Air conditioning
Air conditioner running but not blowing cold air?
A system that will not start is a simple problem. The one nobody expects runs all afternoon, sounds completely normal and leaves the room at 29 degrees, usually on the first properly hot day of the year. Six common faults cause it and you can rule out two of them yourself.
6 min read
Before anything else
Why a system can run and cool nothing
An air conditioner is two machines sharing a box. A fan moves room air across a coil and a refrigeration circuit makes that coil cold. The fan is what you hear.
When the refrigeration side stops working, the fan carries on as before. Air comes out of the vents at the same volume with the same noise, just at room temperature. The sound of a working fan tells you almost nothing about whether the system is healthy.
If air is coming out at all, the unit has power, so the problem sits somewhere on the cooling side.
The six usual causes
These turn up on our residential call-outs in about this order.
| Cause | How it presents | What it takes to fix |
|---|---|---|
| Choked filters | Weak airflow that crept up over a season or two, poor cooling and now and then a frosted indoor coil. | You, in ten minutes, with a vacuum and a tap. |
| Blocked or crowded outdoor unit | Cools well enough in mild weather and gives up on hot days. The outdoor unit runs hot with leaves or a shrub against it. | You, mostly. Clear a metre of space on every side. |
| Low refrigerant from a leak | A gradual decline over months with ice forming on the thin copper pipe at the outdoor unit. | A licensed technician who finds the leak, repairs it and recharges the system. |
| Dirty indoor or outdoor coil | Airflow is fine and the filters are clean but the room still will not come down. Common on coastal installs. | A technician with a proper chemical clean instead of a spray can. |
| Compressor or capacitor fault | The outdoor fan spins but the unit never gets going. Sometimes a hum and a click, then nothing. It often trips the breaker. | A technician. A failed capacitor is a small job but a failed compressor means deciding whether to repair or replace the system. |
| Reversing valve stuck | A reverse-cycle system blows warm in cooling mode or cold in heating mode while everything else seems normal. | A technician. It is a component fault, not a setting. |
The ten minute check
This rules out the two causes you can deal with yourself and gives you something useful to say on the phone if you cannot.
- 1
Feel the air at the vent
Room temperature air means the cooling side is not working. Cold air but not much of it means an airflow problem, which is usually the easier fault.
- 2
Pull the filters
A filter you cannot see light through will strangle the system by itself. Wash them, dry them properly and put them back.
- 3
Look at the outdoor unit
Is the fan spinning? Is there a metre of clear air around it? Leaves, grass clippings, a trellis or a bin packed against the fins stop it getting rid of heat, which is its whole job.
- 4
Check the copper pipes at the outdoor unit
Two copper lines run into it, usually insulated. Ice or heavy frost on either one means stop, because running the system in that state risks the compressor.
- 5
Look at the switchboard
A tripped breaker or a switched-off isolator explains a lot. Reset it once. If it trips again straight away, leave it off and call someone.
- 6
Give it an hour with clean filters
If the filters were the problem, the room will be noticeably cooler within an hour or so of normal running. No change means the fault is further in.
Switch it off now if you see any of this
Does it need a regas?
When a system stops cooling, someone will usually suggest a regas. It is rarely the right fix.
Refrigerant is not fuel and the system does not use it up. A sealed system with a correct charge ten years ago should hold the same charge today. A low charge means it has leaked out through a joint that vibration has worked loose, a pinhole in a coil or a flare that was never quite right.
That makes the leak the fault. A top-up without a leak search puts you back where you started within a season and releases more refrigerant into the atmosphere on the way. Handling refrigerant takes a licence through the Australian Refrigeration Council. Standby holds refrigerant trading authorisation AU43504. Letting refrigerant escape on purpose is against the law.
A leak search takes longer than a top-up. What it usually finds is a single loose joint or a pinhole. Fixing that is what keeps the system cooling.
Why coastal systems fade earlier
The outdoor coil dumps heat into passing air, which only works if air can get through the fins. Along the Illawarra coast salt corrodes aluminium fins and steel casings faster than it does a few kilometres inland. The corrosion swells the fins until they close the gaps between them and strangle the airflow.
The system looks and sounds fine while losing capacity for years, then falls over in the first week of February. A proper coil clean recovers most of it.
If you are on the coast, book a service through air conditioning repair in Wollongong before summer instead of waiting for the day it gives up. If you have not cleaned the filters recently, start there. It costs nothing and fixes a real share of these.
Common questions
- Why is my air conditioner running but not cooling?
- The fan and the cooling side are separate systems and the fan is the part you can hear. If the compressor has stopped, the refrigerant has leaked or the coil is too dirty or iced to move heat, the fan keeps blowing room-temperature air and the unit sounds normal. Check the filters and the outdoor unit first.
- Why is my air conditioner blowing warm air?
- First check the mode and the setpoint. Heating or auto instead of cool is a common cause, as is a setpoint above room temperature. Past that, warm air points at a refrigerant leak, a compressor that will not start or a reversing valve stuck in the heating position.
- What is the 3 minute rule for air conditioners?
- Most systems wait around three minutes before restarting the compressor after it stops. Starting a compressor against leftover pressure is hard on it, so the controller waits for the pressures to even out. If you switch a unit off and straight back on and get only the fan for a few minutes, the protection is doing its job.
- Should I turn my air conditioner off if it is not cooling?
- Yes, if you can see ice on the pipework or indoor coil, if it keeps tripping the breaker or if there is a burning smell. Running an iced system sends liquid refrigerant back to the compressor and damages it. Otherwise it is safe to leave it off while you wait, which also gives any ice time to melt.