Commissioning a New 3-Ton Heat Pump in 45°F Weather
A contractor installs a new 3-ton R-410A split heat pump with a matched indoor coil, a 15 kW electric strip heater, and a 50-ft line set. The outdoor unit is on a pad at ground level; the indoor coil is in an upflow air handler in a closet. The 3/8-in. liquid line and 7/8-in. vapor line are new, with a filter drier at the outdoor unit. Factory charge is 12 lb for 15 ft of line set; the manufacturer says to add 0.6 oz of R-410A per foot of 3/8-in. liquid line beyond 15 ft. Outdoor ambient is 45°F, indoor dry bulb is 68°F, and the installer wants to verify charge before leaving. Measured external static pressure is 0.90 in. wc (return 0.32, supply 0.58); the air handler table shows a maximum of 0.50 in. wc at 1200 CFM. A flow hood reads 1020 CFM. Supply-air minus return-air temperature difference is 24°F. The manufacturer's cooling charging chart requires 70°F outdoor ambient or higher.
1. Calculate the additional refrigerant required and state the charge method you would use at 45°F outdoor ambient. Show your work.
Read the worked answer
Additional length = 50 - 15 = 35 ft. Additional charge = 35 × 0.6 oz = 21 oz, or 1 lb 5 oz. Total charge = 12 lb + 1 lb 5 oz = 13 lb 5 oz. At 45°F outdoor ambient, the cooling charging chart does not apply; the unit cannot be run in cooling long enough to stabilize without low head pressure and unreliable superheat or subcooling. Use the weigh-in method: recover the factory charge if necessary, weigh in the calculated total, or use the manufacturer's heating-mode charging chart if one is provided. Do not add refrigerant by gauge pressure alone. Record the final weight and line-set length on the commissioning label.
2. Evaluate the airflow and static pressure. Is the system ready for charge verification? What is the likely cause and what would you do?
Read the worked answer
1020 CFM ÷ 3 tons = 340 CFM/ton, below the usual 350–400 CFM/ton target. The external static pressure is 0.90 in. wc against a 0.50 in. wc blower rating, so the blower is operating far outside its table. The 24°F temperature difference is also high, consistent with low airflow. Likely causes are a restrictive high-MERV filter, undersized return duct, collapsed flex duct, dirty indoor coil, or the blower set to a low speed tap. Correct airflow before charging; otherwise the low load will distort superheat and the technician may overcharge the system. Check the filter and duct static first, then set the blower to the correct tap and re-measure to about 1200 CFM.
3. After correcting airflow to 1200 CFM, what commissioning checks would you perform before calling the job complete?
Read the worked answer
After airflow is corrected, verify the final charge by weight or the approved heating-mode method; then check the indoor temperature split (roughly 18–22°F), external static below the blower rating, and blower current. Confirm the 15 kW strip heater amperage and voltage balance, defrost initiation and termination, reversing-valve operation, crankcase heater, condensate disposal, and line-set insulation. Leak-check the brazed joints and flare connections. Record all readings and the final weighed charge. If the outdoor ambient is still below the cooling chart range, leave the verified weigh-in as the charge record and schedule a cooling check when weather permits.