Heat Pump With Auxiliary Heat, a Shared White Wire, and a 60 A Circuit
A new 3-ton heat pump with 10 kW electric auxiliary heat is installed in a 2,000 sq ft Carson City office. Nameplate data: compressor RLA 14.7 A, outdoor fan 1.2 A, indoor blower 3.4 A, auxiliary heat 41.7 A at 240 V, MCA 52 A, and maximum overcurrent protection 60 A. The installer used a 60 A breaker, 6 AWG copper NM-B, and a 5-conductor 24 V thermostat cable (R, C, W1, Y1, G). A field check finds the white conductor landed on W1 at the thermostat and on the O terminal at the outdoor unit. On a 40°F morning the thermostat is set to 70°F, the space is 62°F, supply air is 78°F, the compressor draws 14 A, the auxiliary heat draws 41 A, and the thermostat shows “aux heat” continuously. The owner reports high electric bills and says the system “never gets warm.” The installing technician says five wires are enough because “a heat pump only needs R, C, Y, G, and W.”
1. Diagnose the control and sequence-of-operation problem. Why is the system running auxiliary heat continuously and still not heating?
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The white conductor is being used for two different functions. At the thermostat it is landed on W1, which calls for auxiliary heat, but at the outdoor unit it is landed on O, the reversing-valve terminal. When the thermostat calls for aux heat, it also energizes the reversing valve through that same white wire, shifting the refrigerant circuit toward cooling while the 10 kW electric heater is energized. The compressor draws 14 A, the aux heat draws 41 A, and the supply air is only 78°F because the heat pump is fighting the electric heat. Five wires are not enough for a heat pump with independent reversing-valve and second-stage/emergency heat control; the cable needs separate O and W2/E conductors, typically a 7- or 8-conductor cable. The thermostat must also be configured for a heat pump with aux heat and an outdoor sensor or lockout, not as a conventional system.
2. Verify the branch-circuit and overcurrent protection. Is the 6 AWG copper NM-B with a 60 A breaker acceptable?
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Start with the nameplate: MCA is 52 A and maximum overcurrent protection is 60 A. The 6 AWG copper NM-B must be applied at its 60°C ampacity, which is 55 A, not the 75°C column. Because 55 A is at least the 52 A MCA, the conductor is acceptable for this load, and the 60 A breaker is permitted as the next standard overcurrent device above the 55 A conductor ampacity where the nameplate maximum is 60 A. The installer must not assume NM-B can be used at 75°C or that the breaker size alone proves the wire is adequate. Check terminations, the disconnect, grounding, and the auxiliary-heat circuit as well. The electrical issue is not the immediate cause of the 78°F supply air, but it must be verified before the control repair is signed off.
3. What commissioning steps and owner communication are required after the wiring is corrected?
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Correct the control wiring first: provide separate O and W2/E conductors, land them correctly at both ends, and configure the thermostat for heat-pump operation with auxiliary heat. Verify the reversing valve shifts in heating and cooling, check the defrost board and outdoor sensor, and set the auxiliary-heat lockout or balance point so the strip heat does not run continuously at 40°F unless the heat pump cannot maintain capacity. Measure supply and return temperatures in heating, compressor amperage, auxiliary-heat amperage, and static pressure; confirm the temperature rise is within the nameplate range. Verify the 60 A breaker and 6 AWG NM-B terminations, and document the corrected sequence. Explain to the owner that the high bills came from the strip heat running with the system in the wrong mode, not from a lack of capacity, and give them the corrected operating settings in writing.