Most homeowners with a heat pump first notice the auxiliary heat when the house is cold and the electric bill jumps. The thermostat shows an AUX or EM HEAT indicator, the air from the vents runs hotter, and the resistance strips in the air handler start drawing a large load. That is the system working as designed when the heat pump alone cannot keep up. The useful question is whether the backup heat is calling in at the right times and for the right reasons.
A recent ACHR News piece on a contractor testing a heat pump in his own home describes, in its summary, "what a real-world heat pump installation reveals about sizing, training, comfort, and efficiency." Those four words map neatly onto the places where auxiliary heat usually goes wrong in a house. An oversized or undersized system, a thermostat setup nobody explained at install, or one room that never catches up can all push the strips on more often than they should. Read the trade view, then check what your own system is doing.
Start With the Settings
Open the thermostat menu and find the auxiliary or emergency heat settings. Many controls let you set an outdoor lockout temperature or a balance point, which tells the system when the heat pump should carry the load alone and when backup heat may join. Write down every value you find, along with the date. If the installer left a settings sheet, compare it to what is on the display. A mismatch is the most common reason strips run on mild afternoons.
Log When the Strips Come On
Keep a simple log for one cold week. Record the date, the outdoor temperature from a source you trust, the thermostat setpoint, and the times the auxiliary indicator turns on and off. A smart thermostat's runtime history can fill much of this in for you. Read the pattern afterward. Backup heat during a large setback recovery, first thing on a frigid morning, is normal. Strips running steadily through a mild afternoon, or cycling while the house is already at setpoint, points to a setting, a control problem, or a comfort issue in one part of the home that is pulling the thermostat into backup mode.
Use the Meter on the Control Signal
Your multimeter can confirm the basic control signal without touching anything live. The thermostat's 24-volt control circuit runs between the R terminal and the C terminal, and a call for the second stage or auxiliary heat shows up on the W terminals. At the furnace or air handler control board, with the system in a safe state and the cover open under your own supervision, you can check the control voltage across R and C, then check W2 to C while the auxiliary call is active. A healthy control circuit reads near 24 volts AC. A reading that sags well below that, or one that drops out when the strips should be energized, is worth bringing to a technician.
Keep your hands off the line-voltage side unless you are trained and equipped for it. Any work inside the cabinet starts with the power shut off at the disconnect and the breaker, followed by a meter check that the circuit is dead before you touch a terminal. A clamp meter, if you own one, can show current on a single supply leg of the strip heater circuit during a call. Compare that reading to the heater kilowatt rating on the label, using the voltage the heater runs on, and note whether the strips draw the expected load each time they engage. If you are not comfortable inside a live panel, stop at the thermostat and the control board and hand the rest to a licensed technician.
What to Bring to the Tech
A written log, a photo of the thermostat settings screens, and your meter readings make the next service call shorter. Ask the technician to confirm the lockout logic, check whether the strip heat is sized for the house and the heat pump, and explain any setting they change. Keep the paper with the equipment records. The next time backup heat runs on a cold night, you will know whether it is doing its job or quietly covering for a setting that needs attention.