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commercial air conditioning compressor charging gauge set (C) InspectAPedia.com Air Conditioner Compressor & Refrigerant Pressure Readings
Typical HVAC refrigerant pressures & quantities

  • REFRIGERANT PRESSURE READINGS - CONTENTS: What are the Typical Air Conditioner Compressor & Refrigerant Pressure Readings ? A/C Refrigerant Pressure. How to diagnose refrigerant pressure problems; how to determine the proper refrigerant charge quantity. Definitions of HVACR "high side" & "low side" pressures; typical refrigerant pressures; effects of overcharged or under-charged refrigerant
  • POST a QUESTION or READ FAQs about refrigerant pressure readings in air conditioners, heat pumps, & other refrigeration equipment
  • REFERENCES
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Refrigerant charge quantity for air conditioners & heat pumps:

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.

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.



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What are the Typical Air Conditioner or Heat Pump System Pressures During Normal Operation

Photograph of a
commercial air conditioning compressor charging chartMeasuring the refrigerant pressure in air conditioning, heat pump or other refrigerant systems can diagnose a range of operating problems including a refrigerant leak, over charging or under charging.

Refrigerant pressure readings measured at the air conditioning compressor/condenser unit and which are found to be too low on the high pressure side (compressor output) or on the low pressure side (compressor input or suction line) can indicate a problem with the compressor's ability to develop normal operating pressure ranges and thus will affect the cooling capacity of the air conditioning system.

Abnormally high compressor output pressures are possible but less likely. (See Two Basic A/C Refrigerant Pressure Diagnostics below).

Article series contents

Definition of Air Conditioner High Side and Low Side Pressures

First let's explain "low-side" and "high-side" air conditioner compressor motor pressures and what they mean.

Air conditioning service manuals and training refer to:

Temperature Pressure Charts for Common Refrigerants

Example R-410A Refrigerant Pressures

For every refrigerant type you use, you'll want to pick up a Refrigerant Pressure/Temperature chart from your supplier. A typical R-410A refrigerant pressure/temperature chart will give refrigerant pressures at various (ambient) temperatures ranging from below freezing to over 160 degF. Here are a few examples:

Example R-410A Refrigerant Temperature vs Pressure Data

Temperature in DegF Pressure in PSIG
0 48
32.2 102
60.5 172
80 236
101.1 322
150.1 608

Notes:

These psig readings for R-410A are what you'd expect the pressure of the gas to be in an enclosed container at the temperature given and at steady state.

These pressures are not suction or low-side nor compressor output or high-side pressures as those depend on the compressor model and the refrigerant metering device. This operating pressure for R-4140A example was given by ACHRNews in 2002:

... to produce an evaporating temperature of 40 degrees F and a condensing temperature of 115 degrees, the suction and head pressures would be 83 psia and 257 psia in an R-22 system, while they would be 133 and 406 psia, respectively, in an R-410A system. - Checket-Hanks, Barbara A., "Guidelines for Troubleshooting R-410A Systems", ACHR News 16 May 2002, retrieved 2016/08/29 original source: http://www.achrnews.com/articles/87788-guidelines-for-troubleshooting-r-410a-systems

An air conditioner or heat pump using R-410A refrigerant may operate at pressures exceeding 600 psi. A typical Carrier HVACR unit incorporates a high-pressure safety switch that operates at 610 psi and a low pressure (loss of refrigerant detection safety device) that operates at about 20 psi. Low pressure refrigerant switches on Carrier HVACR typically open at 50 psi and close at 100 psi. - Prah, Frank, CMS, "Refrigerant 410A", [PDF] Refrigeration Service Engineers Society, 1666 Rand Road, Des Plaines IL 60016 USA, Tel: 847-297-6464, retrieved 2016/08/29, original source: https://www.rses.org/assets/r410a/R-410A.PDF

Both older R-22 and R-134 operate at lower pressures than R-410A.

Watch out: Excerpts from Carrier’s Puron® Quick Reference Guide

“When charging 410A (liquid refrigerant only), use a commercial-type metering device in the manifold hose when charging into the suction line with the compressor operating.”
“Manifold sets should be a minimum 700 psig on the high side and minimum 180 psig low side, with 550-psig low-sided retard. Use hoses with a minimum 700-psig service pressure rating.”
“Be sure that servicing equipment and replacement components are designed to operate with [410A].” The recovery cylinder service pressure rating must be 400 psig, DOT 4BA400, or DOT 4BW400.

