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Read on as Bob and Btu Buddy move through a residential service call.
Bob is called to a job where the owner complains that the unit ran all night but there is no cooling; the house is still warm and humid. When Bob arrives, he finds the suction line is frozen all the way back to the compressor and the compressor is a solid ball of ice. This looks like a clear case of low refrigerant charge.
A Clear Case?Btu Buddy says, “Ask some questions, Bob.” So Bob asks the owner to describe the sequence of events and the owner says, “We left the unit off all day yesterday while we were at work. When we arrived home last night, we had several people over for a cookout. So we turned the unit on and it ran and seemed to cool for a while, but it ran all night and the house is not cool now. This is the first time we have run the unit this season.”
The outdoor ambient temperature is 75 degrees F and the indoor temperature is 78 degrees F. Btu Buddy says, “The system is not going to perform correctly under these conditions because of the outdoor ambient temperature.” Bob decides to install gauges on the unit.
Btu Buddy says, “This may not be a good idea because the charge will be altered when the 6-foot gauge hose is fastened to the liquid line. This system uses an orifice for a metering device and has a critical charge to plus or minus one ounce. A 6-foot gauge line will hold about 0.21 ounces of R-22 per foot of 1/4-inch line. This figures out to 1.26 ounces of liquid refrigerant that will move into the gauge line. This is enough to affect the system charge during the test. If this refrigerant is not put back into the system, there will be efficiency problems with the unit. If you are going to install gauges, it would be a good idea to use a shut off control valve so the refrigerant may be charged back into the unit when you disconnect the gauges. You could also use a short gauge hose connector on the liquid line.”
Installing GaugesBob installs the short gauge connector on the liquid line and a regular 6-foot gauge line on the suction line. He knows that the charge will not be affected by the 6-foot gauge line because that line only contains vapor. When the small amount of refrigerant is released from the lines, it will meet the de minimis (minimum loss) requirement of the EPA.
When Bob installs the gauges, he is at first alarmed because the suction pressure is 37 psig and the discharge (head) pressure is 165 psig in this R-22 system. The low side of the system is operating at 14 degrees F, which is well below freezing. Btu Buddy says, “The first thing you must do is defrost the coil.”
Bob knows that thawing out the coil will take time so he sets the thermostat to Off and the fan switch to Fan On. This will keep maximum airflow against the coil and after awhile, it will defrost a hole in the ice where air can flow through the coil. He tells the owner that he is going to another service call to give the ice time to melt. He tells the owner to watch for air to flow at the air supply register in the den. When she can feel airflow, she is to call Bob on his cell phone and he will return.
Bob receives a call in about two hours that the air is moving, so he returns. He starts the compressor and observes the pressures and finds the suction pressure to be 55 psig and the head pressure to be 200 psig. The ambient temperature is still 75 degrees F and both pressures are low, still indicating a low charge, so he starts to his truck to get a cylinder of R-22.
Btu Buddy stops him by saying, “This system is not going to operate correctly because the outdoor temperature is nearly the same as the indoor temperature. The condenser is so efficient that it is condensing too much refrigerant in the condenser. You must remember this system has a critical charge. There should be a prescribed amount of refrigerant in the condenser, the evaporator, the liquid line, and the suction line. The system will operate correctly then and only then. If any amount of refrigerant unbalance occurs, it will cause the system to malfunction. In this case, the suction and liquid lines contain the correct charge as one is vapor and the other is full of liquid. The condenser has too much refrigerant, therefore starving the evaporator. When a system is allowed to run for many hours, the suction pressure and temperature will gradually drop to the point that ice will form — more running time and the coil will freeze. This is exactly what happened.
“When the homeowner has a house full of company and the system is not cooling, they have a tendency to turn the thermostat down because they think it will make the unit run faster. When you arrived, the thermostat was set on 60 degrees F and the unit was running all of the time. As it got later and later, the outdoor ambient temperature dropped even further, causing the discharge and suction pressure to drop lower and a freeze condition occurred.
“Now Bob, block some air crossing the condenser and raise the head pressure to about 275 psig, to approximate the head pressure for a 95 degree F day, and watch what the suction pressure does.”
Bob reduces the airflow across the condenser and raises the head pressure to 275 psig and the suction pressure is raised to 70 psig within about 10 minutes. This is close to the design evaporator temperature of 40 degrees F. He then checks the superheat and finds it to be 12 degrees, which is good. Btu Buddy tells Bob, “This is actually simulating a design day for the equipment and can be used to check the charge on most fixed bore metering devices”.
Btu Buddy recaps the call by saying, “ It is always good to take some time to think before fastening gauges to a system. Really Bob, you could have left the gauges in the truck, thawed the coil, blocked the condenser airflow until the air leaving the condenser became warm, and felt the suction line to see if it became cold. If so, you could have declared the system OK. It is not good to apply gauges until you really see a need. Service technicians can alter the charge when needlessly installing gauges and may leave a leak behind when removing the gauge lines. Let’s agree that a service call should not cause a problem, but should solve a problem.”
Johnson has been active in the HVACR industry since the 1950s. He graduated in gas fuel technology and refrigeration from the Southern Technical Institute, a branch of Georgia Tech (now known as Southern Polytechnic Institute). He taught HVAC classes at Coosa Valley Vocational & Technical Institute for four years. He moved on to become service manager for Layne Trane, Charlotte, N.C. He taught for 15 years at Central Piedmont Community College, part of this time as program director. He had his own business for five years doing installation and service work. Now retired, he is the author of Practical Heating Technology and Practical Cooling Technology, and continues as a co-author of Refrigeration & Air Conditioning Technology, 5th Edition, all published by Delmar Publishers. For more information, he can be reached at 704-553-0087, 704-643-3928 (fax), or firstname.lastname@example.org.
Publication date: 04/21/2003