The condensing unit can be covered with a cloth tarp or bedsheet to serve as a barrier against any outside air movement and to contain any leaking refrigerant gas during leak testing.
One might think that with the advanced refrigeration systems of today, and with electronic leak detectors and fluorescent dyes, that finding leaks wouldn't be such a difficult problem for technicians. However, all sealed systems leak and they're not always easy to find. The leak could be 1 pound per second or as low as 1 ounce every million years.

The leak repair requirements, promulgated under Section 608 of the Clean Air Act Amendments of 1990, require that an owner/operator of an appliance that normally contains a refrigerant charge of more than 50 pounds must take corrective action when a leak is discovered that exceeds the applicable trigger rate during a 12-month period. Trigger rates range from 35 percent for commercial refrigeration to 15 percent for comfort cooling appliances.

Since July 1992, it has been unlawful to knowingly vent refrigerants to the atmosphere. In keeping with the intent of the U.S. Environmental Protection Agency, it makes sense to completely understand the causes of leaks in refrigerant systems.

Every pressurized system leaks because ‘flaws' exist at every joint fitting, seam, or weld. These flaws may be too small to detect with even the best of leak detection equipment. But given time, vibration, temperature, and environmental stress, these flaws become larger detectable leaks.

Thus, it is technically incorrect to say that a unit has no leaks. All equipment has leaks to some degree. A sealed system, which has operated for 20 years without ever needing a charge, is called a ‘tight system.' However, the equipment still has leaks - just not enough leakage to read on a gauge or affect cooling performance. No pressurized machine is perfect.

Remember, a leak is not some arbitrary reading on a meter. Gas escapes at different times and at different rates. In fact, some leaks cannot be detected at the time of the leak test. Leaks may plug and then reopen under peculiar conditions.

A leak is a physical path or hole, usually of irregular dimensions. The leak may be a speck of dirt on a gasket or a microgroove between fittings.

Refrigerant vapor can flow under layers of paint, flux, rust, slag, and pipe insulation. Often the refrigerant gas may show up quite a long distance from the leak site. This is why it is important to clean the leak site by removing loose paint, slag, rust, or flux. Remove any pipe insulation. Oil and grease must also be removed from the site because they will contaminate the delicate detection tips of electronic detectors.

Figure 1. Oil will flow with the refrigerant gas and mark the general area of leakage.

Six Classes Of Leaks

There are six classes of leaks:

1. Standing leaks - These are leaks that can be detected while the unit is at rest (off) and high and low side pressures are fully equalized. Standing leaks, fortunately, are the most common of all leaks.

2. Pressure-dependent leaks - These are leaks that can only be detected as the pressure increases. Nitrogen is usually used to pressurize a system's low side to about 150 psig and high sides to about 450 psig. However, always check the refrigeration system's design test pressures. Pressure-dependent leak tests should be performed if no leaks are discovered by the standing leak test.

3. Temperature-dependent leaks - These leaks are associated with the heat of expansion. They usually occur from high-temperature ambient air, condenser blockages, or during a system's defrost period.

4. Vibration-dependent leaks - These leaks only occur during unit operation. The mechanical strains of motion, rotation, refrigerant flow, or valve actuation are all associated with vibration-dependent leaks.

5. Combination-dependent leaks - These leaks are flows that require two or more conditions in order to include leakage. For example, temperature, vibration, and pressure cause the discharge manifold on a semi-hermetic compressor to expand and seep refrigerant gas.

6. Cumulative microleaks - These leaks are all the individual leaks that are too small to detect with standard tools. The total loss over many years of operation slightly reduces the initial gas charge. In practice, the greater the number of fittings, welds, seams, or gasket flanges in a system, the greater the amount of cumulative microleaks.

Figure 2. Oil is always present at the Schrader valves and access ports due to the discharging of refrigerant hoses on the manifold and gauge set.

Oil Spotting

Successful leak detection is solely dependent on careful observation made by the testing technician.

All refrigeration systems internally circulate compressor oil with the refrigerant. Oil will flow off with the refrigerant gas and mark the general area of leakage with oil. Oil spots appear wet and have a fine coating of dust. (See Figure 1.)

The technician must determine if the wetness is due to oil or condensate. This can be accomplished by rubbing the area with a finger and feeling for oil slickness.

Oil spotting is the technician's first quick check, but it is not always reliable for the following reasons:

  • Oil is always present at the Schrader valves and access ports due to the discharging of refrigerant hoses on the manifold and gauge set. (See Figure 2.) Often these parts are falsely blamed as the main point of leakage.

