If you have a heat pump system, you may see that your thermostat will at times go into auxiliary mode (sometimes seen as “aux” on the screen). To understand why this happens, you first need to know some basics of how your heat pump works.
Every heat pump system has an outdoor unit, the heat pump, and an “indoor” unit, which is either an air handler or a gas furnace. The location of the indoor unit can be inside the home, garage, attic, or crawl space. The majority of the time that your thermostat is calling for heat, the heat pump is used to produce it. However, when the temperature drops near freezing or below, the heat pump cannot work as efficiently, so the air handler is used to produce the heat instead. When that happens, you will see “aux” show up on your thermostat screen, which means that that auxiliary heat has been automatically turned on to produce the most comfort in your home.
The most common time people see this happening is during winter mornings when they first get up and the temperatures are still below, at, or slightly above freezing. This is completely normal operation, but there can be concern if it stays in auxiliary heat even when the temperature has risen. Behavior like that can point to a problem with the installation of the system, a mechanical failure in the heat pump or air handler, or a faulty thermostat. You can run the system in auxiliary heat until a technician gets to your house, but you do not want to have it in that mode indefinitely, since it is much more expensive to use the indoor unit to solely heat your home instead of the cost efficient heat pump.