August 26, 2026 · Cairns Aircon Installers
The system is running. You can hear the fan, air is coming out of the vents, but the room never gets cold. It is the most common call we take through a Cairns summer, and the cause is usually one of eight things. Here is how to work through them, starting with the ones you can check yourself in ten minutes.
1. The Filters Are Blocked
This is the single most common cause, and it is the one you can fix yourself. A clogged filter chokes the airflow so the system cannot move enough air across the coil to cool the room. In the tropics filters load up far faster than the manufacturer’s three-month guidance assumes.
Lift the front cover, slide the filters out and look at them against the light. If you cannot see through the mesh, that is your answer. Wash them in warm water with a little detergent, dry them in the shade, and refit them. Our step-by-step filter guide covers it properly.
2. The Coil Behind The Filters Is Filthy
Clean filters do not mean a clean system. The coil sitting behind them collects a mat of dust and mould that a brush cannot reach, and once it is coated the system simply cannot transfer heat. If your filters are clean but the room still will not cool, and there is a musty smell when it starts up, the coil is the likely culprit. That needs a professional deep clean, not a DIY.
3. The Outdoor Unit Cannot Breathe
The outdoor unit dumps the heat it pulls out of your house. If it cannot shed that heat, nothing inside gets cold. Go and look at it:
- Is there a half-metre of clear space around it, or has a shrub grown into it?
- Are the fins matted with grass clippings, palm fronds, dust or mud splash from the wet?
- Is the fan actually spinning when the system is running?
- Is it sitting in full afternoon sun with no airflow, boxed in by a fence or a deck?
Clearing debris and cutting back growth costs nothing and fixes a surprising number of these calls.
4. It Is Set To The Wrong Mode
Worth ruling out before you call anyone. On fan mode the unit circulates air without cooling at all. On dry mode it prioritises removing humidity and cools only gently. Both feel like a system that is running but not working. Check the controller is on cool, the set point is genuinely below room temperature, and no timer or schedule is fighting you.
5. The System Is Undersized For The Room
If it has never really cooled the space properly, this is often why. A unit sized off floor area alone, without accounting for ceiling height, window area, orientation and insulation, will run flat out all afternoon and never reach the set point. In Cairns that shows up worst on west-facing living areas through the build-up.
There is no repair for this one. Either you accept the performance, improve the room (insulation, window shading), or you replace the unit with a correctly sized one and we do a proper heat load calculation first.
6. Low Refrigerant, Which Means A Leak
Refrigerant is not consumed. It circulates in a sealed loop, so if the charge is low there is a leak somewhere. Common signs are ice on the pipework or indoor unit, a hissing or bubbling noise, and cooling that has faded gradually over a season or two rather than failing suddenly.
This needs a licensed technician. Topping up without finding the leak is money burned, and handling refrigerant without a licence is illegal in Australia. On coastal Cairns homes, corrosion on the outdoor coil is a very common source of these leaks.
7. A Failing Capacitor Or Compressor
If the outdoor fan runs but the compressor does not, or the system trips the breaker after a few minutes, you are usually looking at an electrical component or the compressor itself. A capacitor is a cheap, quick fix. A failed compressor on an older unit usually means it is time to talk about replacement rather than repair.
8. It Is Just Very Humid
Sometimes the system is fine and the weather is the problem. Through the build-up, a unit can hold a room at 24 degrees and it still feels sticky, because the humidity has not come down. If the thermometer says the room is cool but it does not feel cool, try dry mode overnight instead of cranking the temperature down. Our wet season guide goes into this properly.
Quick Checklist Before You Call
- Filters washed and dry
- Outdoor unit clear, fan spinning
- Controller on cool, set below room temperature
- No ice anywhere on the pipes or indoor unit
- Breaker has not tripped
If you have worked through all five and it still will not cool, it is a job for a technician. See our repairs service, or get in touch and describe the symptoms. We can often tell you over the phone whether it is something simple before you pay for a call-out.