New R-410A System Commissioning: Charge, Superheat, and Vacuum
A new 10-ton R-410A split refrigeration system is being commissioned for a produce room. The line set is 80 ft of 1/2-inch liquid line and 1-5/8-inch suction, with a 25-ft vertical rise. Factory charge is 15.0 lb. The manufacturer’s additional-charge table calls for 0.60 oz per foot of 1/2-inch liquid line. After pressure-testing with dry nitrogen at 450 psig and evacuating to 480 microns, the technician weighs in the factory charge plus the calculated additional charge. At 95°F ambient and full load, the measured liquid pressure is 360 psig, liquid-line temperature at the condenser outlet is 92°F, suction pressure is 120 psig, and suction-line temperature at the evaporator outlet is 62°F. The sight glass is clear. The manufacturer specifies 10°F subcooling and 10-12°F evaporator superheat.
1. Calculate the additional charge and the total charge to weigh in.
Read the worked answer
Additional charge = 80 ft x 0.60 oz/ft = 48 oz = 3.0 lb. Total = 15.0 + 3.0 = 18.0 lb. Weigh this in; do not use the sight glass as the primary charge method.
2. Calculate subcooling and superheat from the readings. Do they meet specification, and what would you check before recovering refrigerant?
Read the worked answer
At 360 psig, R-410A saturation is about 108-109°F; subcooling = 108.5 - 92 = about 16.5-17°F, above the 10°F target. At 120 psig, saturation is about 41°F; superheat = 62 - 41 = about 21°F, above the 10-12°F target.
That combination matters: high subcooling with high superheat is not a simple overcharge. A restriction downstream of the condenser outlet, such as a partially blocked filter-drier, a closed or partly closed solenoid, a kinked liquid line, or a plugged TXV inlet screen, can back liquid up in the condenser (raising subcooling) while starving the evaporator (raising superheat). Before recovering refrigerant, check the temperature drop across the filter-drier, solenoid, and TXV screen; inspect the 25-ft vertical liquid line for kinks; verify gauge calibration and that both readings are taken at the same points. If no restriction is found and the unit is at full load, recover charge in small increments to the 10°F target.
3. The system reached 480 microns. Is that sufficient, and what else must be documented?
Read the worked answer
480 microns is generally acceptable if the system is clean and dry, but the number alone is not the test. Isolate the pump and perform a standing vacuum test: the micron gauge should remain below the manufacturer's limit (commonly 500 microns, with only a small rise allowed) for 10-15 minutes. A rapid rise indicates a leak or moisture; a slow rise can indicate moisture. Use a micron gauge at the farthest point, not manifold gauges. If moisture is suspected, use nitrogen breaks and triple evacuation. Document the pressure test, vacuum hold, final charge weight, subcooling, superheat, and ambient/box temperatures.