Fixed-Orifice Charge Check With Low Airflow
A 2.5-ton R-410A heat pump with a fixed orifice at the indoor coil is not keeping up on a 95°F day. Indoor dry bulb is 75°F and wet bulb is 67°F; outdoor ambient is 95°F. The technician measures suction pressure 118 psig, suction-line temperature 61°F, discharge pressure 330 psig, and liquid-line temperature 100°F. The unit has a new filter and a clean outdoor coil. Airflow measured with a flow hood is 850 CFM. External static pressure is 0.80 in. wc. Supply-air minus return-air temperature difference is 23°F. The manufacturer's fixed-orifice superheat chart for 67°F indoor wet bulb and 95°F outdoor ambient calls for 10°F superheat. The system has no receiver and the line set is 25 ft. The homeowner reports that the system was charged last summer and worked until the filter was changed to a high-MERV filter two months ago.
1. Calculate the superheat and subcooling. Does the system appear undercharged, overcharged, or restricted? Explain.
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R-410A at 118 psig saturates near 40°F; with a 61°F suction line, superheat is about 21°F. R-410A at 330 psig saturates near 102°F; with a 100°F liquid line, subcooling is about 2°F. The target superheat is 10°F, so the system is running 11°F above target, and the low subcooling supports an undercharge. However, the airflow is only 850 CFM for a 2.5-ton unit, or 340 CFM/ton, and the high-MERV filter has raised static pressure. Low airflow reduces the indoor coil load and can raise superheat even with a correct charge. The correct diagnosis is not to add refrigerant yet; restore airflow first, then recheck superheat.
2. How does the high-MERV filter and measured airflow affect the superheat reading and the charging decision?
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The high-MERV filter increased external static pressure and reduced airflow. With less air across the indoor coil, the coil absorbs less heat, suction pressure falls, and superheat rises. If the technician charges to the 10°F target under low airflow, the system will be overcharged when the airflow is later corrected. The 23°F indoor temperature difference also suggests low airflow rather than a normal 18–20°F split. Replace the restrictive filter with the manufacturer's recommended MERV rating, check the return duct and blower speed, and bring airflow to about 1000 CFM before making a charge decision.
3. Outline the sequence to correct the system and verify efficiency.
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First correct airflow: install the correct filter, verify return static, and set the blower tap to deliver about 400 CFM/ton. Then recover and weigh in the charge, or charge slowly by superheat to the 10°F target, allowing 15–20 minutes to stabilize between adjustments. Verify subcooling, suction and discharge pressures, compressor amps, and the indoor temperature split. Confirm the outdoor unit is clean and the line set is not kinked. Record the final airflow, static, superheat, subcooling, and amp draw. If the system still cannot reach target superheat with correct airflow, leak-check the coil and line set before adding more refrigerant.