  • Oil blotches can originate from motors, pumps, and other sources.

  • Oil residue may be the result of a previous leak.

  • Oil is not always present at every leak site. It may take months, even years, of unit operation to cause enough oil blowoff to accumulate on the outer side.

  • Oil may not be present with microleaks.

  • Oil may not reach certain leak positions.

  • Oil may not be present on new startups.

    Figure 3. Microfoaming solutions will form fine foam or “cocoon” when in contact with a leak.

    Evaporator Suction Leaks

    Many leaks that go undetected are in the evaporator coil. This is because most evaporator sections are contained in cabinets, buttoned up, or framed into areas that do not allow easy access. In order to avoid time-consuming labor to strip off covers, ducting, blower cages, or the unloading of product, an easy electronic leak detecting screening method can be used.

    The steps are:

    1. Turn off all system power including evaporator fan motors.

    2. Pressurize the system to equalization including defrosting of freezer coils.

    3. Warm up and calibrate an electronic leak detector to its highest sensitivity.

    4. Locate the evaporator drain outlet or downstream trap.

    5. Position the detector probe at the drain opening. (Be careful that the detector probe does not come into contact with any water.)

    6. Sniff a minimum of 10 minutes or until a leak is sensed. Recalibrate the device and test again. Two consecutive positive tests confirm an evaporator leak. Two consecutive negative tests rule out an evaporator section leak.

    Remember, refrigerant gas is heavier than air. Gravity will cause the gas to flow to the lowest point. If the evaporator section tests positive, the technician should expose the coil and spray coat all surfaces with a specially formulated bubble/foam promoter.

    Bubble/foamer solutions have been very successful in leak detection because of their price and effectiveness. (By contrast, household detergents often contain chlorides and will pit and corrode brass and iron.) Microfoaming solutions will form fine foam or "cocoon" when in contact with a leak. (See Figure 3.)

    One brand consists of an exclusive composition of esoteric microfoamers coupled into a base of coagulants and wet adhesives.

    All that is required is for the solution to be applied over the suspected leak area. When a leak is found, bubbles or foam will tell the technician of its location. The technician must be patient and let the bubbles stand for at least 10 to 15 minutes if small leaks are suspected.

    Another advantage of bubble testing is that bubbles can be used with nitrogen or refrigerants pressurizing the system. Small but significant leaks of less than a couple ounces per year can often be found with specially formulated solutions. If the evaporator screens negative, continue on to the condensing unit test, which is described next.

    Figure 4. Tuck the probe underneath the pipe insulation.

    Condensing Section Leaks

    To conduct a condensing section leak test, do the following:

    1. Calibrate an electronic leak detector to its highest sensitivity and place the probe at the base of the unit (usually under the compressor). The unit should be fully pressurized and high- and low-side pressures should be equalized.

    2. Cover the condensing unit with a cloth tarp or bedsheet to serve as a barrier against any outside air movement and to trap the refrigerant gas. Do not use plastic material.

    3. Monitor for leakage for 10 minutes or until a leak is sensed. Recalibrate and test again. Two consecutive positive tests confirm condensing section leakage. Two consecutive negative tests rule out a detectable leak.

    4. Use the electronic leak detector to check for leaks on the bellows of the pressure controls. Re-move the control box cover and place the probe within the housing. Cover the control tightly with a cloth barrier and monitor for 10 minutes, as above.

    5. If the results are positive, uncover the equipment and begin spray coating with a bubble/foam promoter. If the results are negative, continue to the suction/liquid line leak test, which follows.

    Suction And Liquid Line Leaks

    The longer the tubing runs between the evaporator and condensing unit, the greater are the chance for defects. Count on all possibilities, whether they involve a typical sight glass/drier, connection leak, or a poor solder joint hidden under pipe insulation.

    1. The suction line can be screened by calibrating an electronic leak detector to its highest sensitivity.

    2. Tuck the probe underneath the pipe insulation. (See Figure 4.) Monitor for 10-minute intervals while the system is at rest and fully pressurized to equalization. It may be necessary to insert the probe at several downstream points.

    3. If a leak is sensed, strip off the insulation and apply a bubble/foam promoter to all surfaces. If no leak was positively found, test the liquid line.

    Preparation of this material was made in conjunction with Refrigeration Technologies and ESCO Press.

    John Tomczyk is a professor of HVACR at Ferris State University, Big Rapids, Mich., and the author of Troubleshooting and Servicing Modern Air Conditioning & Refrigeration Systems, published by ESCO Press. To order, call 800-726-9696. Tomczyk can be reached by e-mail at

    Publication date: 01/10/2005