What Does It Mean When It Says Auxiliary Heat On

Complete Explanation

When a thermostat displays the message “Auxiliary Heat On” (or “Aux Heat”), it indicates that the primary heating source—typically an air-source heat pump—is unable to meet the desired temperature set point on its own. The system has automatically engaged a backup or secondary heating source to supplement the heat pump’s output. This secondary source is most often electric resistance heating strips (located in the indoor air handler) or, in some hybrid systems, a gas or oil furnace. The auxiliary heat is designed to provide additional warmth quickly, especially when outdoor temperatures drop below the heat pump’s efficient operating range (generally below 30–40 °F / -1 to 4 °C) or during the heat pump’s defrost cycle.

• Primary vs. Auxiliary Heat:
  The primary heat source is the heat pump, which extracts heat from outside air and moves it indoors. Auxiliary heat is activated only when the heat pump cannot keep up, ensuring comfort without overworking the heat pump.
• Trigger Conditions:
  Auxiliary heat typically turns on when the indoor temperature is significantly below the set point (e.g., more than 2–3 °F difference), during extreme cold, or when the heat pump enters a defrost cycle to remove frost from the outdoor coil.
• Energy Implications:
  Auxiliary heating is less energy-efficient than a heat pump alone because electric resistance heating uses more electricity per unit of heat produced. Prolonged auxiliary operation can increase utility bills.
• Manual Override:
  Most thermostats allow users to manually select “Emergency Heat” (different from auxiliary heat), which locks out the heat pump and uses only backup heat. This should be used only if the heat pump is malfunctioning.

History / Background

The concept of auxiliary heating arose alongside the development of residential heat pumps in the mid-20th century. Early heat pumps struggled to maintain indoor comfort in very cold climates because they lost efficiency as outdoor temperatures dropped. To address this, manufacturers integrated electric resistance heating strips as a backup. In the 1970s and 1980s, dual-fuel systems emerged, combining a heat pump with a fossil-fuel furnace to improve efficiency and reduce reliance on expensive electric resistance heating. The thermostat display “Auxiliary Heat On” became a standard feature on programmable and smart thermostats to inform homeowners that the system was using backup heat. Over time, improvements in heat pump technology (e.g., variable-speed compressors, cold-climate heat pumps) have reduced, but not eliminated, the need for auxiliary heat in many regions.

Importance and Impact

Understanding when and why auxiliary heat activates helps homeowners manage energy consumption and system maintenance. Overreliance on auxiliary heat can lead to higher electricity bills and increased wear on heating components. Conversely, ignoring the indicator may cause discomfort or strain on the heat pump. HVAC technicians often use the auxiliary heat runtime as a diagnostic parameter: frequent activation suggests the heat pump is undersized, the home is poorly insulated, or the outdoor unit is malfunctioning. The display also serves as a safety feature, ensuring that homes remain warm even when outdoor temperatures are extreme.

Why It Matters

For homeowners and renters with heat pump systems, the “Auxiliary Heat On” message is a practical signal. It alerts them that the heating system is operating in a less efficient mode, prompting potentially cost-saving actions: checking thermostat settings, scheduling maintenance, improving insulation, or considering a cold-climate heat pump upgrade. It also helps troubleshoot issues—if the auxiliary heat runs constantly, it may indicate a problem with the heat pump that requires professional service. In regions with moderate winters, occasional auxiliary use is normal; in very cold climates, frequent auxiliary operation is expected.

Common Misconceptions