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How to add or replace air conditioner or heat pump refrigerant:
This article describes the procedures for charging an air conditioner, heat pump, refrigerator, freezer, or similar equipment - how does an HVAC service tech put the proper amount of refrigerant into the system?
We describe use of a traditional charging cylinder, vaporizing connectors for low-side charging, and modern refrigerant recovery, charging, and vacuum equipment.
We discuss the following: Procedure for evacuating or "pulling a vacuum" on air conditioners & heat pumps or refrigeration equipment, Procedure for Charging the HVAC System or Appliance with Refrigerant Using a Gauge Set,
Current refrigeration service standards including charging precision and refrigerant recovery, Procedure for filling & using a refrigerant charging cylinder, Robinair® example.
Procedure for using a vaporizing connector for low-side refrigerant charging, Kwik-Charge™ example & Using a refrigerant line sight glass or listening for refrigerant line gurgling to indicate charge level.
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As we've explained, charging an empty system will require evacuating it first. For a typical household equipment this takes about 15 minutes but for a larger commercial refrigeration system this process can take an hour.
On residential equipment we always leave our evacuator pump running for at least 1/2 hour to be sure we've evacuated as much moisture and contaminants as possible.
[Click to enlarge any image]
On commercial systems being installed or having major service it would be preferable to leave the evacuator pump running for 12 hours - the longer you evacuate the system the cleaner it will be.
That's because as the system warms up moisture in the system will vaporize further and thus be removed.
Low side charging is also referred to as vapor charging - that is we allow ONLY refrigerant vapor or gas to enter the system.
This is an easy but time consuming method of system charging. The canister supplying refrigerant is connected to the gauge service port and opened, and the HVAC equipment or appliance is run (which pumps refrigerant gas from the supply source in parallel with pumping from the outlet of the evaporator coil).
The refrigerant canister is kept upright so that only gas leaves the canister.
A step by step example of low-side charging of refrigerant on an older R22 system can be seen
at SPLIT SYSTEM AC / HEAT PUMP REPAIRS
When charging an air conditioner, heat pump, or refrigeration appliance from the high side, the system being serviced is turned OFF. The refrigerant gas canister is placed upside down so that only pure liquid refrigerant leaves the canister.
Note that once you start the system running you will not be able to charge on the high side because the head pressure out of the compressor will be higher than the evaporation pressure in the canister - it would push refrigerant back into the canister.
With critically-charged HVAC or appliance refrigeration systems, such as a household refrigerator or air conditioner, you must measure the refrigerant entering the system.
Typically the technician uses a scale that registers in ounces to measure a weighed charge, though we also used other devices such as a charging cylinder that actually shows the volume of liquid refrigerant in the cylinder on a temperature-compensating scale.
A charging board (or charging cylinder just mentioned) that is connected to the high side can accurately measure the liquid refrigerant charge going into the system.
In this case the charging board (Dial-a-Charge charging cylinder produced by Robinaire, Montpelier OH) is loaded with the proper refrigerant charge from the gas cylinder, and the outlet from the charging board is then connected into the high side (perhaps through the gauge service port).
A Vaporizing Connector is an accessory you can add to a charging cylinder or charging board.
The refrigerant vaporizing connector (such as Imperial's Kwik-Charge, assures that liquid refrigerant passing through the device will convert to gas as it exits the device.
This will let you add a measured refrigerant charge to the low side of the system while still making an accurate measurement of the refrigerant measured in ounces.
Liquid refrigerant charging is always faster than low side vapor charging. On the low side you have to charge, then wait for the system pressures to balance, then reexamine the frost line etc.
An alternative to measuring the refrigerant charge when charging air conditioners or heat pumps
As an alternative to making refrigerant measurements by weight or temperature-corrected volume using a scale or charging cylinder, some HVAC service technicians may adjust the refrigerant charge by watching the low side pressure and the exact location of the frost line at the evaporator coil (blower fan not running).
Watch out: do not overcharge or extend the frost line to the compressor or you risk sending liquid refrigerant into the compressor motor - assuring its damage or destruction.
On small refrigeration systems such as a home refrigerator or window air conditioner the refrigerant charge needs to be accurately measured or the system will not work properly.
But on larger HVACR systems and on commercial units that use a liquid refrigerant receiver (a sort of buffer that stores extra liquid refrigerant), you might find a sight glass on the refrigerant piping downstream from the condensing coil.
Some techs add refrigerant while watching that sight glass, adding refrigerant until the gas bubbles just stop. If you see bubbles there either the system is badly contaminated or more likely the refrigerant charge is short. We warned just above - don't overcharge the system - you can damage the equipment.
