Updated: Jul 6
This article covers the ten most common locations where refrigerant leaks may be on an air conditioning system. An electronic leak detector is typically used to find the general area of the leak and non-corrosive bubble leak detector is used to pinpoint the leak. In the following pictures, an Accutrak ultrasonic leak detector and Big Blu non-corrosive bubble leak detector are used to search for and find refrigerant leaks.
A ultrasonic leak detector (see photo below) locates where the turbulent escaping gas is exiting the tubing. This tool can be used during a nitrogen pressure test, or while refrigerant is in the system (R-410A, R-22, or another refrigerant). Any gas escaping through a leak location makes a noise, which an ultrasonic leak detector can pick up. Oil on the inside of the tubing or water on the outside of the tubing creates an even louder sound when a leak is present.
Once the leak location is located, non-corrosive bubble leak detector (see photo below) can be applied to the location to pinpoint the leak. The leak is found where the foaming action or bubbles form.
Ten Common Leak Locations on an Air Conditioning System
Leak Location #1: Valve Core (Schrader Valve) on the Outdoor Unit Port
Check the valve core (Schrader Valve) on the outdoor unit port (see photo below). A cap is placed on the valve core to prevent the refrigerant from escaping from this location and to protect the valve core from depression. This cap may be missing. The type's of caps are listed below.
A plastic cap: This cap may crack, or it may not have an O-ring to create a seal.
A brass cap with an O-ring: If there is no O-ring, it will not seal.
A flared cap: This type of cap needs to have a drop of refrigerant oil on the inside of the flare to seal.
A locking cap: This type of cap should have an O-ring inside to seal.
In order to test the valve core, use a brass end cap with a tiny hole drilled in the flat end surface. This is used temporarily to test for a leak, so that bubble leak detector does not get into the valve core area. (If bubble leak detector gets into this area, it needs to be blown out.) Apply non-corrosive bubble leak detector onto the test cap where the hole is. If a bubble forms here, there is a leak in the valve core (see photo below). (NOTE: A leak at this location is the easiest one to find on the entire air conditioning system. An ultrasonic leak detector is required at all the other possible leak locations.) This valve core may need to be tightened or replaced. Do this by using a valve core removal tool.
Leak Location #2: Indoor Evaporator Coil
A leak may develop at the indoor evaporator coil where the tubing goes through the galvanized tin. This is due to rust forming where the two dissimilar metals meet. Move an ultrasonic leak detector along the entire evaporator coil to find the leak location (see the following two photos below). Noise is heard and the LED display light rises higher on the ultrasonic leak detector as the tip of the tool gets closer to the leak location.
To determine exactly where the leak is, apply non-corrosive bubble leak detector to the area on the tubing where the noise was heard. Wait several minutes for a bubble to form, or for foam to appear on the copper tubing. The bubble(s) or foam shows where the leak is.
Leak Location #3: The Joints Where the Tubing in Evaporator Coil Enters
The joints that have a bad swage, where they enter the tubing, are the most susceptible to having a leak form (see photo below). However, be sure to check all the joints and tubing at the evaporator coil.
Pass an ultrasonic leak detector across these locations to find the leak (see photo below).
Apply non-corrosive bubble leak detector to the area where the leak has been found (see photo below).
View the bubbles forming.
Leak Location #4: The Distributor Tubes at the Evaporator Coil
Examine the distributor tubes at the evaporator coil to determine if they are rubbing against each other (see first photo below). Also, examine where the tubes go into the 3/8 tubes (see second photo below). Use an ultrasonic leak detector to check for a leak along the length of the coils.
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Leak Location #5: In the Middle of the Tubing Run in the Evaporator Coil
There may be a leak in the middle of the tubing run behind the fins (see first photo below). A leak can develop here, if the copper tubing was made too thin (It may have been designed too thin, for energy efficient reasons, i.e. energy transfer), (see photo showing the inside of the tubing). Grooves can be put on the inside of the tubing to increase the surface area. The grooves may be in a circular pattern in the tubing, or straight down the tubing run (see photo showing the inside of the tubing). The grain inside the tubing can cause a leak. (NOTE: The leak was found in the middle of the coil, using an ultrasonic leak detector.)
