Auxiliary and emergency heat: why it's on and what it costs

A heat pump collects heat from outdoor air. As the air gets colder, there is less heat to collect and the house needs more of it. At some point the heat pump alone falls short, and the system brings on a second heat source to help. That helper is auxiliary heat. In most homes it is a set of electric resistance strips inside the air handler. In a dual fuel system it is a gas furnace.

The thermostat decides when to call for it. Common triggers are a large gap between room temperature and setpoint, a long run without reaching the setpoint, an outdoor temperature below a programmed cutoff, and the defrost cycle. When it is active, the thermostat shows Aux, Aux Heat or a similar label.

Emergency heat uses the same backup equipment, with one difference: you switch it on by hand, and it locks the heat pump out completely. The backup then carries the whole house by itself.

Use emergency heat only when the heat pump has failed or the outdoor unit is frozen solid and you are waiting for a technician. It is not meant for ordinary cold days. The system already brings on auxiliary heat automatically when it needs help, and it keeps the heat pump running alongside it.

Electric resistance strips turn electricity straight into heat. A heat pump moves heat, and it can deliver more heat energy than the electricity it uses, as ENERGY STAR notes for air-source models. So every hour spent on strips in place of the heat pump uses more electricity for the same warmth. A winter of nonstop aux heat shows up clearly on the bill.

How much more depends on your electric rate, the size of the strips and how many hours they run. With a gas furnace as backup, the comparison depends on local gas and electricity prices.

Treat these as warning signs:

• Aux heat runs in mild weather.
• Aux heat is on every time the system heats, all day.
• The outdoor unit is not running while the thermostat calls for heat.
• The outdoor unit is covered in ice.
• Your electric bill has jumped with no change in weather or habits.

The usual causes are a heat pump that has stopped working, low refrigerant, a defrost fault, a dirty filter, or a thermostat set up with the wrong cutoff settings. If the outdoor unit is dead or the air is not warm, start with the page on a heat pump not heating.

The tech will check the thermostat configuration and the outdoor temperature cutoffs, confirm the heat pump is producing its share of the heat, read refrigerant pressures, test the defrost cycle and measure the current drawn by each stage of backup heat. Sometimes the fix is a settings change. Sometimes the backup heat has been hiding a failed heat pump for weeks. The heat pump overview links to the repair page.