 

 

Reader Question: what is the pressure in an A/C or heat pump system when the compressor is off?

Hi, I always wondered, car or house A/C, if it is off, compressor off, is there any pressure in the system? Or does the compressor cause and make the pressure? Thanks, J.K [email 26 Sept 2015]

I would assume less pressure in system when compressor is off then when on? Thanks, John Kelly. PS: Just had new condenser / compressor unit put in my house and when running, see small drip on lower line seems to be diluted oil. Possibly just condensation mixed with oil that got on while repairing. It dont seem to get that small drop when I dont run it. Its on the bottom of that weld spot.

Reply:

Absolutely there is pressure in the air conditioning or heat pump system when the compressor is not running.

When the compressor shuts off the high side pressure is high - say 200 and the low side low - say 20 (these are just conceptual as actual pressure depends on the compressor, ambient temperature, refrigerant type).

Then when the motor stays off for a time - say 1/2 hour more or less - *normally* pressure in the whole system equalizes to a lower number which would be the pressure that the particular refrigerant would sit at for the current ambient temperature (at sea level).

E.g. R22 at 80 degF will stabilize at 143.7 psi in the system while R410 would be at 235.9 psi.

Watch out: about that OIL DRIP: is bad if it's really leaking. There is oil in the refrigeration system (to lube the compressor); if there is oil leaking out at a fitting on the refrigerant line then there is most likely also an actual refrigerant leak.

If you clean the joint - wipe it clean of oil and condensation - you should not see more oil there. Condensate yes.

Typical HVAC Refrigerant Pressures

Photograph of a
commercial air conditioning compressor charging chart

Typical residential air conditioning refrigerant pressures vary depending on the model, compressor motor size and design, and the refrigerant used. The design pressures may be provided on labels attached to the equipment but the actual air conditioner operating pressure will vary in part as a function of the incoming air temperatures.

"Charging Charts" (such as the commercial unit charging chart shown here) are provided in service manuals to determine the target suction vacuum (negative) pressure and output pressure for a given compressor motor.

Use of the charging chart for the specific compressor is the correct way to service it. The following example pressures are based on "rules of thumb" that get you in the right "ballpark" if no charging chart is at hand.

Example actual air conditioner compressor high side output pressure: using R-22 refrigerant and assuming an outside air temperature of 85 degF called for 120 degF. inside the compressor (add 35 degF. to incoming air temperature) and an output high-side compressor pressure of about 260 psi.

Example of actual air conditioner low side input or suction line pressure during operation (low-side pressure) during normal operation of the same compressor model and refrigerant and the same outdoor air temperature of 85 degF called for 45 degF. temperature entering the compressor (subtract 40 degF. from incoming air temperature) which on the service chart indicates that the incoming or suction line pressure would be about 75 psi.

Example of a more theoretical air conditioner or heat pump pressure and temperature at the compressor and at the cap tube or thermostatic expansion valve during normal operation: at an outdoor temperature of 72 degF, liquid refrigerant (R12 for example) leaving the outdoor condensing coil and entering the cap tube or TEV might be at 100 psi and 95 degF.

These numbers vary by changes in ambient temperature, compressor model, and refrigerant gas used. On the low side of the TEV or cap tube (in the cooling coil in the air handler) where the liquid refrigerant is changing state to a gas, it may be cooled down to 10 degF. and by the time the refrigerant leaves the cooing coil (evaporator coil) and gets back to the compressor motor it will be all vapor and may be at just 15 psi. [R12 refrigerant changes from liquid to vapor at 14.6 psi at 10 degF.

Air Conditioner or Heat Pump Refrigerant Equalization Pressure - System-OFF refrigerant pressures

When you measure heat pump or cooling system pressures makes as much difference as where you measure it. When an air conditioning or heat pump system has turned off and been off for some time (30 minutes or more) pressures equalize throughout the system between the high and low sides.

At that point the refrigerant pressure in both the high side and low side of the air conditioner or heat pump system will be in accordance with the ambient air temperature and the properties of the particular refrigerant gas present.

