Heat Pump Defrost Complaint in Mild Weather
A 3-ton air-source heat pump with a 10 kW electric auxiliary strip heat package is in a 1,800 sq ft house. At 40°F outdoor and 68°F indoor, the customer complains of cool drafts and high electric bills. The unit defrosts every 45 minutes and the outdoor coil is partially frosted at the bottom. In heating mode, supply air is 88°F and return is 68°F. Suction pressure is 50 psig and liquid pressure is 250 psig. The reversing valve shifts properly. The defrost board is set for a 45-minute interval and 50°F coil termination. The outdoor thermistor reads 58°F when the actual outdoor temperature is 40°F. The auxiliary heat sequencer is energized during defrost and remains on for 10 minutes after defrost ends. The indoor airflow is 1,050 cfm. The homeowner says the problem began after the defrost board was replaced last winter. You need to determine whether the complaint is caused by defrost control, charge, airflow, or auxiliary heat staging.
1. Evaluate the defrost thermistor reading and the aux heat sequence. What is likely wrong?
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The thermistor reads 58°F when the outdoor air is 40°F. That 18°F error is not normal; the sensor or its resistance curve is wrong, or the board is set for the wrong sensor type. The board terminates defrost at 50°F coil temperature, so a false-high sensor can terminate defrost early, leave frost on the coil, and cause frequent defrosts. Aux heat staying on 10 minutes after defrost is excessive; many boards have a fixed 1-3 minute delay or stage aux based on outdoor temperature and time. A stuck sequencer or a board configured for a long aux delay explains the cool drafts and high electric bills. The partial frost at the bottom of the coil can also come from a low charge or poor airflow, so those must be checked.
2. Calculate temperature rise and explain what it indicates. Does it prove the charge is low?
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Temperature rise = 88°F - 68°F = 20°F. At 1,050 cfm, sensible capacity is roughly 1.08 x cfm x ΔT = 1.08 x 1,050 x 20 ≈ 22,700 Btu/h. That is plausible for a 3-ton heat pump at 40°F without aux heat, but low if the 10 kW aux is energized during the reading. A 20°F rise does not prove low charge because heat-pump capacity and temperature rise depend on outdoor temperature, airflow, and aux staging. Low suction pressure at 40°F can indicate low charge, but it can also result from a frosted coil, low outdoor airflow, or a metering problem. Charge must be verified by weight or by the manufacturer's heating-mode subcooling method, not by suction pressure alone.
3. List the tests and corrections you would perform, including aux heat staging.
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Tests: (1) compare thermistor resistance with the manufacturer's temperature/resistance chart at the actual outdoor temperature; (2) verify defrost board pins and sensor type; (3) check the defrost termination and initiation sequence with a forced defrost; (4) measure aux heat current and sequencer timing; (5) verify indoor airflow and outdoor fan operation; (6) check charge by weight or approved subcooling method. Corrections: replace the faulty outdoor sensor, set the board to the correct defrost interval and termination, replace a stuck sequencer or correct the aux heat delay, and correct airflow if low. Do not simply add refrigerant or disconnect aux heat; both could create comfort or capacity problems. After repair, recheck temperature rise, defrost timing, and supply air during and after defrost.