Technical Note: if you see bubbles in the sight glass, or if you hear gurgling in the refrigerant lines indeed those can be indicators that the refrigerant charge in the system is low.
Note that a reading of "zero" on these pressure gauges is not really zero, it's 14 psi or 1 atmosphere.
Also if you look closely at your gauges you 'll see that one permits pressure readings in the "negative" direction - used when pulling a vacuum on the system using an evacuator pump - a step necessary before charging a system that has been opened for service or repair.
To remove all moisture or refrigerant from an air conditioner, heat pump, refrigerator, or other refrigeration equipment your evacuator pump needs therefore to pump past "zero" on the gauge (14 psi) to absolute zero or 29.9" Hg vacuum.
The evacuator pump is attached to the center port or "service port" on the gauge set.
SAE J-2788, requires that all service equipment manufactured after December 31, 2007, must recover 95% of the refrigerant and recharge to within 1/2 ounce.
Charging cylinders such as the 1980's vintage equipment described below are often supplanted now (2011) by refrigerant charging scales and by larger, more sophisticated refrigerant charging machines that combine refrigerant charge measurement, refrigerant evacuation and recovery/recycling, and other service functions.
Newer refrigeration service equipment can automatically recover refrigerant (such as R134A) from a system to be serviced, vacuum test and clean the system to prepare it for a charge, leak test the system, and insert the proper refrigerant charge, all automatically.
These improved refrigeration management functions were required by a combination of legislation and standards that stop the discharge of refrigerants into the environment, and by improvements in refrigeration system design that produced equipment that uses a smaller, but more precisely-measured charge of refrigerant.
Quoting from Robinair's description of their current refrigerant management equipment line:
Robinair 34988, 34788 & 34288 recover up to 20% more refrigerant, which means it will cost less to recharge the system. The best charge accuracy that could be claimed by older generation service machines was +/- 1 ounce, a 3% error on a two-pound system.
That same charge accuracy on a 14-ounce system is over twice the error (7%). Early R-134a systems could still provide some cabin cooling when they were 4-6 ounces (12-18%) low on refrigerant.
However, new designs are so efficient, they do not have reserve refrigerant, and charge accuracy is critical. The 34988, 34788 & 34288 will recharge the vehicle to within 1/2 ounce of the charge capacity, and you will avoid the dreaded “come back”. 
While the new refrigeration system testing and charging equipment described above is fully automatic, to understand the problems that the newer equipment has to solve (and automate), it is useful to review how, traditionally, we inserted a precisely measured refrigerant charge into air conditioners and other refrigeration equipment.
We used the Robinair® Dial-a-Charge® charging cylinder for many years as a way to install an accurate charge of refrigerant into air conditioners, heat pumps, and refrigerators and freezers undergoing service or repair.
As an aid to service technicians who may have lost their instruction sheet we include and comment on the charging cylinder instructions here, using the Robinaire® Dial-a-Charge charging cylinder system as an example.
Robinair offered the following explanation of charging cylinder use along with our 1980's vintage equipment.
With an increase in temperature in any cylinder filled with refrigerant, there is a corresponding increase in pressure and a change in the volume of liquid refrigerant in the cylinder.
To measure out an accurate charge by weight from a cylinder using the liquid level in a sight glass as a point of measurement, it is absolutely necessary to compensate for liquid volume variations caused by temperature variations.
These temperature variations are directly related to pressure variations and accurate measurements by weight can be calibrated in relation to pressure.
The Dial-a-Charge Charging Cylinder is designed to meter out a desired amount of a specific refrigerant by weight. Compensation for temperature variations is accomplished by reading the pressure on the gauge of the cylinder and dialing the plastic shroud, with the calibrated chart, to the corresponding pressure reading for the refrigerant being used.
When charging a refrigeration or air-conditioning system with refrigerant, often the pressure in the system reaches a point where it is equal to the pressure in the [charging] cylinder from which the system is being charged.
In order to get more refrigerant into the system to complete the charge, heat must be applied to the cylinder.
Robinair's Heated Dial-a-Charge Charging Cylinder eliminates the problem caused by equalization of pressure between the cylinder and the system being charged. The [Robinaire] charging cylinder has a heating element installed in the base of the cylinder.
The male plug of the heating element can be plugged into any 110-115 volt AC outlet. [Note that if you want to charge the system on the low side using vapor only, you can safely do so using a refrigerant vaporizer attachment instead of heat to assure that no liquid goes where it's not wanted.]
