Most "not cold" calls in Bangkok end with a wash bag and a photo of a black coil. A dirty evaporator can't move heat, so the unit runs and runs and the room never gets there. That is why a clean comes first.
But sometimes the unit was cleaned last week and it is still blowing warm. Then the coil is not the problem, and it is worth knowing what is before anyone says the word "gas".
First, a one-minute test
Set the unit to 18 °C on high fan and wait ten minutes. Hold a thermometer — a kitchen one is fine — in the airflow at the vent. Healthy supply air is 10–14 °C colder than the room. If the room is 28 °C and the vent reads 26 °C, the unit is not cooling. If it reads 16 °C but the room won't come down, the unit is cooling and the problem is elsewhere: an undersized unit, an open window, a west-facing wall at 4 pm.
Then go and look at the outdoor unit.
1. The compressor isn't running
Stand next to the outdoor unit while the indoor unit is calling for cold. You should hear the compressor hum and feel warm air blowing out of the condenser fan. If the fan spins but the air is not warm, or if nothing is happening at all, the compressor is not working.
The most common cause on a Bangkok unit over five years old is a failed capacitor — the cylinder that gives the compressor and fan motor their starting kick. It fails in the heat, it costs ฿1,500–2,500 to replace, and the unit is cold again within an hour. Less often it is a contactor, a control board, or the compressor itself.
2. Low refrigerant, which means a leak
Signs: ice on the copper pipe at the outdoor unit or on the indoor coil, a hissing sound, and air that is cool-ish but never cold. Pressure gauges confirm it in two minutes.
The important thing: refrigerant does not get used up. A sealed system holds the same charge for the life of the unit. If it is low, it is escaping through a flare joint, a valve or a pinhole in the coil. A top-up without a leak test buys a few months, then you pay again. A proper refrigerant visit tests, finds, quotes, fixes and then charges by weight. Which gas your unit takes matters too — see R32 vs R410A.
3. The outdoor coil is choked
The indoor coil gets all the attention. The outdoor condenser is where the heat actually leaves the system, and in Bangkok it collects road dust, leaves, pollen and — on balconies — laundry lint. A blocked condenser means high head pressure, a compressor working flat out, and weak cooling indoors.
Look at the fins. If you can't see daylight through them, it needs washing. Also check nothing is parked against it: plant pots, a folded drying rack, boxes. A condenser needs clear air on the intake side and space to blow hot air out.
4. A control fault
Wall units have a thermistor that measures room temperature and one on the coil. If either drifts or fails, the unit may decide the room is already cool and idle the compressor. Symptoms are inconsistent — cold for a while, then warm, then cold again — and often an error code on the display. This is a board-and-sensor job, ฿1,500–3,500 depending on the part.
5. It was never big enough
If the unit cools properly at the vent but the room stays warm, check the maths. A rough Bangkok rule is 600–700 BTU per square metre for a normal room, more for a west-facing room with big windows or a kitchen. A 9,000 BTU unit in a 22 m² living room with afternoon sun will run all day and never get there. No repair fixes that; a correctly sized replacement does.
What a repair visit should look like
A CoolRova repair visit starts with a ฿590 diagnostic: gauges on, current measured, capacitor and motors tested, sensors and board checked. You get the fault and the price in writing before any work, and the diagnostic fee is waived if you go ahead. Most capacitor, sensor and drain faults are fixed on the same visit from the van; boards and compressors are sourced and fitted within days, with a 90-day parts and labour warranty.
If someone diagnoses "needs gas" without gauges, or quotes a repair without naming the part, get a second opinion. Not-cold has five causes and only one of them comes in a cylinder.
Frequently asked questions
Why is my AC running but not cooling?
If the indoor fan blows but the air isn't cold, either the compressor outside isn't running (capacitor, contactor or compressor fault), the system is low on refrigerant because of a leak, the outdoor coil is too dirty or blocked to shed heat, or a sensor is telling the unit the room is already cool. A dirty indoor coil also causes it, but that should have been fixed by the clean.
Does my AC need gas after cleaning?
Cleaning does not use or remove refrigerant, so if the gas was fine before it is fine after. A unit that is low on gas is leaking; the right fix is a pressure test to find and repair the leak, then a recharge by weight. Topping up without fixing the leak is temporary.
How cold should the air from an air conditioner be?
Supply air at the vent should be 10–14 °C colder than the room air, so around 12–15 °C in a 26 °C room. If the difference is under 8 °C the unit is not cooling properly.
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Written by the CoolRova Team
We're Bangkok's documented AC crew — everything we write comes from real cleaning, repair and installation visits across condos, homes and short-stay rentals.
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