We got called to a solicitor’s office on High Holborn last September. Water was dripping from a ceiling cassette directly onto a partner’s desk — onto his laptop, his papers, and his patience. The ceiling tile above had gone from white to brown over a few weeks, and nobody had flagged it until it started actively raining indoors.
The fix took 20 minutes. Blocked condensate drain, cleared with a wet vac and treated with biocide. But the water damage to two ceiling tiles, a keyboard, and the dignity of a senior partner — that was the expensive part.
AC water leaks are one of the most common calls we get across London’s offices, restaurants, and retail spaces. Almost all of them are preventable.
Why AC Units Produce Water
Quick explanation for anyone who hasn’t thought about this before: when an AC unit cools the air, it also pulls moisture out of it. Warm humid air passes over the cold evaporator coil, the moisture condenses (exactly like a cold beer glass on a summer day), and the water collects in a drain tray underneath. From there, it flows down a drain line to a waste pipe — by gravity if you’re lucky, or via a small condensate pump if the drain point is above the unit.
On a muggy London day in August, a single ceiling cassette can produce several litres of condensate. In a restaurant kitchen where humidity is already through the roof from cooking, it’s even more. All normal. The problem starts when that water doesn’t go where it’s supposed to.
What Goes Wrong
Blocked Drain Line
Number one cause by a distance. The condensate drain is a narrow-bore pipe running from the unit to a waste point. Over months, algae, mould, dust, and general slime build up inside it. The flow slows, then stops. The drain tray fills up, and water spills over the edge — usually straight through the ceiling.
In offices with ceiling cassettes, the first sign is stained ceiling tiles. Then sagging tiles. Then dripping. And if the unit happens to be above a row of desks with monitors and docking stations, you’ve got a facilities manager fielding angry emails by lunchtime.
In restaurants, it’s worse. A leaking AC unit above a food prep area is a food hygiene issue. An EHO inspector will flag it. It’s not a good look.
The fix is simple: flush the drain line, treat it with condensate tablets or biocide to slow regrowth. It’s a standard check on every PPM visit we do. The fact that we see this fault so often tells you how many buildings aren’t on a proper maintenance schedule.
Condensate Pump Failure
Not every unit can drain by gravity. Basement offices, rooms where the nearest drain point is higher than the unit, installations where the pipework route makes gravity drainage impractical — these all need a condensate pump.
These pumps are small, reliable, and forgotten about until they fail. The pump motor burns out, the float switch sticks, or the reservoir gets clogged with the same biological sludge that blocks drain lines. When the pump dies, the water has nowhere to go.
Most commercial installations have a safety float switch that kills the AC unit before the tray overflows. Good for preventing water damage, bad for the building occupants who suddenly lose cooling in the middle of summer. You get a choice: water damage or overheating.
Frozen Evaporator Coil
This one catches people out. The symptom is a water leak, but the actual cause is ice. If the evaporator coil gets too cold — usually because a dirty filter has restricted airflow, or the refrigerant charge is low — ice forms on the coil during operation. When the unit cycles off or conditions change, that ice melts all at once. The drain tray wasn’t designed for that volume of water in one hit, and it overflows.
If someone reports ice or heavy frost on the coil or pipes, turn the system off and let it defrost. Check the filter — if it’s choked, that’s probably your answer. If the filter’s clean and it still ices up, the system likely has a refrigerant leak. That needs an F-Gas engineer with gauges and a leak detector.
Cracked Drain Tray
Less common but worth knowing about on older units — 10+ years in service. The tray can crack, corrode (metal trays in humid environments), or shift so it’s no longer level. A tilted tray pools water at one end and overflows before it reaches the drain outlet.
What It Actually Costs When AC Leaks in a Commercial Building
The repair itself is usually cheap — a drain flush, a pump replacement, a filter change. But the collateral damage is where the bill lands:
- Ceiling tiles — stained or waterlogged tiles in a client-facing office or meeting room need replacing. Not expensive per tile, but the disruption and appearance matter.
- IT equipment — water on laptops, monitors, servers, network switches. A single soaked network switch in a comms room can bring down connectivity for an entire floor.
- Tenant complaints — in managed buildings, a persistent water leak from the landlord’s AC system is the kind of issue that escalates to formal letters and rent reduction negotiations.
- EHO risk — in restaurants and food environments, water dripping anywhere near food handling areas is a compliance problem.
- Business disruption — desks relocated, meeting rooms out of action, restaurant sections closed while it’s being sorted.
All preventable with twice-yearly maintenance.
How to Stop It Happening
Every one of the faults above gets checked during a proper AC service visit:
- Drain line flushed and treated
- Condensate pump tested and cleaned
- Filters cleaned or replaced
- Drain tray inspected for cracks and correct alignment
- Refrigerant pressures checked to prevent coil icing
We recommend servicing commercial AC at least twice a year — pre-summer and pre-winter. For buildings with a lot of ceiling cassettes in dust-heavy environments (restaurants, retail, older offices), quarterly makes more sense.
A planned maintenance contract covers all of the above and gives you priority response if something does go wrong between visits. Cheaper than one emergency callout with ceiling damage.
Got a Leaking AC Unit?
Don’t leave it. Water damage gets worse, not better, and a dripping unit above expensive equipment or a food area isn’t something to ignore. Call us on 020 3974 1419 and we’ll get it sorted.