Coil and fan cleaning |
| 4 | Blocked airflow | Obstructions at indoor outlets and around the outdoor unit | Fan and coil work |
| 5 | Outdoor conditions beyond design | Compare conditions with the published operating range | Designer, if the range is often exceeded |
| 6 | Low refrigerant | Note falling capacity, ice or oil staining | F-gas engineer |
| 7 | Failed fans or sensors | Whether fans turn; any fault code | Competent engineer |
| 8 | Iced coils | Note and photograph it; report it | F-gas engineer |
| 9 | Undersizing after a change of use | Compare current heat gains with the design brief | Designer |
| 10 | VRF system-level faults | Record the code, unit and time | Installer or F-gas engineer |
Which causes are quick to rule out?
1. Mode and setpoint. Mitsubishi Electric's service manual tells call handlers to check the set temperature first: the outdoor unit cannot operate if the set temperature is not appropriate, and it runs in cooling only when the set temperature is lower than the room temperature. Delays can look like faults. Daikin says its system starts five minutes after being turned on again, to prevent overloading the compressor motor, and after power is switched on the microcomputer runs a communication check with the indoor units that can take up to 12 minutes, during which the fan can run without cooling.
2. Zone controller conflicts. On a Daikin changeover system only the master user interface can select heating or cooling, and slave interfaces follow it, so a zone asking for cooling while the master is set to heat will not get it. Where "Under Centralised Control" is displayed, the local interface cannot be used. Staff can find out which controller is master and whether a central schedule is overriding; changing the configuration is for the installer.
3. Dirty filters. Mitsubishi says clogged filters reduce the airflow and lower the unit's capacity. Daikin lets users clean them with the unit turned off, with a rule of thumb of every six months and more often where the air is extremely contaminated, and says the user interface can show a "Time to clean filter" notification. Planned visits are on the commercial air conditioning service page.
4. Blocked airflow. Mitsubishi says obstacles at the air intake or outlet of indoor or outdoor units block the airflow and lower capacity. Staff can look for stored goods, vegetation or debris and report anything that needs coil or fan work.
5. Outdoor conditions beyond design. Daikin tells users to run its VRV IV system within stated temperature and humidity ranges, and says that beyond them safety devices may operate and the air conditioner may not operate. The ranges are in the unit's technical data. A system can be healthy and still struggle on the hottest days of the year; a designer can say whether the range is regularly exceeded.
Which causes need an F-gas engineer?
6. Low refrigerant. Mitsubishi's table for "the air does not cool well" lists refrigerant shortage first, then filter and heat exchanger clogging. The leakage-checking regulation lists insufficient cooling capacity among the situations that constitute a presumption of leakage. Mitsubishi's countermeasure column shows how an engineer confirms it: with a leak, discharge temperature rises and the expansion valve opening increases, so the readings are inspected and the pipe connections checked for gas leakage. Adding gas is not the answer: Article 3 of Regulation (EU) No 517/2014 requires a detected leak to be repaired without undue delay. The refrigerant leaks guide covers the duties and how leaks are found.
7. Failed fans or sensors. Daikin's code list has separate entries for indoor and outdoor fan motors, and for thermistors on the indoor heat exchanger, suction air and discharge air. Electrical fault-finding belongs to a competent person: Regulation 16 of the Electricity at Work Regulations 1989 bars anyone from work needing technical knowledge unless they have it or are suitably supervised.
8. Iced coils. Ice formation is also on the regulation's presumption-of-leakage list. Low airflow from clogged filters is a second route, so an engineer separates the two. A unit that ices and then drips is covered in the leaking water guide.
When is the design or the system the problem?
9. Undersizing after a change of use. For example, a meeting room converted into a small comms room adds heat the original design never counted, and no fault is needed to explain the complaint. The sizing guide shows how loads are worked out, and the workplace temperature law guide covers the duties when staff are too warm.
10. VRF system-level faults. Daikin's list includes low-pressure and expansion valve faults, supply-phase faults, wiring faults and "system mismatch". A code is a prompt, not a diagnosis. Daikin asks for the code, the unit type and the serial number from the nameplate to be passed to the installer. The commercial air conditioning repair page explains how faults are diagnosed, and readers who want a contractor to attend can send an enquiry through the site's form.
What changes for server rooms and 24-hour spaces?
Daikin's VRV IV manual says not to use the system for cooling precision instruments, food, plants, animals or works of art, to avoid quality deterioration. A server room on comfort VRF that struggles is therefore a design question as well as a fault question. The server room air conditioning page and the server room cooling requirements guide cover the alternatives.
Sources
- Service manual OCH526, PUHZ-SHW outdoor units (September 2012), Mitsubishi Electric, checked 22 October 2026
- VRV IV RXYLQ10~14T7Y1B installation and operation manual, 4P543426-1D (November 2025), Daikin, checked 22 October 2026
- VRV system air conditioner FXFQ20~125BVEB installation and operation manual, 4P540926-1F (June 2023), Daikin, checked 22 October 2026
- Commission Regulation (EC) No 1516/2007, legislation.gov.uk, checked 22 October 2026
- Regulation (EU) No 517/2014, Article 3, legislation.gov.uk, checked 22 October 2026
- The Electricity at Work Regulations 1989, regulation 16, legislation.gov.uk, checked 22 October 2026
Common questions
Why is there no cooling for several minutes after a power cut?
Daikin says the system waits five minutes before restarting to protect the compressor motor, and after power is switched on the microcomputer can take up to 12 minutes to finish a communication check with the indoor units, with the fan running but no cooling. If cooling has not started after that, record any code and report it.
Will topping up the refrigerant fix a unit that is not cooling?
Only for a short time, and not lawfully as a stand-alone step. A detected leak must be repaired without undue delay, and where the equipment is leak-checked it must be re-checked within one month of the repair. The cause needs finding first.
What should be reported when calling a contractor?
Daikin asks for the complete model name, the manufacturing number on the nameplate, the installation date and the symptoms. Add any fault code shown on the user interface, which unit it came from and when the problem started.
Can a building management system stop a unit cooling?
It can, where it has taken central control. Daikin says that when the user interface shows that it is under centralised control, pressing the operation button only makes the display blink and the local interface cannot be used. Check the central schedule and setpoints before assuming a fault.
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