Auxiliary Heat Runs All Winter on a 28°F Day

Work it both ways at the nominal 1,200 cfm:

• Compressor only: 20,000 ÷ (1.08 × 1,200) ≈ 15°F rise
• One 5 kW bank (≈17,000 BTU/h): 17,000 ÷ 1,296 ≈ 13°F rise
• Compressor plus one 5 kW bank: ≈ 37,000 ÷ 1,296 ≈ 28°F rise
• All 10 kW alone: ≈ 34,100 ÷ 1,296 ≈ 26°F rise

The measured 24°F rise, about 31,000 BTU/h delivered, is far above what the compressor can produce at 28°F. Something in the strip circuit is energized.

Do not trust the thermostat display alone. Clamp the strip heater circuit(s) and confirm actual amperage - 5 kW at 240 V is roughly 21 A, 10 kW roughly 42 A. That separates 'the stat says aux' from 'the strips are actually on.' If only one bank is drawing, the diagnosis is a staging or lockout configuration issue; if both are drawing, the thermostat is calling full aux whenever the compressor runs. Also check the rise at a steady 10 minutes of runtime so you are not measuring strip heat staged for recovery.

The settings are the fault, not the equipment.

• At 28°F the load is 26,000 and the compressor makes 20,000, so a 6,000 BTU/h deficit is normal and some aux is expected at that temperature. A 10 kW kit (34,100 BTU/h) is roughly five times the deficit, so it must be staged, not switched on as a block.
• A 40°F compressor lockout is wrong for this climate. Where the capacity curve crosses the load curve is the balance point - here it falls between 28°F and roughly the mid-30s, since capacity at 47°F exceeds the load and capacity at 28°F does not. Locking the compressor out at 40°F forces strip-only operation through the whole range where the heat pump is still cheaper to run, which matches the complaint that aux runs at 45°F.

Changes:

1. Set the compressor lockout at or below the calculated balance point (roughly 30–32°F), not 40°F, and confirm the manufacturer permits operation at that ambient.
2. Set the aux lockout above the balance point so strips are available only below it, plus for defrost and for recovery from a setback of more than 2–3°F.
3. Stage the strips: first bank as aux, second bank only as emergency heat or below a lower outdoor cut-in.
4. Remove any large setback schedule that forces aux recovery in the morning.

Then verify with a 20-minute run and an amp clamp that the strips drop out above the balance point.

Heating-mode charge verification is not the same as cooling mode, and this is where techs misdiagnose heat pumps.

• In heating, the outdoor coil is the evaporator and the indoor coil is the condenser. The small line between the coils is the liquid line, so subcooling is read there, but the indoor metering device and the indoor airflow (which is now the condensing airflow) both affect the reading.
• Take the reading only with the coil clean, the blower at the correct speed, a stabilized 15+ minutes of runtime, and no defrost cycle running or just completed. A reading taken during or just after defrost will show nonsense because the reversing valve and the outdoor fan have just cycled.
• Use the manufacturer's heating charging chart if one exists, or weigh in the charge; do not use compressor superheat as the target on a system with a TXV or EEV.
• Verify the true fault before adding refrigerant. In this case the delivered capacity problem is explained by compressor lockout and strip staging, not by charge. Adding refrigerant to a system that is already correct will overcharge it and can raise discharge temperature and pressure in heating.