Cold-Climate Heat Pump With Auxiliary Heat Lockout
A 2,000 sq ft house in a cold climate (design 0 F, balance point estimated at 22 F) receives a cold-climate variable-speed heat pump rated 36,000 Btu/h at 47 F and about 22,000 Btu/h at 5 F, with a rated coefficient of performance of 1.8 at 5 F. The existing 15 kW electric resistance strip heat is retained as backup. The owner reports that during a 10 F cold snap the strips ran almost continuously and the electric bill spiked, while the heat pump appeared to run at low capacity. The thermostat is a communicating model with an outdoor sensor, and the installer set the auxiliary heat lockout at 35 F and the compressor lockout at 5 F. The homeowner asks whether the heat pump is 'not working' and whether the strips should be disabled.
1. Diagnose why the strips ran continuously at 10 F and identify the configuration error.
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At 10 F the heat pump can still produce substantial capacity, likely 25,000-30,000 Btu/h for a cold-climate variable-speed unit, and the house load at 10 F is below the design load of roughly 36,000 Btu/h. The auxiliary lockout at 35 F means the strips are permitted to run whenever outdoor temperature is below 35 F, so at 10 F the thermostat can call strips alongside the compressor. Combined with a compressor lockout at 5 F, the system is configured to lean on resistance heat across most of the heating season. The correct configuration is to set the auxiliary lockout below the balance point, often 20-25 F, and the compressor lockout at or below the manufacturer's minimum operating temperature, not at 5 F. The strips are not the problem; the lockout setpoints are.
2. Calculate the approximate strip-heat contribution and compare operating cost with heat pump operation at 10 F.
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15 kW of strips delivers about 51,200 Btu/h at full output, far more than the house needs at 10 F, so the strips will stage or cycle, but any strip runtime is expensive. At an electric rate of $0.15/kWh, one hour of full strip operation costs $2.25. The heat pump at 10 F, interpolating between 22,000 Btu/h at 5 F and 36,000 Btu/h at 47 F, delivers roughly 24,000 Btu/h at a coefficient of performance near 2.0, using about 3.5 kW, or $0.53/hour for the same delivered heat. Even accounting for defrost and colder-coil penalties, heat pump heat is roughly one-quarter to one-third the cost per delivered Btu. The fix is to let the heat pump carry the load down to its balance point and use strips only below it or during defrost.
3. What commissioning verification steps confirm the system is operating as intended after the lockouts are corrected?
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Verify the outdoor sensor reads accurately against a calibrated thermometer. Confirm the balance point by comparing measured capacity at several outdoor temperatures to the calculated load. Check that the thermostat's auxiliary lockout and compressor lockout match the intended design and the manufacturer's minimum operating temperature. Measure refrigerant charge and subcooling/superheat per the manufacturer's charging chart at the current conditions. Verify airflow and that the strips stage properly with a clamp meter on each element. Run a defrost cycle and confirm strips engage only as needed. Record the settings, measured capacities, and observed strip runtime in the commissioning report, and explain to the homeowner that disabling strips entirely risks comfort and equipment protection during defrost and extreme cold.