Quick answer
The signs of an AC refrigerant leak are ice on the large copper line or the indoor coil, a hissing sound, air from the vents that is only slightly cool, run times that stretch for hours, and oily residue on fittings or service valves. An air conditioner never uses up refrigerant, so a low charge always means a leak, and in Ontario the leak is repaired before recharging.
The signs of an AC refrigerant leak show up as a pattern rather than one symptom: cooling that weakens over weeks, then ice, then a system that runs all afternoon and never reaches the setpoint. Refrigerant runs in a sealed loop and is not consumed like fuel. If the charge is low, it left through a hole, and topping it up without finding that hole only buys time.
Six signs of a refrigerant leak you can spot yourself
- Ice or frost on the suction line, the thicker insulated copper pipe, or on the indoor coil. A low charge drops the coil temperature below freezing and the moisture in the air freezes onto it.
- Supply air that is cool but not cold. A healthy system delivers air roughly 8 to 11 °C colder than the air going into the return; a leaking one feels like a fan.
- Long run times. The AC runs for hours on a day it used to handle in 15-minute cycles, and the hydro bill climbs with it.
- A hiss or faint bubbling at the indoor coil or the outdoor unit while the system runs.
- Oily residue or a dark, dust-collecting stain. Compressor oil circulates with the refrigerant and comes out with it, so a leak leaves a greasy spot at the joint.
- Higher indoor humidity. A starved coil stops pulling moisture out of the air well before it stops cooling altogether.
None of these alone proves a leak. A plugged filter or a failed blower also ices a coil, which is why the AC freezing guide starts with airflow. Ice that comes back after a new filter and a complete thaw is the pattern that points to refrigerant.
Where residential air conditioners leak
Leaks follow the joints and the thin metal. On a central AC the usual places are the Schrader valve cores and caps on the outdoor service valves, the brazed joints where the line set meets the unit, and the evaporator coil above the furnace. Evaporator coils with copper tubing can develop pinhole leaks from formicary corrosion, a slow chemical attack that leaves holes too small to see. On ductless mini-splits the flare joints behind the indoor head and at the outdoor unit are the first suspects, especially when a flare was over-tightened or under-tightened at install.
Line sets get damaged too: a rub point where the copper touches a joist or the cabinet, or a kink from a condenser that settled on its pad. A leak that shows oil only at the service valves is often a small repair. A leaking evaporator coil usually means replacing the coil.
How a technician confirms a refrigerant leak
- Superheat and subcooling readings. Gauges and pipe-temperature clamps show whether the system is undercharged or whether the symptom is airflow or a restriction. This comes first, because adding refrigerant to a system that is not low causes its own damage.
- Electronic leak detector. A handheld sniffer is passed along every joint, the service valves and the coil, and it sounds when it picks up refrigerant.
- Nitrogen pressure test. The refrigerant is recovered, the system is pressurised with dry nitrogen, and the gauge is watched for a drop. Soap solution on suspect joints bubbles at the hole.
- UV dye. For slow leaks that hide, dye is added to the system and the leak shows under a UV lamp on a later visit.
The Ontario rule: repair the leak first, then recharge
Ontario regulates refrigerants under the Environmental Protection Act. Only a technician holding an Ozone Depletion Prevention (ODP) card may handle refrigerant, releasing it to the air is prohibited, and a system known to be leaking is repaired before it is charged again. A top-up with no leak search is not a service, it is a repeat bill. Ask to see the ODP card; a legitimate technician carries one.
The refrigerant in the unit also shapes the decision. R-22, used in air conditioners made before about 2010, has been phased out and only reclaimed stock is left, so a major leak on an R-22 system usually tips toward replacement. R-410A is still serviceable but is giving way to R-454B and R-32 in new equipment. The three are not interchangeable, and a system is recharged only with the refrigerant named on its rating plate.
What refrigerant leak repair costs
Refrigerant leak repair and recharge runs $300–$800+, on top of a $90–$150 diagnostic that is credited toward the work. Three things move the price:
- Where the leak is. A valve core or an accessible brazed joint is at the low end; a coil buried in the plenum is at the high end or becomes a coil replacement.
- How much refrigerant was lost, and which type. Finding and sealing the leak costs more than the refrigerant itself, though current refrigerants are not cheap.
- The age of the unit. On a system past 12 years with a leaking coil, compare the repair against replacement before approving it.
A leak is rarely an emergency for the people in the house, and whether a refrigerant leak is dangerous is answered separately. It is urgent for the compressor, so turn the AC off if you see ice. ZK Mechanical pressure-tests before recharging and puts the price in writing first: see AC repair or request a visit.