Heat Pump Contactor Chattering When Auxiliary Heat Stages
A 3-ton heat pump with a 10 kW electric auxiliary heat package serves a 1,800 ft² house. The homeowner reports the outdoor unit 'short-cycling' and a loud clicking from the air handler whenever the auxiliary heat is energized. On arrival, the compressor contactor pulls in and drops out repeatedly while the sequencer is energized. Control voltage is 26 VAC at the transformer with only the thermostat connected, but falls to 18.5 VAC at the contactor coil when the compressor, reversing valve, and both 5 kW sequencers are energized. The installing contractor used a 40 VA transformer and 18 AWG thermostat cable on a 150 ft run. No C terminal was available at the thermostat, so a power-stealing thermostat was installed. Coil loads: contactor 8 VA, reversing valve 12 VA, each sequencer 10 VA, control board 8 VA, thermostat 3 VA.
1. Calculate the connected control load and determine whether the transformer is adequately sized.
Read the worked answer
The transformer is overloaded.
- Contactor: 8 VA
- Reversing valve: 12 VA
- Two sequencers at 10 VA: 20 VA
- Control board: 8 VA
- Thermostat: 3 VA
- Total = 51 VA against a 40 VA transformer - about 128% of rating.
A transformer loaded beyond its rating cannot hold secondary voltage. The 18.5 VAC measured at the contactor is below the usual ±10% tolerance of 21.6–26.4 VAC. The contactor coil drops out, the compressor stops, and the cycle repeats because the sequencer remains energized. The power-stealing thermostat adds load and is itself unreliable at low voltage.
2. Explain the chatter mechanism and describe a test procedure that confirms the diagnosis rather than guessing at parts.
Read the worked answer
Confirm by measuring under load, not open-circuit.
- Measure transformer secondary with all loads energized. 18.5 VAC confirms a supply problem; 26 VAC with only the thermostat connected shows the transformer is fine at no load.
- Measure at each load (contactor coil, sequencer, reversing valve) to see where the drop accumulates.
- Separate transformer sag from wire drop: compare voltage at the transformer secondary with voltage at the load. A 150 ft, 18 AWG control run adds measurable resistance, but it cannot explain a 7.5 V drop by itself.
- Verify actual sequencer VA from its label; a 5 kW sequencer may draw more than assumed.
- Rule out a marginal contactor coil, a shorted thermostat wire, or a loose neutral before replacing parts.
3. Recommend a repair and the verification steps to prove the chatter is gone.
Read the worked answer
Fix control capacity, then re-verify every load.
- Replace the 40 VA transformer with a 75 VA unit and fuse the secondary.
- Pull a cable with a C wire so a conventional thermostat can be used; the power-stealing thermostat is a poor match for a heat pump with aux heat.
- If a second transformer is impractical, use a relay to energize the aux sequencers from a separate supply.
- Verify 24 VAC ±10% at every load with everything energized, check transformer temperature after 15 minutes, and confirm no chatter through a full heating cycle including defrost and aux staging.
- Confirm the outdoor thermostat or board lockout is set so auxiliary heat does not stage unnecessarily.