Every summer, around mid-June, our phones start ringing with the same call: “The AC’s running but the office is roasting.” It happened 40-odd times last July alone.
The thing is, when a single wall-mounted split stops cooling, it’s usually a simple fix. But when you’re managing an office building with a Daikin VRV system running 30 ceiling cassettes off two outdoor units, or a restaurant with a ducted Mitsubishi City Multi setup — and the whole thing stops pulling its weight — the diagnosis is different, the stakes are higher, and you need someone who actually understands commercial systems.
First Question: One Zone or the Whole Building?
This is the most important thing you can figure out before calling us. It tells the engineer whether they’re looking at a single indoor unit fault or a system-level problem that affects the entire refrigerant circuit.
One Zone Down, Everything Else Fine
If one ceiling cassette is blowing warm while the rest of the floor is cold, the fault is almost certainly on that individual indoor unit. The usual suspects:
Blocked filters. Ceiling cassettes in commercial spaces swallow dust. In restaurants near the kitchen extraction, it’s worse — cooking grease gets pulled into the return air and coats the filter media. A clogged filter chokes the airflow across the evaporator coil, and the unit can’t cool even though the refrigerant circuit is working fine. We’ve pulled filters out of ceiling cassettes in City restaurants that looked like they’d been dipped in cooking oil. Change them quarterly at minimum.
Failed fan motor. The indoor fan pulls air across the evaporator coil. If it dies, the unit looks like it’s responding to commands from the controller, but nothing’s actually moving. On ceiling cassettes this is hard to spot because you can’t see the fan from below — you just notice the room getting warm.
Stuck expansion valve. On VRF systems, each indoor unit has its own electronic expansion valve (EEV) that meters refrigerant flow. If it sticks closed, that unit gets no refrigerant and can’t cool. The rest of the system carries on as normal. This is one of the most common single-zone faults on Daikin VRV and Mitsubishi City Multi. You won’t diagnose this one without a refrigeration engineer and a set of gauges.
Wiring or comms fault. VRF systems use a communication bus between indoor and outdoor units. A damaged cable, a dodgy PCB, or an addressing error on one unit can stop it from talking to the system. After office refurbishments — ceiling tiles moved, data cabling run, partitions shifted — we regularly find that someone’s nicked through a comms wire.
Whole Building or Multiple Zones Down
This is the urgent one. If several indoor units or the entire system is blowing warm, the fault is almost certainly on the outdoor unit or the shared refrigerant circuit.
Outdoor unit trip. The outdoor unit houses the compressor and condenser. If it’s tripped on a safety protection or locked out on an error code, every indoor unit connected to it loses cooling. On a VRF system serving an entire office floor, that’s 10, 20, sometimes 30+ units going warm simultaneously. Check if the outdoor unit’s fan is running — if it’s silent, that’s your answer.
Refrigerant leak. A gradual leak shows up as a system that copes on mild days but can’t keep up when it’s 30°C outside. Over weeks, performance degrades until the system trips on low-pressure protection. Under F-Gas regs, we have to find and fix the leak before recharging. No shortcuts.
Condenser blockage. The outdoor unit needs clear airflow to reject heat. Rooftop units are usually fine. Ground-level or balcony units in London? They get buried in leaves, litter, and whatever the wind blows in. We attended a Mayfair office last year where someone had stacked delivery boxes against the outdoor unit. Head pressure through the roof.
Inverter board failure. Modern commercial systems use inverter-driven compressors. If the inverter board goes, the compressor shuts down entirely. On VRF units with twin compressors, you’ll get reduced capacity rather than a total loss — every zone runs lukewarm instead of cold.
VRF Systems: A Different Animal
VRF (or VRV if it’s Daikin) dominates London’s commercial buildings. It’s the right technology for the job — efficient, flexible, simultaneous heating and cooling across different zones. But the complexity means you need a contractor who works on these systems daily, not someone who usually does domestic installs and had a go.
A VRF system might have 40 indoor units, a bank of outdoor units, electronic expansion valves on every head, oil management systems across long pipe runs, and a communication network tying it all together. When something goes wrong, the outdoor unit displays an error code — but reading that code and tracing the actual root cause requires VRF-specific training and diagnostic software.
We’re a Daikin D1 partner, which means direct access to Daikin’s technical support line, their diagnostic software, and their parts supply chain. We also work on Mitsubishi City Multi, Fujitsu Airstage, Samsung DVM, and Carrier systems across London’s commercial stock every week.
Ceiling Cassettes and Ducted Systems
Ceiling cassettes are the workhorse of commercial AC in offices and retail. Recessed into the suspended ceiling, four-way air distribution, out of sight. When one stops cooling, it’s usually filters, condensate drain (more on that below), or the fan motor.
One thing people miss with ducted systems: the unit itself might be working perfectly, but the ductwork is the problem. Damaged or disconnected ducts behind the ceiling, closed fire dampers that nobody reopened after testing, or insulation that’s degraded so the cooled air warms up before it reaches the room. An engineer who only checks the mechanical unit and ignores the distribution side will miss this.
What to Tell Us When You Call
This genuinely saves time and often means a first-visit fix:
- How many zones are affected — one cassette, one floor, or the whole building?
- Is air coming out of the vents? Warm air = refrigerant or compressor issue. No air = fan or power issue.
- Any error codes on the wall controllers? Take a photo. Saves 15 minutes of diagnosis.
- Is the outdoor unit running? Fan spinning? Compressor humming?
- Sudden or gradual? Dead this morning = component failure. Getting worse all week = leak or blockage.
- Any recent building work? Electrical changes, ceiling work, BMS updates — all can affect the AC.
Maintenance Stops This Happening
We say this every time because it’s true every time: the majority of these callouts are preventable. Dirty filters, blocked condensers, low refrigerant, failing fan bearings — all detectable during a routine service visit, all fixable before they take a zone offline.
On multi-unit VRF systems, maintenance is even more critical because one undetected fault — a slow leak, a failing EEV — can cascade into a system-wide problem that costs five times more to fix.
AC Down? Give Us a Call
If your building’s overheating and the AC isn’t playing ball, call us on 020 3974 1419. We work on VRF, multi-split, ducted, and cassette systems across offices, restaurants, hotels, retail, and server rooms all over London.