Auxiliary / backup electric resistance heat

Definition

Auxiliary electric resistance heat, often called 'backup heat,' refers to heating elements (like those in a toaster) that supplement or replace a heat pump's heating function. These coils generate heat directly from electricity, typically activating when the outdoor temperature drops below a heat pump's efficient operating range, often around 30-35°F, or if the heat pump struggles to meet the thermostat setting. Homeowners encounter this term when reviewing heat pump specifications or energy bills, as it's a standard component in most cold-climate heat pump systems.

Why it matters

The frequency and duration of auxiliary heat operation significantly impact your energy bills. While heat pumps are highly efficient, auxiliary electric resistance heat operates at 100% efficiency, meaning every dollar of electricity generates one dollar of heat – far less efficient than a heat pump, which can deliver 2-4 dollars of heat per dollar of electricity. If your system frequently cycles into auxiliary heat, especially above 35°F, it suggests your heat pump might be undersized for your home's heating load or its 'lockout temperature' (the setpoint at which auxiliary heat takes over) is configured too high. This can lead to substantially higher winter utility costs. Conversely, a properly sized and configured heat pump minimizes auxiliary heat use, maximizing your savings and comfort.

Where you'll see it

You'll typically see 'auxiliary heat' or 'backup heat' mentioned on a heat pump quote, an AHRI (Air-Conditioning, Heating, and Refrigeration Institute) certificate, or a manufacturer's specification sheet. While the heat pump's capacity (e.g., 3-ton, 36,000 BTU/h) is primary, the auxiliary heat capacity is usually expressed in kilowatts (kW), often ranging from 5 kW to 20 kW. A common residential setup might include 10-15 kW of auxiliary heat. While the presence of auxiliary heat is normal, excessive sizing (e.g., 20 kW for a small, well-insulated home) isn't necessarily a red flag if properly controlled, but it highlights the potential for high operating costs if frequently engaged. Premium systems aim to reduce reliance on auxiliary heat through enhanced low-temperature performance.

Common mistake

A common mistake is assuming auxiliary heat is a primary heating source or that its frequent activation is normal. Homeowners often misinterpret the 'whoosh' of warmer, drier air from auxiliary heat as superior performance, not realizing it's a less efficient mode. Contractors sometimes set the auxiliary heat lockout temperature too high (e.g., 40°F), causing the system to switch to expensive resistance heat prematurely, even when the heat pump could still operate efficiently. This inflates energy bills and masks potential issues like an undersized heat pump or refrigerant problems, which should be addressed rather than compensated for by relying on backup heat.

See also

• Coefficient of Performance (COP)
• Balance Point
• Lockout Temperature
• Supplemental Heat