Refrigerant charge diagnosis:
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.This air conditioning repair article series discusses the the diagnosis and correction of abnormal air conditioner refrigerant line pressures as a means for evaluating the condition of the air conditioner compressor motor, which in turn, is a step in how we evaluate and correct lost or reduced air conditioner cooling capacity.
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Low head pressure: If the head pressure at the compressor is low we figure that there is a short charge - that is, the system has lost refrigerant.
Condenser coil plugged: High head pressure: if the head pressure at the compressor is abnormally high we figure that the condenser coil is plugged and needs replacement.
Short refrigerant charge: Low pressure on both the High and Low sides of the system typically means that there has been a loss of refrigerant or a short charge.
Frozen TEV: Low side pressure or zero pressure on the low side of the air conditioner or heat pump system may mean that the metering device such as a TEV is frozen or jammed and is not releasing any refrigerant into the cooling coil. In this condition the high side pressure may go up, then down. You can test and temporarily cure this condition by warming the TEV or cap tube.
Plugged or saturated drier: by comparison with the above conditions, a partially clogged drier will form a restriction in the refrigerant line so that the low side pressure drops and the high side pressure increases. You may also notice that the refrigerant line temperature is significantly different on the inlet and outlet sides of the drier.
High side pressures in the air conditioner or heat pump system that are too low (100 psi for example) can indicate that the compressor is failing (cannot pump up to pressure) or that the refrigerant metering device is stuck wide open and the system is not developing enough pressure difference between the high and low sides.
High side pressures in the air conditioner or heat pump system that are too high can also mean serious trouble: a blocked condensing coil, blocked filter/dryer on the high side, or a refrigerant metering device (TEV) that is stuck closed.
A small refrigerant pressure change on the high pressure side of a refrigeration system will make a big change on the low side. A common field diagnostic step is to quickly look at the system low-side pressure since if that reading is bad you know that there is a problem on the high side.
Low side pressures in the air conditioner or heat pump system reflect the compressor's ability to draw refrigerant through the system and the rate of metering and evaporation of refrigerant in the cooling coil.
While newer higher efficiency air conditioners and heat pumps run at higher suction pressures than older units, a rule of thumb used by many HVAC techs is that the low side pressure should be well under 90 psi. If you are seeing 90-100 psi (or higher) on the low side of the system then either the compressor is damaged (not able to pump down to a low enough pressure) or the refrigerant metering device is stuck wide open and flowing too much refrigerant through the system.
The exact refrigerant charge: The manufacturer specifies the quantity of refrigerant that should be placed into any system: air conditioner, heat pump, refrigerator, freezer.
Watch out: equipment and ambient temperatures affect how you measure and install a charge of refrigerant into cooling equipment or heat pumps. Especially on residential systems installing the proper total refrigerant charge, which has to take into account not just refrigerant liquid volume but also ambient temperatures, is critical. [On many commercial refrigerant systems there is a receiver that holds a larger buffer quantity of refrigerant, so you'll notice the effects of refrigerant loss only after quite a bit has leaked out.]
Changes in air conditioner or heat pump operating pressure can be effected by adding or removing refrigerant from the system.
Changing the amount of refrigerant will cause a pressure change at the point where the refrigerant changes state. Normally an HVAC technician will charge the system to its recommended pressure and we won't vary the total refrigerant charge away from what the system manufacturer recommends.
Technical detail: refrigerators and some other equipment have a data tag that give a test pressure. Ignore this number when charging the system. This is a leak test pressure.
The data tag also gives the type of refrigerant that should be used in the system (no you cannot substitute). And the data tag will give the proper refrigerant charge quantity, typically in ounces. For a small residential equipment (a refrigerator) this may be 5-11 ounces.
Remember that the location of the frost line (on the cooling coil and refrigerant piping) can indicate evidence of overcharging.
