No Heat After an Outage: Stuck Reversing Valve or Failed Compressor?
A 3-ton R-410A split heat pump, installed in 2019, serves a single-family house. After a six-hour utility outage, the homeowner reports that the outdoor unit runs but the house gets no heat. Outdoor ambient is 38°F; the thermostat is calling for heat; the indoor blower runs continuously. The technician finds the outdoor fan running, the compressor humming and tripping its internal overload after about 20 seconds, and both the vapor and liquid service valves warm to the touch. Suction pressure is 115 psig and discharge pressure is 225 psig; the two pressures equalize within a minute after shutdown. Compressor RLA is 16.5 A; measured running current before trip is 8.9 A. The reversing valve solenoid has 24 VAC at its coil. The defrost board is not in defrost, and the outdoor coil is clean. The homeowner says the system heated normally before the outage.
1. What do the nearly equal pressures, warm service valves, and low compressor current indicate about the refrigeration circuit? Explain the normal heating-mode flow path through the reversing valve.
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In heating, the reversing valve is energized. Compressor discharge goes to the indoor coil through the vapor line; the outdoor coil is the evaporator, and suction returns from the outdoor coil. If the valve is stuck mid-position, hot discharge leaks into the suction side. That explains the low pressure differential, both service valves warm, and compressor current below RLA. The compressor is not necessarily failed; it has little mass flow because the high and low sides are bypassing. The 24 VAC at the solenoid only proves the board is commanding heat, not that the valve has shifted. A failed compressor would normally still show a temperature split between discharge and suction at the compressor shell, but a mid-position valve can look similar, so the next step is to separate valve bypass from compressor valve failure.
2. How would you determine whether the reversing valve or the compressor is at fault, using only electrical and temperature measurements available at the unit?
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First verify the solenoid: 24 VAC present, coil resistance within manufacturer range, and listen or feel for a shift when the solenoid is energized and de-energized. Then measure temperatures at the four reversing-valve ports. In heating, the compressor discharge port should be hot, the indoor-coil port hot, the outdoor-coil port cold, and the suction port cold. If all four ports are near the same temperature, the valve is bypassing internally. To check the compressor, use the manufacturer's pump-down or block-off test: if the compressor can pull the low side down and build discharge pressure when the valve is isolated or in a known position, the compressor is pumping. Low amp draw alone is not proof of compressor failure because it is expected when the pressure differential is low.
3. The solenoid reads 18 ohms and has 24 VAC, but the valve does not shift. List three mechanical or system causes and the next action.
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Mechanical causes include a stuck valve spool from debris or sludge, a clogged pilot orifice, a failed solenoid coil that no longer moves the pilot, loss of pilot pressure because of low charge, or a damaged valve body. Electrical causes are largely ruled out by the 24 VAC and 18-ohm coil, but the coil could still be weak magnetically. Next action: recover the refrigerant, replace the reversing valve and the liquid-line filter drier, purge with nitrogen during brazing, evacuate to the manufacturer's micron level, and weigh in the charge. Then verify the valve shifts in both modes and check superheat and subcooling before returning the system to service.