Why does it say Auxiliary Heat on my Honeywell thermostat?

The "Auxiliary Heat" indicator on your Honeywell thermostat means that your heating system is using a backup heating source, typically electric resistance heaters, to help warm your home. This usually occurs when the primary heat pump is struggling to maintain the desired temperature, often in colder weather. It can also activate during defrost cycles when the heat pump is temporarily unable to provide adequate heating. If this light stays on for extended periods, it may indicate a problem with your heat pump.