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.

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.

If your air conditioning or heat pump system has lost its cooling capacity or won't start

see REPAIR GUIDE for AIR CONDITIONERS.

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Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

Good Low Pressure on Low Side but Very Low Pressure on High Side of my AC unit

Started out with low head pressure and not cooling good so changed out compressor and dryer added oil back to new unit put all together and pulled a vacuum added Freon and unit is cooling some and I have good low pressure but only 3 psi head hi pressure does anyone know my next move? Thanks - On 2018-05-26 by Jack

Reply by (mod) -

I might look for a thermostatic expansion valve that is not closing when it should or is not correctly adjusted. If the thermostatic expansion valve isn't closing it keeps sending so much refrigerant back to the low side that the high side never develops pressure.

If the TEV is, on the other hand, working, then the problem is probably the compressor motor itself.

Review the low head pressure causes above on this page for a more complete diagnosis

 

Cause of zero gauge head pressure on a heat pump, compressor running

Getting no guage pressure on York heat pump in cooling mode but compressor is running. Is compressor bad? - On 2020-06-11 by Anonymous

Reply by (mod) -

Anon

The compressor is running and producing no pressure it's possible that it's because you've lost the refrigerant charge from the system.

High side pressure is very low on Lennox Compressor - bad TXV?

I have a 3 ton 410a Lennox with correct suction pressure however my high side pressure is way low.

Pressures are 120headover 118 suction low side. What would be the cause. Bad txv or tev - On 2019-05-07 by Cory Calhoun

Reply by (mod) - bad compressor

Cory, thank you for an interesting question. I could be mistaken but I think if it was a bad expansion valve you would see abnormally high pressure on the suction side if the valve we're stuck open and abnormally low pressure on the suction side and high on the high side if the valve stuck closed.

So unless you've simply lost the refrigerant charge (so there's no gas to compress) I'm afraid I suspect the compressor.

Cause of equal refrigerant pressures on High & Low Sides

I just installed a 3 ton mini split unit pressures are equal the liquid line is not warm the compressor sits above the cassette up on the roof my line is 15 foot long I'd appreciate any help thank you - On 2019-07-16 by I just install a 3 ton mini split

Reply by (mod) - Normal operation - system is OFF

Its normal for the refrigerant high side and low side pressures to equalize when the equipment has been off for a time.

What is "normal" head pressure for a Two Ton heat pump?

What is the normal head pressure for a 2 ton heat pump? - On 2019-07-03 by Freeman

Head Pressure isn't a function of heat pump tonnage or size, its' refrigerant and temperature

@Freeman, it’s not about tonnage. On a 85 degree day:

R22 - 200 roughly
R410 - 300 roughly

- On 2019-07-04 by Anonymous

What should be the low side refrigerant pressure in a 13 SEER condenser unit?

What should the low side pressure be if the condensing unit is 13 seer and the air handler is 10 seer - On 2016-07-30 by Rich olsen

Reply by (mod) -

Neither the AC unit's SEER rating nor its tonnage or size or capacity determine the low side or suction side refrigerant pressure. The pressure you should see depends principally on the specific refrigerant in use and the ambient temperature.

Take a look at the temperature/pressure chart for your HVACR system for the specific refrigerant that you are using; you'll need to know the ambient temperature as well.

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Continue reading at REFRIGERANT PROBLEM TYPES or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

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