Commercial Air Conditioning Repair: diagnosing it properly the first time

Faults by symptom — what each one usually means

"It runs but it isn't cooling"

The most common callout, and it has three usual causes. Low refrigerant charge from a leak is the first suspect — the system runs, the compressor works, but there is not enough refrigerant to move the heat. A fouled condenser coil is the second and the most preventable: an outdoor coil blocked with leaves, pollen, dust or kitchen grease cannot reject heat, so the system runs continuously and still loses ground on a hot day. Airflow restriction indoors is the third — blocked filters, a failing fan, or closed-off grilles.

The diagnostic distinction matters commercially, because the three have wildly different costs. A coil clean is routine maintenance. A leak repair on a long pipe run in an occupied building is not.

"It keeps tripping out"

Repeated tripping is the system protecting itself, and the protection that operates tells you a lot. High-pressure trips point to heat rejection problems — fouled condenser, failed condenser fan, restricted airflow, or overcharge. Low-pressure trips point the other way — undercharge, a blocked filter drier, or a failing expansion valve. Electrical trips point at the compressor, the contactor or the supply. Repeatedly resetting a tripping system without diagnosing why is how a recoverable fault becomes a compressor replacement.

"There's water dripping from the ceiling unit"

Almost always condensate, not refrigerant. An indoor unit pulls moisture out of the air and that water has to drain away. Blocked drains, a failed condensate pump, insufficient fall on the drain line, or a dry trap letting air break the flow are the usual causes. It is disruptive and it damages ceilings, but it is generally a cheap fix — and it is worth knowing that a refrigerant leak does not produce dripping water, so this symptom rarely means what people fear it means.

"It's making a noise it didn't used to make"

Worth acting on early. Fan bearing noise, loose panels vibrating, compressor rumble, or the distinctive gurgle of a system running low on refrigerant. Noise is usually the cheapest stage at which to catch a developing fault, and the stage at which it is most often ignored.

"Some rooms are fine, others aren't"

On a multi-zone system this points to controls, balancing or a single failed indoor unit rather than to the central plant. It is also the classic symptom of a system that was never properly commissioned — if it has been uneven since installation, the problem is balancing, not a fault.

Why a regas is not a repair

This deserves its own section because it is the single most common bad practice in the market. Air conditioning runs on a sealed refrigerant circuit. Refrigerant is not a consumable — it is not burned, used up or worn out. If the charge is low, it has escaped, and it will carry on escaping.

The GB F-Gas Regulation is explicit about the consequence: where a leak is detected it must be repaired, and the system must be re-checked within one month of the repair to verify it was effective. Simply topping the system up and leaving is not compliant, and as the operator it is your duty, not the contractor's, that is being breached.

There is a commercial argument too, quite apart from the legal one. Refrigerant costs money, high-GWP gases are getting more expensive as the phase-down bites, and a system that is leaking is also running inefficiently and stressing its compressor the whole time. Repeated top-ups are more expensive than the leak repair almost every time.

Work on F-Gas equipment must be carried out by a certified engineer working for a certified company. Records of leak checks, repairs, verifications and refrigerant quantities have to be kept. Our F-Gas guide covers the duties, and current guidance is on GOV.UK.

How a leak actually gets found

Leak detection is a craft. The methods used, roughly in order of escalation: visual and UV dye inspection, since oil traces at a joint often mark the spot; electronic detectors swept along pipe runs and joints; bubble solution on accessible joints; pressure or vacuum decay testing on an isolated section to prove whether a length of pipework holds; and nitrogen pressure testing with the system isolated where the leak is stubborn.

The awkward reality is that leaks on long concealed pipe runs can take time to find, and time is what you are paying for. A contractor who gives you an honest estimate of the search effort — and a decision point at which it becomes cheaper to replace a pipe run than to keep hunting — is being more useful than one who quotes a flat price and then discovers the problem.

