If your heat pump’s auxiliary (or "aux") heat won’t kick in during cold snaps—leaving your home chilly even when the thermostat says it’s running—you’re likely dealing with a failed component, not a settings issue. This isn’t just an inconvenience; it can strain your compressor and spike energy bills. The good news? Many aux heat failures trace to one of three replaceable parts—and you can often fix it yourself in under two hours.
Quick Diagnosis
Before grabbing tools, rule out simple causes:
- Thermostat set to "Heat" mode (not "Emergency Heat" or "Cool")
- Auxiliary heat lockout temperature exceeded (most units disable aux below ~15°F or above ~65°F)
- Blower motor running but no heat—points to faulty heating elements, contactor, or sequencer
- Burnt smell or visible scorch marks on the indoor air handler’s electric heat strips
- Multimeter reading of 0 volts across aux heat terminals while system is calling for heat
Tools & Materials Needed
| Item | Purpose | Estimated Cost |
|---|---|---|
| Digital multimeter | Verify voltage to aux heat strips and continuity of components | $25–$65 |
| Insulated screwdrivers (Phillips & flat) | Safely remove panels and terminal covers without shorting live circuits | $12–$28 |
| Replacement electric heat strip kit | Exact OEM or compatible match (e.g., 5 kW, 240V, 3-wire) | $180–$420 |
| Wire nut assortment (red & yellow) | Secure 10–12 AWG heater circuit connections | $6–$12 |
| Non-contact voltage tester | Confirm power is OFF before touching wiring—critical safety step | $15–$35 |
Step-by-Step Fix
Most aux heat failures involve one of three parts: the heating element itself, the sequencer (on older units), or the contactor (on newer models). Here’s how to address each:
- Power down completely: Turn off both the indoor air handler breaker (usually 60A) AND the outdoor unit breaker (typically 30A). Verify zero voltage at the aux heat terminals with your non-contact tester and multimeter.
- Access the heating section: Remove the front panel of the air handler. Locate the electric heat strip assembly—usually mounted vertically behind the blower, with heavy-gauge wires (10–12 AWG) connected to brass terminals.
- Test heating elements: Disconnect one wire from each strip. Set multimeter to ohms (Ω). A functional 5 kW strip reads ~11.5 Ω at 240V. An open circuit (OL) means replacement is needed.
"Over 73% of aux heat failures in residential heat pumps stem from burned-out heating elements—not control board issues," says HVAC Technician Certification Board’s 2022 Field Data Report.
- Replace the strip: Unscrew mounting brackets, unclip old strip, and install new one using same orientation. Reconnect wires tightly—loose connections cause arcing and premature failure. Double-check torque on terminal screws (25–30 in-lbs).
- Test operation: Restore power, set thermostat to "Heat" with a 5°F rise above room temp, and watch for aux heat engagement after 5–7 minutes. Confirm warm air output with an infrared thermometer (should hit 110–130°F at supply register).
When to Call a Pro
Stop and call a licensed HVAC technician if you encounter any of these:
- No voltage at the aux heat terminals—even with breakers ON and thermostat calling for heat (points to control board, transformer, or wiring fault)
- Tripped 60A breaker that won’t reset after replacing heat strips (indicates shorted wiring or grounded element)
- Corroded or melted wiring inside the air handler cabinet (requires full circuit inspection)
- Your heat pump uses a dual-fuel setup with gas backup—aux heat integration is more complex and safety-critical
According to the U.S. EPA, improper electrical repairs on HVAC systems account for nearly 12% of residential fire incidents linked to heating equipment (EPA Indoor Air Quality Guidelines, 2023).
Prevention Tips
Aux heat components last longer when maintained properly:
- Clean or replace air filters every 30 days during heating season—restricted airflow overheats strips
- Inspect heat strips annually for discoloration or warping; replace proactively at 8 years (even if working)
- Ensure blower speed is set correctly for heating—low CFM causes heat strip overheating
- Install a programmable thermostat with adaptive recovery to reduce aux heat runtime
Can I bypass the sequencer to test if it’s bad?
No—bypassing the sequencer risks simultaneous activation of all heating stages, overloading the circuit and tripping breakers. Instead, use your multimeter to check for 24V AC input to the sequencer coil and 240V output to the strips when the thermostat calls for aux heat. If input is present but output isn’t, the sequencer is faulty.
Why does my aux heat only run for 90 seconds then shut off?
This points to high-limit switch cycling—often caused by dirty evaporator coils, clogged filters, or low airflow. Check filter first, then inspect coil cleanliness. If airflow is adequate, the high-limit switch itself may be weak or improperly calibrated.
Is it safe to replace a 10 kW heat strip with a 15 kW unit?
No. Oversizing creates dangerous overcurrent conditions. Your breaker, wiring gauge, and blower capacity are engineered for the original rating. Upgrading requires rewiring, larger breakers, and blower motor verification—work only a licensed electrician should perform.
Do I need to vacuum the refrigerant lines when replacing aux heat components?
No—aux heat components are electrically isolated from the refrigerant circuit. Refrigerant work requires EPA Section 608 certification and specialized equipment. Focus only on the air handler’s electrical compartment.
What’s the difference between auxiliary heat and emergency heat?
Auxiliary heat engages automatically during defrost cycles or when outdoor temps drop below the heat pump’s effective range—it’s part of normal operation. Emergency heat is manually selected and disables the heat pump compressor entirely, relying solely on electric strips. Use emergency heat only if the outdoor unit fails.
How do I know if my heat pump even has auxiliary heat?
Check your air handler’s nameplate—it’ll list “Electric Heat” or “Aux Heat” with a kW rating (e.g., “10 kW”). If your system is a straight cool-only air handler paired with a heat pump, it almost certainly includes aux heat. Ductless mini-splits rarely do unless specified as “cold climate” models.
Replacing a failed auxiliary heat component restores comfort and protects your heat pump’s longevity—but it’s not a job for guesswork. Stick to verified diagnostics, match OEM specs exactly, and never skip the voltage verification step. For ongoing reliability, pair this repair with regular heat pump maintenance checklist and consider upgrading to a smart thermostat for heat pumps that optimizes aux heat staging. Most importantly: when in doubt about wiring, voltage, or load calculations, reach out to a certified technician—your safety and system warranty depend on it.