AC Not Cooling? A Diagnostic Tree for Commercial Rooftop Units

Your commercial AC is not cooling, a tenant is emailing about it, and a contractor has already quoted a refrigerant top-up. Before you approve it, it’s worth knowing that “no cooling” is a symptom, not a diagnosis. It can come from a thermostat override, a clogged filter, a stuck damper or a real refrigerant leak, and only one of those is fixed by adding refrigerant.

This diagnostic tree is written for property managers who want to follow the logic and for technicians who want to keep it disciplined. It targets the commercial air conditioning systems and rooftop units common across Greater Montréal, though the same reasoning applies to split systems.

AC not cooling: where do you start?

Start with what is simplest and most common, and finish with the refrigerant circuit. Check the call for cooling (setpoint, schedule), then airflow, the economizer, the condenser and the compressor. Only then do you measure superheat and subcooling. A first-visit recharge skips every one of those checks.

Tools you’ll need

• Dual-probe digital thermometer (supply and return air);
• Multimeter with clamp ammeter;
• Digital gauges and pipe-clamp temperature probes (qualified technician only);
• Access to the unit controller or BAS, and to the maintenance log.

⚠ Safety: Lock out the unit before opening an electrical panel or reaching near a fan. On a flat Montréal roof in November, ice and wind are part of the hazard, so nobody goes up alone. Connecting to the refrigerant circuit is reserved for a qualified person.

Step 1: Is the unit actually being asked to cool?

A good share of “it’s not cooling” calls end at the thermostat. Check setpoint, mode (cool, auto, fan only) and the occupancy schedule. In a building run on a BAS, look at the command point itself. A setpoint stuck under a forgotten override, or a schedule nobody updated after a tenant change, looks exactly like a breakdown.

Step 2: Is air moving at the right flow?

A clogged filter, a slipping belt or a dirty evaporator coil cuts airflow. Cooling capacity stays trapped in the unit, and the coil can ice over. The occupant then feels a weak, lukewarm breeze. Measure the temperature split between return and supply air and write it down, because you’ll need it at step 6. If you find ice, let it melt completely before any refrigerant readings. Pressures off a frozen coil are meaningless.

Step 3: Why won’t a rooftop unit cool in the fall?

This is the seasonal question in Montréal. In shoulder season, a rooftop unit cools with outdoor air first, through its economizer. Many controllers also lock out the compressor below an outdoor temperature set at the factory or by the programmer. If the outdoor-air damper stays shut (failed actuator, seized linkage, drifting sensor), nothing takes over. Interior zones, heated year-round by lighting, equipment and people, overheat even at 5 °C outside.

Command the damper in test mode and watch it move. The full procedure is in our guide to checking a rooftop unit economizer.

Step 4: Is the condenser rejecting heat?

The heat pulled from indoors has to be dumped outside. Confirm every condenser fan runs and the coil isn’t packed with June cottonwood fluff, October leaves or old debris. In summer, a dirty condenser drives head pressure up until the high-pressure safety trips. In cold weather the opposite tends to happen: without working head-pressure control, pressure drops and the evaporator can freeze.

Step 5: Does the compressor start?

Read the alarm codes and controller history before resetting anything. A compressor tripping on high or low pressure tells you where to look; resetting it three times without knowing why is how compressors get damaged. Then check the contactor, supply voltage and running amps against the nameplate. A compressor that hums without pumping is possible, but it’s the last hypothesis, not the first.

Step 6: What do superheat and subcooling tell you?

Only now do the gauges go on. You record four values (suction pressure, head pressure, superheat and subcooling) and compare them with manufacturer data. On a TXV system, a common field rule of thumb when no manufacturer data is available is about 10 °F ± 3 °F of subcooling at the condenser outlet. On a fixed orifice, use the manufacturer’s superheat chart. The pattern of all four readings points to the fault; a single number does not:

Look at the third row: a restriction and a shortage of refrigerant both produce high superheat. Only subcooling tells them apart, which is exactly where “charge by feel” goes wrong.

Step 7: Fix the cause, not the symptom

A refrigerant circuit is sealed: if charge is missing, there’s a leak. In Québec, the Regulation respecting halocarbons requires everyone who handles these substances to hold an environmental qualification. It also requires refrigerant recovery during repairs. Find the leak, repair it, pull a vacuum and charge to manufacturer data: that’s the only defensible sequence. Then log the before-and-after readings in the unit record.

Brossard: the recharge that never happened

Early November, a medical clinic in a commercial building in Brossard. The exam rooms in the core of the building hit 27 °C every afternoon while the perimeter offices stay cool. The tenant already had a quote to “add gas” to the rooftop unit serving the zone.

The technician works the tree. The thermostat setpoint is fine and the call for cooling is active. The filter is clean, but the temperature split is small: the unit isn’t cooling anything. On the roof, the compressor is off, locked out by the controller on low ambient, exactly as designed. The surprise is the economizer. The damper stays shut despite the call, because its actuator no longer responds. The unit can cool neither with outdoor air nor with its compressor.

With a new actuator, the exam rooms are back to setpoint within the hour. The refrigerant circuit was never opened. A recharge would have cost a visit and refrigerant, and changed nothing, since the compressor was locked out anyway.

Common mistakes

• Charging before measuring: you mask a leak, or overcharge a system whose real problem was airflow.
• Reading a frozen coil: every pressure is skewed until the ice is gone.
• Ignoring the season: in November, a rooftop compressor that won’t start may be doing exactly what it’s told. Check the economizer.
• Skipping the log: without past readings, you can’t tell whether today’s superheat is drift or normal for that unit.

Before you call: three things to have ready

If a zone in your building has stopped cooling, write down the unit’s model and location, any codes on the controller, and the times and rooms where the heat builds up. Those three details often let the technician show up with the right part. Server rooms and other critical spaces are a different problem; our article on server room cooling covers the redundancy they need.

Montréal Combustion runs this seven-step tree on every cooling call. Managers pay for a fixed cause instead of another top-up, and occupants get comfortable rooms back on the first visit.