by Phil Rains
An R-410A air conditioner’s ability to operate as designed is dependent upon the amount of refrigerant it contains. U.S. Environmental Protection Agency (EPA) studies suggest that approximately 75 % of installed air conditioners possibly have incorrect refrigerant levels, which can reduce system capacity and efficiency by 20 percent or more.
The level of refrigerant charge is unique to each R-410A air conditioner and is determined by every component, including the outdoor coil and compressor, the indoor coil, and the refrigeration lines that carry the refrigerant between the coils. Correct refrigerant charge and proper refrigerant line sizing protect the compressor from damage, ensure efficiency, and improve performance. You should always verify the refrigerant charge for proper installation of an R - 410A air conditioner.
R-410A air conditioners should be leak-checked during the installation and during each service call. Most R-410A air conditioners are charged with refrigerant at the factory, and are seldom incorrectly charged. R-410A air conditioners that have the correct refrigerant charge and airflow typically perform very close to manufacturers listed cooling efficiencies. Over-charging or under-charging refrigerant however, reduces R-410A air conditioner performance and efficiency.
For satisfactory performance and efficiency, an R-410A air conditioner should be within a few ounces of the correct charge, specified by the manufacturer. You must measure airflow prior to checking refrigerant charge because the refrigerant measurements aren't accurate unless air flow is correct. Several simple methods have been utilized in the field for years by technicians to estimate the Cubic Feet per Minute (CFM) per Ton crossing the evaporator. They include velocity multiplied by area for registers and grilles, and pressure drop across a system or coil. These particular types are useful when you check for airflow across an evaporator during cooling and you should become familiar with these in lieu of simply guessing. Airflow should be between 350 and 450 CFM/Ton per Air-Conditioning, Heating and Refrigeration Institute (AHRI) standards.
When the charge is correct, specific refrigerant temperatures and pressures listed by the manufacturer will match temperatures and pressures measured in the field. Always verify these measurements. If the manufacturer's temperatures and pressures don't match the measured ones, refrigerant should be added or recovered, according to standards specified by the EPA.
R-410A air conditioners charged with refrigerant at the factory are shipped with the refrigerant charge typically noted on the unit nameplate. This charge is for a typical application of between 15 to 25 feet of equivalent line length, depending on the particular manufacturer. Occasionally, you may have to field charge or adjust charge when servicing existing systems. The best method to insure that the R-410A air conditioner is properly charged is by weighing in the amount of refrigerant specified on the outdoor sections nameplate, or per installation and operation manuals. Many contractors and technicians utilize the superheat method (for orifices/pistons), and the sub cooling method (for TXVs) when charging in the field.
R-410A air conditioners installed with more than 25 feet of refrigerant line should be charged following the charging method described in the installation and operation "long-line application" instructions provided by the particular manufacturer, or the superheat method, and/or sub cooling method, as necessary. No additional refrigerant oil charge is usually required for these applications.
Many field variations exist which may affect the operating temperature and pressure readings of an R-410A air conditioner. Some R-410A air conditioners utilize fixed orifice refrigerant control devices prior to the evaporator. The following procedure is for this type of refrigerant control device:
1. Check the condition of coils, blower wheels, and the blower motor speed. Measure airflow. The airflow calculation is very important because it helps you determine evaporator load, and therefore will have a significant effect on system pressures. Correct airflow if necessary prior to performing this check.
2. With both valves fully open, connect a set of manifold gauges to the valves' service ports, being careful to purge the lines.
3. Allow the system to operate at least 10 minutes or until the pressures stabilize.
4. Temporarily install a thermometer on the suction (large) line near the condensing unit's service valve. Make sure that there is good contact.
5. Determine the systems superheat as follows:
a. Read the system's suction pressure on the compound gauge.
b. Using the compound gauge (or a P/T chart) determine the system's saturated
c. Read the suction line temperature with a temperature measurement device.
d. Superheat = the suction line temperature - the saturated liquid temperature.
6. Adjust the charge as necessary to meet the manufacturer’s requirements by adding refrigerant to lower the superheat, or recovering refrigerant to raise the superheat. Superheat charts are provided by manufacturers.
