Why does my aircon lose cooling weeks after a gas top-up?
Gas does not get used up. If cooling fades again after every top-up, the refrigerant is going somewhere, or it was never the fault. Repeat top-ups treat the pressure and leave the cause running.
By Team Snowflake | Updated 19 Aug 2026
1. Slow leak at a joint or service port
Cooling that returns fully after each top-up, then fades on roughly the same interval, points to a slow leak the top-up never touched. Joints work loose as the pipe expands and contracts over years. A worn service port core can also let gas slip past every time gauges are connected.
How to tell
This path shows itself at an accessible joint. Unlike a concealed leak, there is usually an oil trace or dust patch you can see on the outdoor pipework. Unlike a fouled condenser, the fade interval holds steady rather than tracking the weather.
- Cooling comes back fully after a top-up, then fades on the same rough interval.
- An oily film or dust-caked patch sits at a pipe joint on the outdoor unit.
- The fade happens whatever the weather is doing.
- Nobody has ever shown you where the gas was going.
How we confirm it
We pressure-test the circuit and trace the loss to the joint or port that is passing gas. That point is repaired, then the system is held under pressure to confirm it seals before recharging.
A top-up without a leak test is not a fix. It restores pressure and leaves the leak open, so you pay twice on the same cycle.
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2. Leak in a concealed pipe run
The same repeat pattern, but with clean dry joints everywhere anyone has looked, usually means the loss sits in pipework inside a wall or ceiling. Moisture around buried copper corrodes it from the outside in until a pinhole opens. Losses that small only show up when the hidden section is pressurised on its own.
How to tell
This path hides from visual checks. Unlike an accessible joint leak, there is no oil trace to find outdoors, which is why repeat visits keep clearing the system. Unlike a fouled condenser, the charge really is dropping between visits.
- Every joint anyone has checked looked clean and dry.
- More than one contractor has refilled it and found nothing.
- Cooling holds for weeks after each top-up, then fades the same way.
- The pipe run disappears into a wall or ceiling between the two units.
How we confirm it
We isolate the concealed section and pressurise it separately from the accessible pipework. That localises the loss without opening walls on guesswork, so the repair is quoted against what the test shows.
Do not approve hacking or re-piping before the concealed section has been isolated and pressure-tested. Opening the wrong wall is expensive and does not narrow anything down.
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3. No leak, the condenser cannot reject heat
Cooling that sags on hot afternoons and recovers at night, with the charge testing close to normal, is a heat rejection limit rather than a gas loss. A condenser choked with dust, grease, or salt corrosion still moves some air, just not enough under full load. The top-up appears to help because the weather changes.
How to tell
This path tracks the weather. Unlike either leak path, the refrigerant charge tests close to normal, so nothing is escaping. The symptom follows outdoor heat and load rather than the time elapsed since the last top-up.
- Worst on hot afternoons, noticeably better at night.
- The outdoor coil looks dirty, furred, or crusted with salt.
- The fade does not line up neatly with when the last top-up happened.
- No oil trace anywhere, indoors or outdoors.
How we confirm it
We measure operating pressures against ambient conditions. That separates a charge problem from a heat rejection problem, and the condenser is cleaned or assessed on what the readings show.
Adding gas to a system that is not short of gas overcharges it and makes cooling worse under load. If nobody has inspected the outdoor coil, another top-up misses the actual limit.
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Tell us what’s going on. Symptoms, setup, photos, anything we should know. We’ll assess and come back with the right next step.