The static or equalized system refrigerant pressure will be defined by the refrigerant gas type (which defines its boiling point and pressure at various temperatures).

For example with that cute old R12 refrigerant, as long as there is just about any refrigerant in the system - enough so that there is some liquid refrigerant, i.e. it's not all just gas) then in equalized condition at 70 psi ambient temperature the refrigerant pressure will be 70 psi.

With a temperature correction chart you can read the static or equalized refrigerant pressure for any refrigerant gas and the actual ambient temperature.

Reminder: this refrigerant gas behavior means that if you use pressure test gauges (GAUGE, REFRIGERATION PRESSURE TEST) to measure the refrigerant pressure in the static or equalized air conditioning or heat pump system, the gauges only tell you the refrigerant pressure, not the quantity of refrigerant that is present in the system.

Question: refrigerant pressure differences in an inverter-type air conditioenr system?

2016/08/29 Y.W. asked [ by private email to editor ]

Hi, firstly I want to thank you for creating such a helpful site - keep it up!
I just have a few questions regarding inverter A/Cs - In a split-type home air conditioner which use an inverter compressor, does this effect the PSI needed on either the high or low side of the system? If so how do I know what it is supposed to be? Is the method for adding refrigerant different for inverters than a standard system? and is there a way to know at any point in time if the compressor is running at full speed or only partly?
Thank you

Reply:

The psi you'd expect to see in any HVACR system is a function of:

1. the specific refrigerant used
2. the ambient temperature
3. the compressor having reached steady state or run long enough that the low side and high side pressures are being controlled by the refrigerant metering device such as the TEV

For any specific refrigerant you'll find a chart that gives the relationship of refrigerant pressures to temperature - as the tech has to consider that when charging the system.

The data tag and specs for a given compressor model and brand will also often give the standard working pressure range.

For other readers, an "inverter" in an air conditioning system is a variable speed control that changes the pumping speed of the AC compressor motor in response to the temperature in the conditioned space. By varying the compressor speed we vary the rate at which refrigerant is dispensed into the cooling system. In my OPINION this is a sophistication of an air conditioning design that relies only on the refrigerant metering device (TEV or cap tube) to dispense refrigerant into the cooling system.

Inverter designs do not only rely on varying the compressor motor speed. Motor on/off cycling, configurations that bypass refrigerant hot gas output from the high-side over to the low side or suction side of the system, mechanically operated refrigerant control valves, and other approaches allow for an inverter type air conditioner or heat pump design.

The purpose of an inverter type air conditioner system is to match the compressor's output to variations in the cooling load on the system, presumably producing a more efficient cooling system.

These systems may also be described as VRF or variable refrigerant flow designs.

I've researched this question further but not yet found specifiations indicating that the refrigerant operating pressures are different with an inverter or VRF air conditioning or heat pump design. I have found warnings that lower refrigerant velocity can result in lubrication problems in the compressor and a reduced compressor life.

Heatcraft warns:

Compressor Operating Limits:
Compressors should not be operated outside of the operating limits shown on the published compressor specification sheets, and low pressure controls should not be set below the minimum settings without prior written approval of the Copeland Application Engineering Department.
Operating below the allowable minimum suction pressure may result in excessive discharge temperatures which can cause cylinder and valve damage, and may result in lubrication problems. Since high discharge temperatures can occur even though the motor is cool, the motor thermostat will not protect the compressor against these conditions. Operation of motor-compressors beyond the established recommended operating limits will be considered misuse and abuse, and damage may result.

- source: Heatcraft, "Why and How we Use Capacity Control " [PDF], Heatcraft Refrigeration Products – North America, 2175 West Park Place Blvd. Stone Mountain, GA 30087
United States of America Tel: (770) 465-5600 Toll-Free: (800)-321-1881 hrpd.feedback@heatcraftrpd.com retrieved 2016/08/29, original source: www.heatcraftrpd.com/res/pdfs/faqs/Capacity_Control.pdf
This company has offices world wide.

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.

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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Continue reading at REFRIGERANT PRESSURE DIAGNOSIS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see REFRIGERANT PROBLEM TYPES

Or see FROST BUILD-UP on AIR CONDITIONER COILS

Or see REFRIGERANT CHARGING PROCEDURE

Or see LOST COOLING CAPACITY

Or see these

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