How to Charge by Subcooling

Michael E Carpenter

The correct charge of an air conditioner requires the right amount of refrigerant to be placed into the system. Having too much or too little refrigerant in the system results in higher utility costs, shorter compressor life and reduced efficiency.

The subcooling method of charging determines if the system needs more or less refrigerant in the system.

Many air conditioning systems utilize a high efficiency system that utilizes thermostatic expansion valves, or TXV, which require subcooling to properly charge the system. If you are unfamiliar with working with HVAC systems, contact a licensed contractor to check the level of refrigerant in your system.

  1. Find the required subcooling temperature of the air conditioning system. This information is listed in the owner's manual and may also be located on the nameplate attached directly to the condenser. Most system temperatures are between 10 and 15 degrees Fahrenheit. This is the difference in temperature the refrigerant should lose between entering and leaving the condenser coil. The manual or nameplate will also provide if the system is a R-22, R-410A or R-404A condenser.

  2. Attach the HVAC manifold gauges to the air conditioning unit. Ensure the gauges have a tight fit and the readings are accurate. Small mistakes can result in the calculations being inaccurate and worthless.

  3. Note the liquid pressure as the liquid going into the condenser pressure reading. Many gauges will provide various readings for R-22, R-410A and R-404A. Make certain your reading is in line with the type of condenser you have.

  4. Convert the pressure reading into temperature using a pressure temperature chart, also called a P/T chart. Make sure the pressure reading and conversion table match for the type of condenser you have. For example, a pressure reading of 232 PSIG (pounds per square inch gauge) on a R-22 results in a temperature of 112 degrees Fahrenheit.

  5. Subtract the manufacturer's required subcooling from the temperature reading. For example, if the subcooling is 10 degrees Fahrenheit and the pressure conversion was 112 degrees Fahrenheit, the result is 102 degrees Fahrenheit. This is the temperature the refrigerant should be as it leaves the condenser.

  6. Attach a digital thermometer to the liquid line leaving the condenser and write down the reading. Compare the reading to the temperature result from Step 5. If the temperatures match within 3 degrees, the system is properly charged.

  7. Add or remove refrigerant from the system to properly charge the unit. Add refrigerant if the temperature in Step 6 was too high, to bring the temperature of the liquid down. Remove refrigerant from the system if the temperature was too low, to increase the temperature of the refrigerant.