New Heat Pump Runs Constantly and Does Not Dehumidify
A newly installed 3-ton variable-speed heat pump is being commissioned in cooling mode. Outdoor ambient is 95°F; indoor return air is 75°F dry bulb and 63°F wet bulb. The air handler is set for 1,200 cfm. Measurements: suction pressure 112 psig, liquid line pressure 365 psig, suction line temperature 58°F, liquid line temperature 102°F, supply air 61°F, external static pressure 0.92 in. w.c. The installing contractor added 12 oz of R-410A because the line set is 60 ft, but the nameplate charge is for 25 ft and the manual calls for 0.6 oz/ft for 3/8 in. liquid line. The customer complains that the system “runs constantly and doesn't dehumidify.” The unit has a TXV and a variable-speed blower.
1. Calculate superheat and subcooling, and compare the measured charge with the required charge.
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R-410A at 112 psig has a saturation temperature near 37°F, so superheat = 58 − 37 = 21°F. At 365 psig, saturation is about 108°F, so subcooling = 108 − 102 = 6°F. For a TXV system at 95°F, the target is typically 8–12°F subcooling; 6°F is low. Superheat of 21°F is high.
Low subcooling plus high superheat points to an undercharge or a liquid-line restriction, not an overcharge. The required additional charge is 60 − 25 = 35 ft × 0.6 = 21 oz. The contractor added only 12 oz, so the system is about 9 oz short by the length calculation alone.
The 0.92 in. w.c. external static is also above a typical 0.80 in. w.c. limit, so actual airflow is probably below 1,200 cfm, which raises superheat and lowers capacity.
2. What is the most likely cause of the complaint, and in what order should the corrections be made?
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Do not simply add 9 oz and leave. First correct or at least measure airflow. At 0.92 in. w.c., the blower is off its curve; verify the filter, coil, and duct sizing, and set the blower to the correct tap or correct the duct restriction. Low airflow raises superheat and makes the charge look low, and it also explains the “runs constantly and doesn't dehumidify” complaint.
After airflow is corrected, check for a liquid-line restriction by measuring temperature drop across the filter drier and at the TXV inlet; a restriction causes low subcooling downstream and low suction. If no restriction is found, weigh in the remaining 9 oz (21 − 12) and verify subcooling at the target. Then check delta T and amp draw. If subcooling is still low after the full weigh-in, recover and weigh the total charge; do not guess.
3. What commissioning data should be recorded and what should the homeowner be told?
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Record the final airflow in cfm, external static, return and supply dry/wet bulb, delta T, suction and liquid pressures, superheat, subcooling, compressor and fan amps, and the exact charge added. Document the line-set length and size, the filter drier replacement, the evacuation micron result, and the outdoor ambient.
Give the homeowner a short explanation: the system was undercharged and the duct static is high. The immediate correction is charge and airflow, but the long-term fix may be duct work. Set the thermostat configuration for the correct equipment type and stages, and verify auxiliary heat lockout and defrost operation. A commissioning sheet with these numbers is also the baseline for the next service call.