Due to the variety of 220V. receptacles throughout the world, a male plug must be field-added to the 220V heating element in order to fit the particular style in your area [outside the U.S., Canada, Mexico]. Either voltage heating element will work on 50 or 60 Hz power.
Watch out: CAUTION: Heating elements can be destroyed if plugged into an electrical outlet when the [charging] cylinder is empty [of refrigerant].
The heating element is of the correct wattage to increase [refrigerant] pressure sufficiently in a relatively short time, to a level that is above the equalization pressure between the cylinder and the system [being charged]. The higher the pressure in the cylinder, the less time it takes to force the refrigerant into the system.
The heating element should be turned off before the pressure is above the highest pressure on the Dial-a-Charge® shroud of the refrigerant being used.
EXAMPLE: Refrigerant R22 is being used - the pressure at no time should exceed 230 psi.
The Robinair Dial-a-Charge® has a relief valve, for added protection, set to relieve the refrigerant at approximately 320 psi.- 
Carson Dunlop Associates' sketch illustrates a very simple check that can indicate a problem with an air conditioner, heat pump, or other refrigeration equipment: visible gas bubbles seen in the sight glass. The sight glass, if present, is usually located in the condenser unit on the liquid refrigerant line.
Watch out: CAUTION: When working with refrigerants, goggles should always be worn. Contact with refrigerants may cause injury.
As we introduced above at Measuring the Refrigerant Charge, a vaporizing connector is an accessory connected to the refrigerant canister, refrigerant gauge set, to a charging cylinder or charging board to provide a safe, fast way of low-side refrigerant charging without any risk of slugging the air conditioning compressor with liquid refrigerant - an event likely to damage the compressor.
The refrigerant vaporizing connector (such as Imperial's Kwik-Charge illustrated here), converts liquid refrigerant to a gas as the refrigerant flows through the vaporizing device, thus making sure that liquid refrigerant passing through the device will convert to gas as it exits the device.
This will let you add a measured refrigerant charge to the low side of the system while still making an accurate measurement of the refrigerant measured in ounces.
(Aug 28, 2012) Usman ibro said:
Distinquish between vapour charging and liquid charging
If we are charging a refrigeration system from its low-pressure side we ONLY want to send a gas or vapour into the system. That's because sending liquid refrigerant into the suction side of a compressor motor is likely to destroy the motor in seconds - liquids are not very compressible and motor parts or valves are likely to be damaged in most motor designs.
When we are charging a refrigeration system from its high pressure side we can safely introduce liquid refrigerant into the piping or reservoir (the receiver) on that side of the system without damaging it.
Some refrigerant charging systems including one that I used metered the liquid refrigerant into a charging cylinder so that we can see the precise charge volume to be introduced - a measured refrigerant charge - into the refrigeration system.
The charging cylinder uses tables and scales for different refrigerants at different temperatures so that we can make a very accurate refrigerant charge. This is critical for cooling or heat pump systems that do not use a receiver and that must be charged with exactly the refrigerant volume specified by the manufacturer.
In this case we add a vaporizing device like the Kwik Charge - described here - on the outlet of the charging cylinder that converts liquid refrigerant into gas as it exits the charging cylinder and before it enters the HVAC equipment on the low side, thus guaranteeing that we don't damage anything.
[Quoting and paraphrasing], Using the Kwik-Charge™ vaporizing connector provides a safest, faster method of charging fluorinated hydrocarbon refrigerants into the low pressure side of a refrigeration system.
Vapor charging is also within the capacity of this unit, although it is primarily intended for liquid charging - that is, liquid refrigerant is fed into the vaporizing connector, and only refrigerant vapor exits the device.
The Kwik-Charge™ vaporizing connector may remain attached to the low-side port of the charging manifold while performing the usual refrigeration equipment service and diagnostic operations involved in maintaining a refrigeration system, such as pulling a vacuum to clean the system and test it for leaks.
An automatic bypass valve inside of the vaporizing connector permits full flow during any reverse-cycle operation such as pulling a vacuum or removing refrigerant that is already in the system.
As an example for technicians unfamiliar with vaporizing connectors we quote from the Kwik Charge instructions for the device we illustrate here. 
[Quoting and paraphrasing],
This device provides fast, safe refrigerant charging when using tracing dye [or not] with these properties:
Watch out: CAUTION: a minimum hose length of three feet should be maintained between the Kwik-Charge and the low side charging port on the refrigeration system. Be sure to observe all safety practices regarding handling of refrigerants, including the wearing of eye protection.
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