Bottom line about overcharging refrigerants: even if you don't destroy the compressor motor by overcharging the system will be operating at a higher temperature and thus will be operating less efficiently. For residential equipment such as air conditioners, heat pumps, refrigerators, freezers, to work properly you must have exactly the correct charge in the system.
The effect of too much refrigerant in the system - overcharging, over-metering, or other high refrigerant pressure situations are detailed at EFFECTS of OVER CHARGING of REFRIGERANT
Watch out: over-charging refrigerant can damage or even destroy the compressor unit in some HVACR systems.
Effect of too little refrigerant in the system are described in detail at EFFECTS of UNDER CHARGING of REFRIGERANT
Separately at FROST BUILD-UP on AIR CONDITIONER COILS we also explain that in a properly tuned and adjusted refrigeration system there will be liquid refrigerant found all the way to just at the end of the evaporator coil .
Put another way, high temperature or high pressure on the air conditioner low side is a sign of a problem. That is, as pressure on the high side goes way up, low side pressure will increase as well, and we may exceed the operating temperature of the system. The Low side temperature must be low enough to get transfer of heat from the indoor air into the evaporator coil. The High Side temperature must be high enough to get transfer of heat into the outdoor air.
See SEER RATINGS & OTHER DEFINITIONS for additional definitions of the high side and low side of an air conditioning system.
See COMPRESSOR CONDENSER for an explanation of how and why high side and low side pressures in the cooling system enable an air conditioner to move heat from indoors to outdoors.
See OPERATING TEMPERATURES HVAC for a discussion of the typical temperatures at which various types of cooling systems operate.
This discussion has moved to a separate article now found at REFRIGERANT HIGH HEAD PRESSURE DIAGNOSIS
This discussion has moved to REFRIGERANT LOW HEAD PRESSURE DIAGNOSIS
If your air conditioning or heat pump system has lost its cooling capacity or won't start see REPAIR GUIDE for AIR CONDITIONERS.
Continue reading at REFRIGERANT PROBLEM TYPES or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.
Or see LOST COOLING CAPACITY
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(Jan 30, 2015) ed said:
have a 4yr old american standerd a/c heat pump unit.cycles every 10sec.tech serviced 12/06/2014 had to shut unit down01/20/2015.live on a island only way to get here in winter is by plane. expensive! put gauge on high side,unit off temp out side 20deg f read50#psi
Ed you are right to shut off the system; it may be that at low outside temp the unit wants to switch to backup heat anyway.
Short cycling could be a shot or frozen compressor, a burned out compressor base heater, a failed start/run capacitor, a failed relay or control board, a frozen expansion valve that's causing a high pressure safety switch to shut off the system, or something else. With just your note I'm doubtful one could actually diagnose the system.
Is your backup heat running?
Start by checking the control relays and check for a clogged or frozen TEV
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Our recommended books about building & mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.
Complete List of Air Conditioning & Heat Pump Design, Inspection, Repair Books at the InspectAPedia Bookstore.
Modern Refrigeration and Air Conditioning, A. D. Althouse, C.H. Turnquist, A. Bracciano, Goodheart-Willcox Co., 1982
Principles of Refrigeration, R. Warren Marsh, C. Thomas Olivo, Delmar Publishers, 1979
"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]
Refrigeration and Air Conditioning Technology, 5th Ed., William C. Whitman, William M. Johnson, John Tomczyk, Cengage Learning, 2005, ISBN 1401837654, 9781401837655 1324 pages
Carson Dunlop, Associates, Toronto, have provided us with (and we recommend) Carson Dunlop Weldon & Associates' Technical Reference Guide to manufacturer's model and serial number information for heating and cooling equipment ($69.00 U.S.).
Air Conditioning Inspection, Diagnosis, Repair, Efficiency all the basics for home owners, inspectors, new repairmen
Asbestos HVAC Ducts and Flues field identification photos and guide
Fiberglass: Indoor Air Quality Investigations: Fiberglass in Indoor Air, HVAC ducts, and Building Insulation