Most R-410A air conditioners manufactured today are equipped with a TXV prior to the evaporator. You can’t check a unit’s charge by using the previous Superheat Charging Method if this is the case. You must use the Sub-Cooling Charging Method.
TXVs control refrigerant flow by maintaining a constant superheat (for instance, 8ºF to 10ºF). With constant superheat values, the condition of the system charge cannot be determined using superheat. You must look to the condenser and the liquid side to verify proper charge. The following procedure is for this type of refrigerant control device:
1. Check the condition of coils, blower wheels, and the blower motor speed. Measure airflow by using the temperature rise method. Check pressure drop across coils using the manufacturer’s coil specification sheets. Or, use the velocity pressure to calculate airflow. The airflow calculation is very important because it helps you determine evaporator load, and therefore will have a significant effect on system pressures.
2. Check the system operating pressures. Connect the hoses from your manifold gauge set to the pressure taps on the liquid and suction service valves. Measure and record the liquid (discharge) and suction pressures.
3. Measure and record the outdoor ambient temperature.
4. Measure the wet bulb and dry bulb of the air entering the indoor unit in the return duct. This step is very important because it also helps you determine the evaporator load, and therefore will have a significant effect on system pressures.
5. Measure the liquid-line temperature so that sub cooling can be calculated. Use a good thermometer with a probe that can be strapped tightly to the line. Install the probe on the liquid line about 6-in. from the liquid service valve, then measure and record the liquid-line temperature.
6. Measure the high side pressure at the liquid-line service valve pressure tap. Using the discharge gauge (or a P/T chart) convert high side pressure to saturation temperature. Then simply subtract the liquid-line temperature from the saturation temperature of the refrigerant in the condenser to determine the sub cooling value.
Always refer to manufacturer’s data sheets to find the proper operating pressures for the conditions of the air that you’ve measured. Do the same for required sub cooling levels. Some manufacturers have tested their systems in laboratories and developed specific sub cooling requirements. If this has been accomplished, the sub cooling target will typically be visible on the label on the condenser. You should attempt to stay within 3 degrees of the target sub cooling.
If sub cooling is too low, there may be an insufficient amount of refrigerant. Add refrigerant as necessary.
If sub cooling is too high, there may be too much refrigerant in the outdoor coil. Recover refrigerant as necessary.
Zeotropic refrigerants like R-410A must be charged as a liquid from a canister due the possibility of fractionation of the blend of refrigerants it contains. You must consider its temperature glide, which refers to the range of temperatures at which components in a blended refrigerant boil or condense at a given pressures. R-410A’s temperature glide is < .3 º F, making it a near-azeotropic refrigerant mixture. Liquid charging is much faster than vapor due to the density of liquid refrigerant. R-410A must be “liquid charged” into the high side of the system if it is empty, so the components in the blend do not separate. Charging by weight is the preferred method of admitting the liquid charge. If you are “topping off” the charge, it is necessary to charge R-410A refrigerant into the low side of an operating system. You will need to invert most R-410A refrigerant cylinders, as most do not have dip tubes anymore. This will allow liquid refrigerant to flow freely from the cylinder. Connect the service hose to a commercially available throttling device and then to the suction service valve to charge the system. Using the throttling device is recommended, but some technicians have simply “hand throttled” the refrigerant into the system with the low side valve, with very little problem. Either way, you must avoid the possibility of fractionation. Pressures are 50% to 70% higher within an R - 410A air conditioner than what you were used to finding with R-22 refrigerant air conditioners. Always practice safe procedures when working with R-410A refrigerant. For more information about our R-410 online course, Click Here.
Copyright © Phil Rains
About the Author: Phil Rains is Master Trainer/Technical Developer for HVACReducation.net. He has over 35 years of HVAC and Refrigeration experience in installation, service, and training. He is NATE-certified in 5 areas, a member of ASHRAE and RSES, and ACCA EPIC-Certified in Residential and Commercial Design. He also holds a Universal Classification in EPA 608.