Diagnostics: what a competent engineer measures

Beyond the obvious, a proper diagnosis rests on a few measurements. Superheat and subcooling together indicate whether the charge is correct and whether the expansion device is metering properly — the two figures read as a pair tell you far more than a pressure gauge alone. Suction and discharge pressures converted to saturation temperatures show how the system is behaving against ambient. Air-on and air-off temperatures across the coils show whether the heat exchange is actually happening. Compressor current draw against nameplate indicates mechanical health. And on anything modern, the error and event log in the controller usually contains the history that the person reporting the fault does not have.

If an engineer diagnoses a compressor failure without having taken electrical readings, ask how they reached that conclusion. It is an expensive component to replace on a guess.

Repair or replace? The honest decision

There is no single rule, but a few triggers shift the answer decisively toward replacement.

• R22 systems. R22 has been banned for use in servicing, including recycled gas, since 2015. A significant refrigerant loss on an R22 system is effectively terminal — you cannot legally recharge it. See replacement.
• Obsolete parts. Once a manufacturer stops supporting a range, lead times stretch and prices climb. A three-week wait for a board on a system serving a trading floor is a business problem, not just a maintenance one.
• Repeat compressor failure. A second compressor failure on the same circuit usually means an underlying cause — acid in the system, persistent moisture, chronic undercharge — that a third compressor will meet as well.
• Repair cost approaching a large share of replacement. The usual working rule is that once a repair passes roughly 40–50% of replacement cost on plant already well into its life, replacement is normally better value, particularly given the efficiency gap between a fifteen-year-old fixed-speed system and a modern inverter one.
• An R410A system with a large leak. Not banned, but the phase-down makes recharging progressively more expensive, so the economics shift earlier than they used to.

Set against that, a well-maintained system with an isolated fault and available parts is usually worth repairing — commercial plant regularly runs 15–20 years, and replacement carries disruption as well as capital cost.

Response times and what a callout covers

Response expectations should be agreed in advance rather than negotiated during an outage. Attendance windows are typically tiered — same-day or four-hour for critical plant such as a server room or a kitchen, next-working-day for comfort cooling. What matters as much as the window is what "attendance" includes: whether first-visit diagnosis is covered, whether common parts are carried on the van, and what the escalation path is if the fault needs a return visit.

For genuinely critical spaces the honest answer is that no response time replaces resilience. If a comms room going down stops the business, the fix is N+1 plant, not a faster callout. That is covered on the server and comms room page.

The most effective way to reduce callouts is to stop having them. Planned maintenance catches fouled coils, blocked drains, developing bearing noise and slow leaks before they become failures — see AC servicing and whole-building PPM.

Frequently asked questions

Why does my air conditioning keep needing a regas?

Because it has a leak. Refrigerant runs in a sealed circuit and is not consumed, so a charge that keeps dropping is escaping. Under the F-Gas rules the leak must be found and repaired and the repair verified by a re-check within one month, rather than the system being repeatedly topped up. Repeated top-ups usually cost more than the repair.

How much does commercial air conditioning repair cost?

It depends entirely on the fault. A condensate blockage or a coil clean is a routine visit. A leak repair depends on how long the leak takes to find and how accessible the pipework is. A compressor replacement on a larger system is a significant job and is the point at which replacement should be considered instead.

Water is dripping from my ceiling cassette — is that refrigerant?

Almost certainly not. Refrigerant leaks as gas and does not produce dripping water. Water from an indoor unit is normally condensate, caused by a blocked drain, a failed condensate pump, insufficient fall on the drain line or a dry trap. It is disruptive but generally an inexpensive fix.

Can you still repair an R22 air conditioning system?

You can carry out repairs that do not involve adding refrigerant, but R22 has been banned for use in servicing since 2015, including recycled gas. In practice that means any fault involving significant refrigerant loss ends the system's life, because it cannot legally be recharged.

When is it better to replace rather than repair?

The clear triggers are an R22 system with refrigerant loss, obsolete parts with long lead times, a second compressor failure on the same circuit, or a repair cost approaching roughly 40–50% of replacement on plant already well into its life. A modern inverter system will also typically run more efficiently than the fifteen-year-old fixed-speed unit it replaces.

We repair and maintain commercial air conditioning across the UK. If the plant is at end of life, see replacement; to prevent the next callout, see servicing and TM44 or planned maintenance. Background on the systems themselves is on the commercial air conditioning overview, or request a quote.