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Low HVACR compressor head pressure diagnosis:
What are the causes of low head pressure at the air conditioner or heat pump compressor? What are the effects of low refrigerant pressure at the compressor or in the system?
Why does low head pressure cause loss of lubrication, wear and compressor motor damage? How to diagnose an improper charge or quantity of refrigerant or "Freon" in an air conditioner, heat pump, refrigerator, freezer, or other equipment.
In this article series we explain how overcharging or undercharging of refrigerant in an air conditioner or heat pump is detected and we list the effects of overcharged or undercharged refrigerant. We also explain the various causes of liquid slugging a compressor motor.
We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.
Refrigerant Pressure Diagnosis: Causes of Low Head Pressure in HVACR Systems
Low head pressure may occur simulataneously with high pressure on the suction side of an air conditioner, heat pump or refrigeration system compressor system.
If the system is continuing to cool it may be doing so at reduced capacity, observed as a loss of up to ten degrees of temperature drop across the coil. The two most common causes of low head pressure at an air conditioner or heat pump compressor motor are:
Refrigerant under-charge, due to a refrigerant leak or improper charge.
TXV (TEV) malfunction, over-metering refrigerant. This condition is not likely to occur on smaller air conditioners or heat pumps that use a capillary tube to meter refrigerant unless a technician has installed an improperly-sized cap tube (too short or too large in diameter).
Less common causes of low head pressure
Leaky compressor motor valves prevent the compressor from reaching full pressure. Leaky or damaged compressor motor valves are in turn often caused by an overcharge, a bad or mis-adjusted TXV, or other conditions that result in refrigerant liquid slugging at the compressor. Some texts also cite acid, moisture and heat-caused sludge contamiantion in the system. Any condition that sends liquid refrigerant into the compressor motor can damage valves.
Worn compressor motor piston rings or on a rotary compressor, worn seals. We explain below that low head pressure at the compressor motor can in turn cause reduced lubrication in the motor and increased wear as well as permanent compressor damage. An early sign of this wear may be reduced high side pressure and reduced cooling capacity of the system.
Leaks in the oil separator return line
HVACR compressor motor overheating is cited by most texts and sources we reviewed as the most serious and most-often occurring problems faced by refrigeration repair technicians. An overheating compressor motor can in turn cause damage resulting in wear and low head pressures.
Effects of Low Head Pressure at the HVACR Compressor
Effects of low head pressure in a refrigeration system incude more than reduced cooling capacity of the system. On commercial systems (and maybe some residential systems) the reduced head pressure can cause refrigerant oil migration or more specifically, the system may be unable to push oil through the cooling coil (the evaporator coil) until it accumulates as a bolus of oil that suddenly returns in one shot tothe compressor.
The effects of poor oil return to the compressor include first reduced cooling capacity and later damage to the compressor motor. If the liquid line is not warm on your system you may face these problems. (A-1 Compressors 2009).
Smaller air conditioners or refrigeration equipment whose design does not include a refrigerant accumulator are at increased risk of damage from overcharge, liquid slugging, or oil circulation problems. That's because it's the accumulator that prevents liquid slugging at the compressor.
Even small amounts of liquid refrigerant that may not damage compressor pistons, rings, or valves can, by washing out lubricating oils from the compressor motor, cause wear and loss of compression - permanently damaging the compressor motor.
References for Low HVACR Compressor Motor Head Pressures
"Refrigeration & Air Conditioning Guide: A-1 Compressor: Compressor troubleshooting guide, understanding the cooling cycle", A-1 Compressor, Inc., Horace Park, 2009, original source: http://www.rpctubes.com/images/CompressorTroubleshootingGuide.pdf
Chandra, Arora Ramesh. Refrigeration and air conditioning. PHI Learning Pvt. Ltd., 2010.
Dexter, Arthur L., and Mourad Benouarets. A generic approach to identifying faults in HVAC plants. No. CONF-960254--. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., Atlanta, GA (United States), 1996.
Jayanth, Nagaraj. "Air-conditioning servicing system and method." U.S. Patent 6,324,854, issued December 4, 2001.
Excerpts from this patent description: An air conditioning servicing system utilizes a number of sensors which monitor various operating parameters of a malfunctioning air-conditioning system. These operating parameters are provided to a hand held computer along with an identifier of the malfunctioning air-conditioning system. The hand held computer contains the normal operating parameters for a plurality of air-conditioning systems. The hand held computer compares the measured operating parameters with the normal operating parameters for the specific air-conditioning system to provide diagnostic results for the malfunctioning system and possible service procedures. If the hand held computer does not recognize the malfunctioning air-conditioning system identifier, a wireless connection is made through the Internet to a master computer which has a larger data base.
Ling, Jiazhen, Yunho Hwang, and Reinhard Radermacher. "Theoretical study on separate sensible and latent cooling air-conditioning system." international journal of refrigeration 33, no. 3 (2010): 510-520.
Riffat, S. B., C. F. Afonso, A. C. Oliveira, and D. A. Reay. "Natural refrigerants for refrigeration and air-conditioning systems." Applied Thermal Engineering 17, no. 1 (1997): 33-42.
Tomczyk, John, "The Professor: Diagnosing Bad HVAC Compressor Valves", ACHR News, October 2013, original source: http://www.achrnews.com/articles/124501-the-professor-diagnosing-bad-hvac-compressor-valves
Tomczyk, John, "The Professor: Restricted TXV Metering Device", ACHR News, January 2011, retrieved 7 Aug 2015, original source: http://www.achrnews.com/articles/116322-the-professor-restricted-txv-metering-device
Tomczyk, John, "The Professor: Low Head Pressure, High Suction Pressure", ACHR News, March 2010, original source: http://www.achrnews.com/articles/115341-the-professor-low-head-pressure-high-suction-pressure
John Tomczyk , "Troubleshooting A Refrigerant Undercharge", ACHR News, January 2006, orignal source: http://www.achrnews.com/articles/96496-troubleshooting-a-refrigerant-undercharge
Refrigeration & Air Conditioning Technology. Delmar Thomson Learning, 2009.
Yoshida, Harunori, Tatsuhiro Iwami, Hideki Yuzawa, and Masami Suzuki. Typical faults of air conditioning systems and fault detection by ARX model and extended Kalman filter. No. CONF-960254--. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., Atlanta, GA (United States), 1996.
Whitman, Bill, Bill Johnson, John Tomczyk, and Eugene Silberstein. Refrigeration and air conditioning technology. Cengage Learning, 2012.
"Air Conditioning & Refrigeration I & II", BOCES Education, Warren Hilliard (instructor), Poughkeepsie, New York, May - July 1982, [classroom notes from air conditioning and refrigeration maintenance and repair course attended by the website author]
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