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Photograph of a commercial air conditioning compressor charging gauge set (C) InspectAPedia.com Air Conditioner Compressor & Refrigerant Pressure Diagnosis

Troubleshoot HVAC refrigerant pressures & quantities

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.

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A/C or Heat Pump Refrigerant Pressure Problem Diagnostic Checklist

Photograph of a commercial air conditioning compressor charging chartArticle series contents

Illustration: this refrigerant chart excerpt that we found on a commercial cooling system gives suction line pressures at various ambient ("outdoor") temperatures.

Diagnose a Refrigerant Leak or a "Short Charge" of HVAC Refrigerant - undercharged

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

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

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 at 90 psi or up mean trouble

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.

How Much Refrigerant is in the Proper Charge?

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.

How to Use the Equipment Data Tag Information to Charge Air Conditioners, Heat Pumps, or other Refrigeration Equipment

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.

Effects of Overcharging the Refrigerant Level in an Air Conditioner, Heat Pump, or other refrigeration equipment

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.

Effects of Undercharging the Refrigerant Level in an Air Conditioner, Heat Pump, or other refrigeration equipment

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 .

More Notes About Residential Air Conditioner Compressor Pressures

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.

Refrigerant Pressure Diagnosis: 12 Causes of High Head Pressure at an Air Conditioner, Heat Pump or other Refrigeration System

This discussion has moved to a separate article now found at REFRIGERANT HIGH HEAD PRESSURE DIAGNOSIS

Refrigerant Pressure Diagnosis: Causes of Low Head Pressure in HVACR Systems

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.

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Reader Comments, Questions & Answers About The Article Above

Below you will find questions and answers previously posted on this page at its page bottom reader comment box.

Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

Question: short cycling A/C heat pump at 10 second interval

I have a 4yr old american standerd a/c heat pump unit.cycles every 10sec.tech serviced 12/06 had to shut unit down 01/20

I live on a island only way to get here in winter is by plane. Expensive! I put gauge on high side,unit off temp out side 20deg f read 50#psi - Ed.

Reply:

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 the information given 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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