ACHRNEWS

Btu Buddy 90: Underground Refrigerant Leak

September 20, 2010

Bob is a service technician who is well trained and nationally certified. However, he sometimes suffers from the same confusion that all technicians occasionally do - the facts that he gathers may or may not point to the obvious cause of the problem or the best solution. But Bob has something that no one else has. He recalls his long-time HVACR mentor and imagines him accompanying him as “Btu Buddy,” someone who reminds him to take time to stop and think before rushing to judgment, helping keep him on the right track, even with facts that are confusing.

Bob was called to a job that had a no cooling complaint. This was a new customer and Bob wanted it to be a good experience for the customer. It was a small stand-alone store building with a 3-ton cooling only unit.

When Bob arrived, he went in and talked to the store owner. The owner explained, “The system has not been cooling its best for the last three days. This morning, it just stopped cooling. The unit outside is running, but the air temperature is not cool.”

Bob asked where the indoor unit was located and was told the unit is in a closet on an outside wall.

Bob pulled his truck to the back of the store to the condensing unit and noticed something different. The condensing unit was on the other side of a driveway with the piping running underground.

Figure 1. This internal overload on this compressor serves as overheat protection for the motor. Overheating can be caused by overload from high head pressure or unit overload and from low charge. Remember that refrigerant suction gas cools the compressor and a low charge condition will cause overheating. Therefore the internal overload serves as overload protection, high pressure condition and low charge. (Figures are from Refrigeration & Air Conditioning Technology, 5th Edition, by William Whitman, William Johnson, and John Tomczyk, published by Delmar Cengage Learning.) (©Delmar Cengage Learning.) (Click on the image for an enlarged view.)

Bob found the unit’s fan running, but the compressor was not running. He shut off the disconnect and removed the compressor access door and felt the compressor. Just as he suspected, the compressor was hot, but not running. It was shut down by the compressor internal overload (Figure 1). This was a simple low charge problem.

Bob installed his gauges and discovered that the unit did not have any refrigerant in it. This must be a sizable leak, he thought. He connected a cylinder of refrigerant to the gauges and let in enough R-22 to build the pressure to 100 psig. He figured that the leak was large enough that he would be able to find it with his electronic leak detector that could detect a leak down to ½ ounce a year. This should be simple.

Bob went over the field connections at the condensing unit and did not find a problem. He then went in to the indoor unit and checked the field connections. These were the most likely place for a leak. He still found nothing. Then he removed the indoor coil cover where he could reach the coil connections and didn’t find anything. He them moved to the condensing unit and explored all around the unit with no results. He was getting worried when Btu Buddy appeared and asked, “Are you confused, Bob?”

Bob said, “I might have known you would find me in a moment of confusion.”

Btu Buddy said, “You seemed to have checked the places that may be obvious and easy. Have you explored where the piping goes underground?”

Bob said, “No, I was afraid to even think about that.”

Bob then noticed that the piping was run in a plastic sleeve. He started to put the leak detector probe down in the sleeve when Btu Buddy said, “You need to take a flashlight and see what is down in the sleeve. It may be full of water and you don’t want to put the leak detector probe in the water. That particular probe has a small pump that pulls in an air sample and water will damage the instrument.”

Bob looked down in the sleeve with a flashlight and said, “It’s a good thing that you warned me. This pipe is half full of water.”

Bob let the probe down inside the tube so that the probe did not get in the water and said, “Wow, the probe is showing a leak. Now what?”

Btu Buddy said, “I think you should bring the owner out here and show her the problem.”

Bob brought the store owner out, showed her the problem, and said, “There is no way to know what is going on underground except to either dig it up or pull the pipe out. It is very likely that the refrigeration system has moisture inside it because when the refrigerant has leaked out, the large pipe in the underground system goes into a vacuum, and the pipe is laying in water.”

She told Bob to do whatever it takes to do a professional repair.

Btu Buddy said to Bob, “Water should never be allowed to stand around a refrigerant suction line for two reasons: (1) the water will exchange heat with the refrigerant and hurt the system capacity and, (2) if there is ever a low side leak, moisture can be pulled into the system.”

Bob then asked, “Now what?”

Btu Buddy explained, “First you should try to drain the pipe. The company that installed this did a good job. It is professional looking. I believe that they would have had a drain system for this pipe. Let’s dig around each end of the pipe and see if there is a drain. Second, you should see if the pipe can be cut loose on each end and pulled from the underground sleeve rather than dig the whole system up.”

Bob looked at both ends of the system and said, “I believe that the pipe is slanted downward toward the condensing unit. The drain should be on this end and the tube sleeve goes through the wall at the inside unit. The tubing is not “hard drawn,” it is soft. When the tubing is pulled out, a line set of ¾ inch tubing can be unrolled and fastened to the liquid line and gradually pushed through the sleeve to the condensing unit. I believe this was the way it was installed in the beginning.”

Btu Buddy said, “Well, your days spent with the installation crew gave you some good experience. I believe that will work.”

Bob called the shop for some help and a 30 foot line set.

By the time the help had arrived, Bob had cut the line set loose on both ends and taped the pipe closed at the condensing unit and the air handler. He had pulled the old tubing out into the building, coiling it as he pulled it out.

Figure 2. This illustration shows how hard flux is formed on top of any connection using brazing material as the filler material. This flux should be chipped away because it can cover a spot that won’t leak until the flux eventually comes loose. Expansion and contraction can cause the flux to loosen over time. (©Delmar Cengage Learning.) (Click on the image for an enlarged view.)

Now to find the leak. He cut the insulation off of the old pipe and found a field connection under the insulation that was obviously where the leak occurred. It was a braze connection made with flux and the flux was still on the connection. Bob had chipped the flux off of the connection to uncover a pin hole in the brazing fill material. Bob then said to Btu Buddy, “Whoever made this connection did not chip the flux away. The flux eventually came loose in this spot and here is where the leak is (Figure 2).”

Btu Buddy then said, “That is a good deduction. That glassy glaze of flux has been holding and probably has not leaked for years, then it finally loosened.”

Bob then began to dig around the condensing unit piping connection to try to find a drain. He found a “T” in the sleeve where a ¾ inch line was piped off of the bottom. That would complete a good installation, but where did it terminate? Bob kept digging, following the ¾ inch pipe and found that it was going toward a flower bed. He dug around in the end of the flower bed and found that the end of the ¾ inch drain had been covered by the landscapers. He uncovered it and ran a wire up in it to clear it, and water from the unit drained out. He used some landscaping blocks to make the drain termination point very obvious and returned to the condensing unit to finish that job.

The line set and helper arrived and the helper helped Bob run the new line set. They connected it to the unit, leak checked the connections, and pressured the system with a trace of R-22 and the pushed the pressure up to 150 psig with dry nitrogen. It was then time to quit for the day. Bob said, “I am going to leave the system under pressure overnight to see if the pressure holds and come back in the morning.”

Btu Buddy then said, “In the morning you may have to deal with moisture in the system.”

To be continued in the October 18, 2010 issue.

Publication date: 09/20/2010