Service & Maintenance / Extra Edition

Btu Buddy 35: Heat Pump With a Stuck Check Valve

February 20, 2006
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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 receives a call from the dispatcher that a good customer seems to be having a problem with his heat pump. The weather is cold and the unit's auxiliary heat light is staying on most of the time.

Bob arrives at the job and talks to the customer. He is an engineer and really keeps a close eye on how his heat pump operates. He tells Bob, "The auxiliary light at the thermostat almost never goes off and when it does go off, the heat pump continues to run, but the air is not warm like it used to be."

Bob goes out to the heat pump and does a touch test on the lines. The hot gas line is cool. He removes the compressor compartment door and feels the compressor. It is cold all over, even the crankcase, which should be warm no matter how cold it is outside. From the touch test, Bob believes the compressor has liquid refrigerant entering the suction line. He observes that the compressor is a reciprocating compressor and there is an accumulator in the permanent suction line (between the four-way valve and the compressor). He then calls the office to see if anyone other than him has worked on this unit before. The dispatcher pulls up the records and tells Bob that he is the only one who has serviced the unit.

Bob is sure that the unit has too much refrigerant and he can't figure out how. He goes to the house and talks to the owner. The owner assures Bob that no one else has serviced the unit. Bob tells the owner about the symptoms before going back to the unit and applying gauges.

The gauge reading shows a high suction pressure and a low discharge pressure. This looks like a defective compressor now. Bob's head is really spinning.

Btu Buddy shows up about this time and asks Bob what he thinks.

Figure 1. A diagram of a heat pump cycle with two capillary metering devices with their check valves. (Figures are from Refrigeration & Air Conditioning Technology, 5th Edition, by William Whitman, William Johnson, and John Tomczyk, published by Thomson Delmar Learning.)
Bob says, "The compressor is getting liquid from somewhere and I don't believe there is too much refrigerant in the system because of the records. This system has always performed correctly."

Btu Buddy says, "Explain the refrigerant circuit for this unit in the heating cycle."

Bob says, "Let's start with the compressor discharge." He produces a diagram from his truck (Figure 1). "The hot gas leaves the compressor through the hot gas line at the bottom of the compressor, then proceeds through the four-way valve to the indoor coil, where hot gas is condensed to a liquid and leaves at the bottom of the coil. The liquid line carries the liquid through the indoor coil metering device and its check valve, which is open with this direction of flow. The liquid then flows to a metering device at the outdoor coil where there is a check valve in one line and a metering device in the other line. The liquid cannot flow through the check valve and is forced through the metering device, and liquid is metered into the outdoor coil where it absorbs heat from the outdoor air. The liquid refrigerant in the outdoor coil is boiled to a vapor, which leaves the outdoor coil to return to the compressor as a vapor. The compressor compresses the vapor to a hot gas to go around the circuit again."

Figure 2. This illustration shows a compressor that is hot with the internal overload in the open position.

THE CRITICAL QUESTION

Btu Buddy then asks the critical question, "Bob, explain again what happens at the outdoor metering device."

Bob says, "The liquid refrigerant is forced through the metering device by the check valve. I see what you are getting at. What if the check valve were stuck open? Then there would be a straight flow through the check valve and no metering would occur."

Btu Buddy says, "Why don't you check the amperage at the compressor?"

Bob checks the amperage and it is high. Bob then says, "The compressor is pumping. Why doesn't the overload shut the compressor off?"

Btu Buddy then asks Bob, "What does the internal overload on this compressor respond to?"

Bob says, "The high amperage of the motor causes it to become hot and thermal contacts open the circuit to the common terminal in the compressor wiring (Figure 2)."

Btu Buddy then asks another critical question, "Do you believe the internal overload is hot with all of that liquid refrigerant flowing over it?"

Bob says, "I see what you mean. It is running overloaded but cool."

Bob then asks, "Is there anyway to repair the valve?"

Btu Buddy says, "Sometimes you can tap on it with a soft face hammer and reverse it a few times while tapping on it. That may free it up, but this will likely occur again. The valve is either dragging internally from wear or it has a particle stuck in it. The best advice is to replace it and forget it."

Bob goes to the owner and explains the situation. The refrigerant will have to be removed from the unit to make the repair. The owner says to go ahead and make the repair.

Btu Buddy makes a suggestion. "Turn the unit to emergency heat which will shut off the compressor. Let the unit sit here like that with the crankcase heat on the compressor overnight to boil the liquid refrigerant out of the compressor crankcase, and come back tomorrow and remove the refrigerant and change the valve."

Bob then tells the owner the plan of action.

Bob returns the next morning and recovers the refrigerant. He then changes the valve and adds a liquid line filter-drier. He evacuates the system and weighs in a measured charge which includes the extra amount for the new filter-drier. He then starts the heat pump up. The hot gas line gets hot and Bob goes into the house to check the air temperature with the auxiliary heat turned off. The air temperature is 100 degrees F, so Bob explains to the owner that the system is operating correctly.

The owner then says to Bob, "Thanks for a professional job well done."

Bob is driving away from the customer's home when Btu Buddy says, "Good job, Bob. The owner was impressed with the way you did your work."

Bill 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 Thomson Delmar Learning. For more information, he can be reached at 704-553-0087, 704-643-3928 (fax), or billj@carolina.rr.com.

Publication date: 02/20/2006

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