An ultrasonic leak detector is used to detect any leaks in the tubing in an evaporator coil (see photos below).
Note: Ultrasonic leak detectors work even better when the coils are wet, such as on this wet evaporator coil. To get the evaporator coil wet, simply turn the system on in air conditioning mode for a few minutes and then turn it off, prior to leak searching.
I cut the aluminum fins back to pinpoint the leak on the copper.
Leak Location #6: At the Service Valves
When the service valves are installed, if the torch is too close to them, they may overheat. If this happens, it is possible that the O-ring inside the service valve may partially melt or burn. The cap on the service valve only has a rim (see photo below), and not a flare. The overall result is that refrigerant may leak out of the top of the service valve and cap.
Leak Location #7: On Micro Channel Coils
Micro channel coils use very thin aluminum fins in the heat transfer. Because of this, micro channel coils should not be installed near coastal areas. The thin aluminum fins break down fairly quickly when exposed to salt air (see photo below). As such, a refrigerant leak can develop very quickly.
In the pic below, on the ultrasonic leak detector, you can see the red indicator at level 9 of 10, signaling a leak (see photo below).
Non-corrosive bubble leak detector has been added to both sides of this micro-channel coil and bubbles are forming at the leak spot.
Leak Location #8: The Filter Drier
Anything made of steel is prone to rusting, especially in a coastal area. When a filter drier is brazed on both sides, the paint gets burned and eventually flakes off. This area of exposed metal ends up rusting (see photo below). The metal is not very thick, so this becomes a potential leak spot. Installing the filter drier inside the building is one way to reduce the likelihood of a leak since the filter drier will not be exposed to the outdoor elements.
Leak Location #9: The Accumulator Tank
The accumulator tank (see photo below) is made of thin metal, and is installed in the outdoor unit. Both the top and bottom of the accumulator tank are potential leak spots. Leaves and other debris accumulate at the bottom of the outdoor unit. This debris sits around, and is in contact with, the bottom of the accumulator tank. It is likely that the paint on the accumulator bottom will be compromised and flake off the accumulator. It is possible to clean and seal both the top and the bottom of an existing accumulator tank, with wheel well spray paint or Plasti-Dip.
(NOTE: The accumulator tank is normally found in outdoor heat pumps. Unless it has been field installed by a technician, an accumulator tank is not usually found on an air conditioning system. There may be another steel item in an outdoor heat pump that resembles a filter drier. It is called the muffler, and this is located on the discharge gas off the compressor. This is another possible leak location to be aware of.)
Leak Location #10: The Tubing at the Bottom of the Outdoor Condenser Coil Unit
As stated above, leaves and other debris accumulate at the bottom of the outdoor unit. This debris sits around and basically rots the tubing at the bottom of the unit. (It doesn’t matter whether the tube is aluminum or copper, the leak tends to develop because of the location of the tube.) It is a good idea to hose down or use a pump sprayer, to remove leaves and other debris from the base of the outdoor condenser coil.
It helps for the outside of the tubing to be wet when using an ultrasonic leak detector. Additionally, it helps when the system is turned on before leak searching and then is turned off, because the oil that has been slung throughout the inside of the tubing, will be present at the leak location. The gas has to push through the oil in order to escape through the tube. This makes a turbulent noise which the ultrasonic leak detector picks up (see photo below). This photo shows a high noise reading on the leak detector.
In order to locate the exact location of the leak, remove the grill, then move the ultrasonic leak detector across the tubing (see photo below).
Once the area of the leak has been found, apply some non-corrosive bubble leak detector on the tube. Watch for bubbles or foam to form (see the bubble on the aluminum section of the tube in the photo below).
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Author: Craig Migliaccio
About the Author: Craig is the owner of AC Service Tech LLC and the Author of the book “Refrigerant Charging and Service Procedures for Air Conditioning”. Craig is a licensed Teacher of HVACR, Sheet Metal, and Building Maintenance in the State of New Jersey of the USA. He is also an HVACR Contracting Business owner of 17 years and holds an NJ HVACR Master License. Craig creates educational HVACR articles and videos which are posted at https://www.acservicetech.com & https://www.youtube.com/acservicetechchannel