Arkansas HVACR NewsMagazine May 2025 Issue

HVACR NewsMagazine May 2025

Tech News

Subcool Charging

Note: This is only a basic guide for charging. There are innumerable conditions that are not related to the refrigerant charge but can still alter refrigerant pressures, superheat, subcool, and saturation. This guide is not intended to cover the complete diagnosis of the refrigerant circuit. To charge a system using superheat, you will need to monitor the actual temperature of the low-pressure suction line, the saturation temperature of the low-side suction gauge, and the indoor and outdoor temperatures entering the unit(s). Most, if not all, manufacturers have a charging chart available with their respective units. With the information you have gathered on indoor and outdoor temperatures, you can calculate the recommended superheat. In a pinch, you can also use a superheat calculator, such as the Trane superheat calculator, a free app like our superheat calculator (top left), or the MeasureQuick app. A good calculator will require that you determine the wet-bulb temperature in the return air stream. Without a sling, digital psychrometer, or hygrometer, you will not be able to determine wet-bulb temperature. Once you know the target superheat, you can adjust the system charge to hit it. Let's say the recommended superheat was 18 degrees; you would add/remove refrigerant to the system until the actual temperature of the suction line was 18 degrees above the indicated saturation temperature from your low-pressure gauge. Adding charge will decrease the superheat, and recovering refrigerant will increase the superheat. Superheat Charging

To charge a system using subcool, you will need to monitor the actual temperature of the liquid line and indicated saturation temperature on the high-pressure gauge. Information on the entering temperatures is not necessary to charge the unit by the subcooling method. Most manufacturers have recommended subcool charging information with the unit if it is designed for a TXV (TEV). If there is no information on the unit for some reason, or if it has worn off, you can set a typical residential air conditioner charge to 10 to 12 degrees of subcooling, which is a relatively safe range to use.

Let's say, for example, the manufacturer's recommended subcool is 14 degrees. You would add enough refrigerant to the system so that the actual temperature of the liquid line was 14 degrees less than the saturation temperature, as indicated on the high pressure gauge for that particular refrigerant.