Finding Refrigerant Leaks
Pressure test with dry nitrogen, choose a leak detection method and confirm a leak before you repair or recharge a system.
Table of contents
Refrigerant does not wear out, so a system that has lost refrigerant has a leak. Adding more without finding that leak wastes money, lets refrigerant escape and brings the customer's problem right back. Good leak work is slow, orderly and repeatable. This guide covers how to find leaks on a charged system and how to prove a repaired or new system is tight.
What You Will Be Able to Do
- Spot the signs that a system is leaking.
- Pick the right leak detection method for the situation.
- Run a nitrogen pressure test safely.
- Confirm a suspected leak with a second method.
Signs of a Leak
- Oily spots on coils, fittings or service valves, because refrigerant carries oil with it.
- Low pressures with high superheat and low subcooling.
- Ice on the evaporator or suction line.
- A history of repeated top-offs.
- Bubbles in a sight glass on systems that have one.
- A hissing sound at a joint.
Where Leaks Usually Hide
| Location | Why it leaks |
|---|---|
| Schrader cores and service valves | Worn seals, missing caps |
| Flare fittings | Poor flare, wrong torque, vibration |
| Evaporator coils | Corrosion in copper or aluminum, U-bend wear |
| Condenser coils | Vibration, corrosion, damage from outside |
| Line sets | Rubbing on sheet metal, nail or screw damage |
| Brazed joints | Poor heat or poor fit |
| Tubing near the compressor | Vibration cracks |
Choosing a Method
| Method | Works best for | Limits |
|---|---|---|
| Electronic detector | Scanning a charged system, finding a general area | Wind and cleaners can fool it, so it needs a check |
| Bubble solution | Confirming a leak at a joint or fitting | Slow to cover a whole system |
| UV dye and light | Small or intermittent leaks found over time | Needs manufacturer approval, and dye must be compatible |
| Nitrogen pressure test | Proving a new or repaired system, finding larger leaks | The system must have no refrigerant in it |
| Ultrasonic detector | Noisy places, pressurized leaks | Weak on very small leaks |
Use at least two methods when you can. Scan with an electronic detector, then confirm with bubbles. Never trust one alarm by itself.
Using an Electronic Detector
- Check the detector is rated for the refrigerant you are hunting, including A2L refrigerants.
- Turn it on in clean air and let it warm up.
- Shut off fans if possible, since moving air spreads the gas.
- Move the tip slowly, about an inch per second, close to the surface.
- Work from the top down on a piping run or under coils, since refrigerant is heavier than air.
- When it alarms, move away, let it clear, then return slowly and pinpoint the spot.
- Confirm with bubble solution.
The Nitrogen Pressure Test
Use a nitrogen test on a new installation or after a repair, before evacuation.
Warning: Use only dry nitrogen with a pressure regulator. Never use oxygen, which can explode with oil, or compressed air, which adds moisture and can form a flammable mix with oil. Never pressurize above the lowest rated part of the system.
- Make sure the refrigerant has been recovered, or that the factory charge is closed off with the service valves shut.
- Find the test pressure on the data plate or in the installation manual. Do not exceed the low-side test rating.
- Attach a nitrogen regulator and hose to the manifold. Set the regulator low and open the cylinder valve slowly.
- Raise pressure in steps. A common approach is about 150 psig first, then up to the manufacturer's test pressure, which is often around 300 psig on a residential split.
- Wait for pressure to settle, then record the pressure and temperature.
- Apply bubble solution to every joint you touched.
- Hold the pressure for the time your shop and the manual require. Longer tests find smaller leaks.
- Release the nitrogen slowly and safely.
Pressure changes with temperature. A line set in full sun will rise, and one that cools in the evening will drop. At around 300 psig, expect a change of roughly 0.6 psi for each 1°F. If pressure moves more than temperature explains, keep looking.
Adding a small amount of refrigerant as a trace gas to nitrogen is an old trick some shops still use to let electronic detectors work. Follow your shop's policy and the current rules. Never add nitrogen to a charged system and then vent the mix to skip recovery, because that is illegal venting.
After You Find the Leak
- Recover the refrigerant if the repair needs the circuit open.
- Make the repair.
- Run a nitrogen test on that repair.
- Evacuate and recharge.
- Record the location and the repair for any system that falls under leak repair rules.
Some systems must be repaired by a deadline after their leak rate passes a limit. Those rules depend on the size and type of system, and on the refrigerant.
Common Mistakes
| Mistake | What it costs |
|---|---|
| Topping off without looking | The leak keeps running, and the customer pays twice |
| Trusting one detector alarm | A false alarm sends you to the wrong coil |
| Skipping bubbles on new joints | A slow leak surfaces after you leave |
| Testing on a windy roof with fans running | Gas blows away from the tip |
| Exceeding a coil's test pressure | You create a leak where none was |
Check Your Understanding
You find an oily spot under a coil and a low system. Your detector alarms near it. What is your next step? Confirm with bubble solution before you decide on a repair.
A new line set holds 300 psig for an hour, but the pressure dropped 6 psi as the evening cooled by about 10°F. Is that a leak? Probably not by itself. That drop fits the temperature change. Check again at a steady temperature and bubble each joint.
A helper suggests pressure testing with shop compressed air to save nitrogen. What do you say? No. Air adds moisture, can react with oil and is not allowed as a test gas. Use dry nitrogen with a regulator.
A customer wants their system topped off every spring. What does that history tell you? There is a leak. Find it rather than keep topping off.
Key Takeaways
- A low charge means a leak, so find it before you add refrigerant.
- Look for oil stains and start with common leak spots.
- Use more than one method, and confirm electronic alarms with bubble solution.
- Pressure test only with dry nitrogen through a regulator, and stay below the system's rating.
- Temperature changes pressure, so account for it during a hold test.