Aux Heat Not Turning On — Heat Pump Diagnosis

Common Symptoms

• House cannot reach set temperature when outdoor temps are below 35°F
• Heat pump runs continuously but indoor temperature drops during cold weather
• No increase in airflow temperature when aux heat should be active
• Thermostat shows AUX indicator but supply air feels only mildly warm
• Ecobee or Nest shows aux heat as engaged but heat output is unchanged

Most Likely Causes

1. 1

   W2 Terminal — No 24VAC Signal Reaching Air Handler

   Aux heat activates when the thermostat sends 24VAC on the W2 (or Aux) wire to the air handler terminal board, triggering the heat strip sequencers. If this 24V signal never arrives — due to a broken wire, loose terminal connection, or thermostat configuration error — the sequencers never energize and the strips stay cold. Verify 24V at the W2 terminal on the air handler board during a second-stage heating call.

2. 2

   Failed Heat Strip Sequencers

   Electric heat strips are powered by sequencers — thermal relays that stagger strip activation 15–30 seconds apart to prevent voltage spikes. A sequencer has a 24VAC heater coil that, when energized, bends a bimetal strip to close line-voltage contacts powering the strip. Failed sequencers (open coil, open contacts, or welded contacts) cause one or more strips to remain off entirely, reducing or eliminating heat output.

3. 3

   Heat Strip Contactor — Open Coil or Burned Contacts

   Some larger residential heat pump air handlers use a contactor (rather than individual sequencers) to power all heat strips simultaneously via a single 24VAC coil. A failed contactor coil (open circuit at 24VAC coil terminals) means the contactor never pulls in and no line voltage reaches the strips. Burned or pitted contactor contacts pass line voltage to the strips but with high resistance, causing overheating without full heat output.

4. 4

   Tripped 240V Heat Strip Breaker

   Electric heat strips draw 15–40 amps at 240VAC and are typically on a dedicated double-pole breaker separate from the air handler's control power. A tripped heat strip breaker cuts 240V to the strips while leaving the 24VAC control circuit (and fan) fully operational — the system appears to run but produces no strip heat. The breaker may have tripped due to a sequencer failure, a shorted strip element, or a loose connection causing an arc.

5. 5

   Thermostat Aux Heat Lockout Temperature Too Low

   Ecobee and Nest thermostats have a configurable Aux Heat Maximum Outdoor Temperature setting. This prevents aux heat from engaging when outdoor temps are above the configured threshold. If this is set too low (e.g., 20°F), the thermostat will not trigger W2 even when the balance point is exceeded at 35°F. Verify this setting is at or above your system's balance point.

Quick DIY Checks

1. 1Check the thermostat aux heat lockout temperature setting first — it's the easiest fix. On Ecobee: Main Menu → Settings → Installation → Thresholds → Aux Heat Maximum Outdoor Temperature. This should be set to at or above your balance point, typically 40°F. If it's set to 20°F, aux heat won't engage until outdoor temps drop below 20°F — far below where most systems need it. On Nest: Settings → Nest Sense → Aux Heat (you may need the Nest app on mobile for full access to lockout temp).
2. 2Force a second-stage call by lowering the thermostat set point 5–8°F below current indoor temperature. Wait 10–12 minutes for the thermostat staging delay to expire. During this time the thermostat is in first-stage heating with the heat pump only. After the delay, the thermostat should send a second-stage call on W2. Watch the thermostat display: it should show AUX.
3. 3Measure 24VAC at the W2 terminal on the air handler terminal board during the second-stage call. Access the air handler control compartment (usually a panel held by two screws). With the thermostat calling for second-stage, measure voltage between the W2 terminal and the C common terminal. You should read ~24VAC. If you read 0V at W2 but the thermostat shows AUX, the W2 wire has a break or the thermostat is not outputting on W2 — test directly at the thermostat's W2 terminal. If you read 24V at the board's W2 terminal but no heat, the sequencers or contactor are the issue.

1. 4Check the heat strip 240V breaker at the electrical panel. Locate the breaker labeled 'Air Handler', 'Heat Strips', 'Backup Heat', or similar — it will be a double-pole breaker (240V circuit). Toggle it fully to OFF then back to ON even if it doesn't appear tripped. A breaker can be in a tripped state without visibly moving to the center position. A tripped breaker resets by first moving fully to OFF before returning to ON.
2. 5Test the heat strip sequencer coil resistance. With power off and the W2 wire disconnected, measure resistance across the 24VAC coil terminals on each sequencer (marked with low-voltage screw terminals — typically two small screws separate from the line-voltage contacts). A working sequencer coil reads 100–500 ohms depending on model. Open circuit (infinite resistance/OL on multimeter) = failed coil — replace the sequencer. If all coils test good, test the line-voltage contacts: with power restored and W2 energized, measure 240VAC across the line-voltage output terminals of each sequencer — a working sequencer will show 240V on its output terminals after the 15–30 second thermal delay.
3. 6Check for defrost board aux heat inhibit signal. During an active defrost cycle (outdoor unit fan off, steam coming from coil), the defrost board typically sends a signal to the air handler that inhibits aux heat. This prevents the strips from heating the supply air while the refrigerant circuit is reversed for defrost. If you tested during a defrost cycle and got no aux heat, wait 10–15 minutes until defrost completes and re-test. On Carrier and Trane integrated defrost boards: the 'W2 inhibit' terminal is labeled on the board schematic.
4. 7Test the heat strip contactor coil (if equipped instead of individual sequencers). With power off, measure resistance across the 24VAC coil terminals on the contactor (two small wires, usually 24V). Should read 20–80 ohms. Open circuit = failed coil — replace the contactor. With power restored and W2 energized, measure 240VAC across the contactor's line-voltage contacts (the large terminals where the heavy wires connect). A pulled-in contactor will read near 0V across its contacts (contacts closed, voltage drops to near zero). A contactor with burned contacts will read partial voltage (e.g., 50–100V) indicating high contact resistance — replace the contactor.

Repair vs Replace

Recommended Tools & Parts

• Electric Heat Strip Sequencer Relay

  Thermal time-delay relay that staggers heat strip energization. Available in 24VAC coil models. Match to your air handler brand and model — Carrier, Trane, Lennox, Goodman, and Rheem all use different sequencer ratings.

  $15–$40

  Buy on Amazon →

• Heat Strip Contactor (24V Coil, 40A/240V)

  Replacement contactor for heat pump air handler heat strips. 24VAC coil, 40A line-voltage contacts. Verify amp rating and pole count match your existing contactor data plate.

  $25–$60

  Buy on Amazon →

• Non-Contact Voltage Tester

  Essential for verifying 240V power is off before touching heat strip components. Detects both 120V and 240V without direct contact.

  $15–$30

  Buy on Amazon →

• Digital Multimeter

  For measuring 24VAC control signal at W2 terminal, sequencer coil resistance, and 240V contact output verification.

  $15–$35

  Buy on Amazon →

Links are Amazon affiliate links (tag: fixitfastai-20). Prices are estimates.

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