How to Diagnose a Refrigerant Leak

Symptoms That Point to Low Refrigerant

Before you pull out the manifold gauges, the system itself will often tell you something is wrong. These symptoms together are a strong indicator of low charge:

The most obvious sign is warm or barely-cool air from the supply vents even though the system is running. The compressor is working but there's not enough refrigerant to transfer heat effectively.

Ice forming on the suction line or the evaporator coil is another tell. Low refrigerant causes suction pressure to drop, which lowers the evaporator temperature below freezing. You'll sometimes see frost or a solid block of ice on the copper line coming out of the air handler.

Longer-than-normal run times are common — the system struggles to hit setpoint. High electricity bills follow. The customer may have noticed this over the past season without connecting it to a refrigerant issue.

Gauge Readings That Confirm a Leak

Connect your manifold gauges to confirm what you're dealing with. For R-410A systems in cooling mode at typical ambient temperatures (25–30°C outdoor, 21°C indoor), you're looking for:

Finding the Leak

Never recharge a system without finding the leak. There are three primary methods, and the best techs use them in order:

1. 1
   Electronic Leak Detector

   Sweep slowly around all joints, flare connections, schrader valve cores, service valves, and the coils. Move the probe tip at about 2–3 cm/sec. False positives happen near oils and cleaners — verify any hit before calling it confirmed.
2. 2
   UV Dye Inspection

   If UV dye was previously injected, scan with a UV light. Look for yellow-green staining at joints and on the coils. If no dye is present, inject one OEM-compatible cartridge, run the system for 15 minutes to circulate, then inspect.
3. 3
   Nitrogen Pressure Test

   The most reliable method. Recover all refrigerant first. Pressurize the system to 150–200 PSI with dry nitrogen. Apply soap bubbles to all connections and watch for bubbling. Hold pressure for 30 minutes — if it holds, no major leak. Document pressure at start and end.

Common Leak Locations

Experience tells you where to look first. Save time by starting with the most likely spots:

Schrader valve cores are the #1 cause of slow leaks. They vibrate loose over time. Test by applying a small soap bubble to the port cap — if it grows, replace the core.

Flare connections at the line set terminations are a common failure point, especially on older installs or where the line set was disturbed. Look for oil staining at the fitting.

Evaporator coils in residential equipment fail frequently — especially aluminum coils in corrosive environments (near the ocean, near pools). Formicary corrosion creates tiny pinholes that are almost impossible to find without pressure testing.

Condenser coils take mechanical damage from debris, hail, and lawnmowers. Inspect carefully at the bottom where damage is most common.

Service valves at the unit (both the suction and liquid valves) can develop leaks at the valve stem packing, especially on older valves or if someone has been cranking on them.

After Finding the Leak — The Right Fix

Once you've confirmed and located the leak, here's the correct sequence every time:

1. 1
   Recover all refrigerant

   Required by law in Canada before opening the system. Document recovery weight on your service ticket.
2. 2
   Repair the leak

   For flare connections: re-flare or replace the fitting. For coils: seal with approved HVAC epoxy or recommend coil replacement depending on severity.
3. 3
   Pressure test the repair

   Re-pressurize with nitrogen to 200 PSI. Hold for 30+ minutes. Verify the repair holds before proceeding.
4. 4
   Pull vacuum

   Target 500 microns or lower. Hold vacuum for 30 minutes after closing the manifold valves — any rise indicates moisture or remaining leaks.
5. 5
   Recharge by weight

   Charge to the nameplate specification in pounds or kilograms — not by pressure alone. Verify superheat and subcooling after charge.