AC Running But Not Cooling — The Real Causes, In Order

First: measure the split yourself

Before you think about gas or compressors, one simple test tells you whether the unit is actually cooling. Hold a thermometer at the return air intake (where the unit pulls room air in), then at the supply vent where cold air comes out. Let the AC run fifteen minutes before you measure.

The normal difference between those two readings is 8 to 12°C. If the gap is 5°C or less, the unit is not cooling and there is a genuine fault. If the gap is a healthy 10°C and you are still hot, the unit is working — but its capacity is undersized for the room, or heat is leaking in through windows and doors.

This split is the first thing we measure when we arrive. Anyone who tells you it "needs gas" before measuring anything is guessing.

Cause one: the filter and evaporator are dirty

In a city like Riyadh this is by far the most common cause, and not close to second. The fine dust that arrives with every storm settles on the filter, passes through it, and packs onto the fins of the indoor evaporator coil.

A blocked filter starves the coil of airflow. Too little air means the coil gets colder than it should until ice forms on it, and that ice blocks the path completely. The result is exactly what you describe: the fan spins and air comes out, but that air never actually passed over a cold surface.

The tell-tale sign is that the unit cools well for the first hour after you switch it on, then fades. Turn it off for two hours, switch it back on, and it cools again — because the ice melted.

• Check the filter yourself: open the front cover and slide it out. If you cannot see light through it, it is blocked.
• Washing the filter solves only part of the problem. The layer packed onto the evaporator fins needs pressure or steam cleaning.
• If you see ice on the copper lines or on the coil, switch the unit off and let it thaw before running it again — running the compressor with ice present damages it.

Cause two: the outdoor unit cannot dump its heat

An AC does not create cold. It moves heat from inside the house to outside it. The outdoor unit is what throws that heat away. If it cannot, cooling stops.

We see this constantly in Riyadh villas and apartments: the unit sits in a narrow light-well between two walls, or behind a closed decorative panel, or draped with cloth to shield it from the sun. The hot air it discharges gets pulled straight back in, its own temperature climbs, and the system cuts out on high-pressure protection.

The pattern here matters: the AC cools beautifully at night and in the morning, and fails only between midday and late afternoon. That pattern means a heat-rejection problem, not low gas.

• Leave at least 30cm clear behind the outdoor unit and a metre in front of it.
• Never cover the outdoor unit while it is running, and do not stack things on top of it.
• Condenser fins clog with dust and lint fast in Riyadh. Washing them restores cooling in many cases with no parts at all.

Cause three: low refrigerant — which means a leak

We need to be blunt here, because this is where customers get taken advantage of most. A refrigeration circuit is a sealed loop. An AC does not burn refrigerant or consume it the way an engine consumes fuel. If the gas is low, there is somewhere it is getting out.

Topping up without finding the leak means paying again in two or three months, and again after that. We find the leak first — with nitrogen pressure, an electronic detector, or dye — then repair it, then charge the correct amount by weight according to the unit’s data plate.

Signs of low charge: ice on the thin copper line at the outdoor unit, a bubbling or hissing sound from the indoor unit, and weak cooling that stays weak all day rather than improving at night.

Cause four: run capacitor or compressor

If the indoor fan runs but the outdoor unit is completely silent, or hums and then stops, the run capacitor has most likely failed. It is a small, relatively inexpensive part, and its service life is cut short considerably by Riyadh heat.

If the compressor itself runs and gets hot but pressure never builds, it has lost efficiency internally. That turns into a repair-or-replace decision that depends on the age of the unit and the condition of the rest of the circuit, and we explain both options before starting anything.

What separates the two: a failed capacitor produces an audible start attempt every few minutes. A weak compressor runs continuously and quietly, but achieves nothing.

Cause five: the unit is undersized, or the room leaks heat

Sometimes there is no fault at all. The AC is working at full capacity and is simply smaller than the room’s heat load. This is common in rooms with an exposed west-facing wall, in large majlis rooms, and in rooms added later without insulation.

We also find houses where the unit is perfectly healthy but the room door opens onto an uncovered stairwell, or the window is single glazing, or an extractor fan pulls cooled air outside all day.

If the intake-to-outlet difference is 10°C or more, the machine is doing its job. The problem is heat load, not the unit, and the answer lies in capacity or insulation rather than repair.

How we approach this call

When you call us with a "runs but does not cool" fault, the technician measures before opening anything: the intake-to-outlet split, circuit pressure at the service port, compressor draw in amps, and the visible condition of the filter, evaporator and condenser.

Then we tell you the numbers we found and what they mean, and agree the work with you before we start. We do not open the circuit or add gas and inform you afterwards. And we stand behind the work we hand over.

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