Heat Pump That Won't Hold Temperature at 25°F
A 2.5-ton single-stage R-410A heat pump with a 10 kW electric strip kit (two 5 kW banks, each on its own 30 A breaker) serves a house in north Alabama. Outdoor design is 22°F; today it is 25°F with wind. The owner complains the system "blows cool air and runs nonstop," and the electric bill doubled last month. Indoor temperature is 68°F with the thermostat set to 70°F. The compressor is running, suction and head pressures are in the normal heating range, and the outdoor coil is clear of frost. Manufacturer's expanded rating for this unit at 70°F return / 25°F outdoor is 18,000 Btu/h. A block load calculation puts the house heat loss at 26,000 Btu/h at 25°F and 32,000 Btu/h at the 22°F design condition. With the thermostat calling for auxiliary heat, a clamp meter on both strip circuits reads 0.0 A, while W2 at the thermostat shows 24 V to C.
1. Using the capacity numbers, determine whether the strip heat should be operating at these conditions and how much auxiliary capacity is actually needed.
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- Building load at 25°F: 26,000 Btu/h.
- Heat pump output at 25°F: 18,000 Btu/h.
- Deficit the strips must cover: 26,000 − 18,000 = 8,000 Btu/h ≈ 2.3 kW (8,000 ÷ 3,413).
- At the 22°F design point the deficit is 32,000 − roughly 16,500 ≈ 15,500 Btu/h ≈ 4.5 kW.
So the 10 kW kit is generously sized for the design load (it also needs margin for defrost cycles and pickup), but only about 2.3 kW is needed right now. Because the strips are electrically staged in 5 kW banks, a properly functioning system would bring on one 5 kW bank and cycle it to make up the deficit - not run it continuously. The clamp reading of 0.0 A on both banks during a known aux call is a hard failure, not a sizing issue: with no auxiliary heat, the system can only deliver 18,000 of the 26,000 Btu/h required, which is exactly why the house drifts to 68°F in a 25°F ambient.
2. List the checks you would make, in order, to find the reason no current is flowing, and identify the most likely fault given the evidence.
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Work from the control signal outward to the load:
- Confirm the call: 24 V AC between W2 and C at the thermostat is present (already verified) - also verify it at the air handler terminal strip, ruling out a broken thermostat wire.
- Breakers: check both 30 A strip breakers at the panel and any disconnect at the air handler. A tripped breaker is the simplest explanation for 0.0 A on both banks at once, and it is common for one bank to trip first and be reset while the other stays off.
- Sequencers/relays: with 24 V at the sequencer heater terminal, check for line voltage across the contacts. 24 V in and no line voltage out on both sequencers points to the control side.
- Limits/ECO: a tripped high-limit or energy-cutoff on the strip assembly will open the line side. Verify continuity and that the limit has not been bypassed.
- Load side: verify 240 V at the element terminals and confirm element resistance (a 5 kW, 240 V element is about 11.5 Ω); an open element reads infinite.
Given 24 V present at W2, both banks reading 0.0 A simultaneously, and normal compressor operation, the highest-probability fault is an open line-side path shared by the strips - a tripped/miswired breaker or a failed sequencer/relay coil. A single failed element would still leave the other bank drawing current.
3. The owner says, "The air coming out is only about 92°F - that's not heat, my old system blew 120°F." How do you explain the delivered air temperature without dismissing the complaint?
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Delivered air temperature is a function of the heat added and the airflow, not of the type of system:
ΔT ≈ Btu/h ÷ (1.08 × CFM)
- Compressor only at 25°F: roughly 18,000 Btu/h ÷ (1.08 × 1,200 CFM) ≈ 14°F rise, so about 84°F supply air.
- Compressor plus one 5 kW bank (17,000 Btu/h) plus the compressor: about 35,000 Btu/h ÷ 1,296 ≈ 27°F rise, so about 97°F supply air.
- A gas furnace at 120°F supply is adding far more heat to a lower airflow - not a fair comparison.
So 92°F supply air from a heat pump with partial strip heat is normal, and 84°F air (compressor only) feels cool against skin because it is below body temperature even though it is warming the house. The real complaint is that the house cannot hold setpoint, which the load math confirms and the 0.0 A strip reading explains. Also note: during defrost the indoor coil becomes a condenser-less evaporator and cold air will blow until the strips come on; verify the strip-on-during-defrost function and any blower delay. Explain the numbers, fix the strip circuit, then re-measure supply temperature and confirm the house reaches setpoint.