Heat Pump Auxiliary Heat Not Kicking On: Thermostat Wiring, Strip Heat, and Control Board

If your heat pump isn’t pulling in auxiliary heat when temperatures drop, the most common culprits are a wiring issue at the thermostat, a tripped breaker on the electric strip heaters, or a failed sequencer or control board. A couple of those you can check yourself. The rest need a tech.

Why Aux Heat Exists (and When It’s Supposed to Kick On)

Heat pumps get less efficient as outdoor temps fall. Aux heat, usually electric resistance strips in the air handler, kicks in automatically when the heat pump alone can’t keep up. The outdoor temperature at which that happens is called the balance point, and it varies by home. A well-insulated house might not need aux until it’s 25°F or lower. A drafty older home might need it at 40°F. It also runs during defrost cycles and whenever you set the thermostat more than a few degrees above the current indoor temp.

If you’re seeing no heat on a cold night, or the thermostat shows “Aux” but the strips aren’t actually running, something in that chain has failed.

Most Likely Causes, in Order

Tripped breaker on the strip heaters. Electric strips run on a separate 240V breaker from the heat pump compressor. It trips more often than people expect, especially on older panels or at the start of heating season when the strips haven’t been used in months. Go to your breaker panel and look for a double-pole breaker labeled “Air Handler,” “Heat Strips,” or “AHU.” If it’s tripped, reset it once. If it trips again within a few hours, stop. A breaker that keeps tripping means something is drawing too much current, possibly a shorted element, and continuing to reset it is a fire risk. Call a tech.

Thermostat wiring, specifically the W2 or Aux terminal. The aux heat signal comes from the W2 or Aux wire at your thermostat. If that wire is loose, corroded, or was never connected during a prior thermostat swap, the strip heaters never get the call. Pull the thermostat off the wall and check that the wire labeled W2 or Aux is seated firmly in the correct terminal. Do this with the air handler powered off. The thermostat side runs at 24V, so it’s safe to inspect.

Bad sequencer. The sequencer is a relay that staggers the electric strips so they don’t all come on at once, which would spike current and trip breakers. When one fails, it may take out one or more stages of heat. The heat pump still runs, the strips don’t. Replacing a sequencer means opening the air handler cabinet and working at 240V line-voltage terminals. That’s a tech job.

Control board failure. The air handler’s control board interprets the thermostat signal and fires the sequencers. A failed relay on the board means the W2 signal arrives but nothing happens downstream. You’ll sometimes see indicator lights on the board that point toward the problem. A technician can use those blink patterns alongside voltage measurements to narrow it down quickly.

Defrost board or outdoor unit control. Less common. During defrost, the system reverses the refrigerant cycle and the strips run to temper the cold air coming off the indoor coil. If the defrost control is acting up, aux behavior gets erratic. Usually accompanied by other symptoms like ice buildup on the outdoor unit.

How a Tech Diagnoses It

A technician will put a meter on the W2 terminal at the air handler to confirm the thermostat is actually sending the signal. Then they check voltage across the sequencer outputs and at the strip heaters themselves. It’s a methodical voltage trace from the thermostat signal all the way to the heater elements. Takes 20-40 minutes on most systems. If the sequencer or board is the issue, the tech usually carries common parts and can often repair it the same visit.

What You Can Check Yourself

Resetting the breaker once: yes. Checking the W2/Aux wire at the thermostat terminal block with the air handler powered off: yes.

Opening the air handler cabinet to test or replace sequencers, elements, or boards: no. Strip heater wiring runs at 240V, breakers can be mislabeled, there may be multiple circuits feeding one cabinet, and the blower motor capacitor holds charge after power is cut. Not worth the risk.

A Note on Thermostat Settings

Some smart thermostats have a setting that limits when aux heat can run, either a minimum outdoor temperature lockout or a balance-point setting. If you recently installed a new thermostat and aux heat stopped working right after, check the advanced settings before assuming a hardware failure. On Ecobee, look under Installation Settings > Thresholds for “Aux Heat Max Outdoor Temp.” The factory default (35°F) is a reasonable starting point. If that value was pushed lower by a prior installer or an energy-saving adjustment, the thermostat may stop calling for aux before you actually need it.

When to Call

Call right away if the breaker trips again after you reset it. Repeated trips mean something is drawing too much current, and continuing to reset it is a fire risk.

Call if the thermostat wiring looks fine and the breaker is good but strips still won’t run. At that point the problem is inside the air handler and needs a meter and a tech comfortable with 240V panels.

Don’t wait until January to find out your aux heat is broken. We handle heat pump and air handler repairs throughout the Bay Area. Call (925) 999-4095 or schedule at bayareahvacservice.com.

Key takeaways

• Check the air handler breaker first. Strip heaters run on a separate 240V circuit and that breaker trips more often than people expect.
• A loose or missing W2/Aux wire at the thermostat is a common cause after a thermostat swap. Worth a visual look at the terminal block, but if the wires look seated and strips still won't run, the problem is inside the air handler.
• If the breaker trips again after you reset it, stop and call. Repeated trips mean something is drawing too much current and resetting it is a fire risk.
• Sequencer and control board work requires a voltage meter and comfort with 240V line-voltage panels. Leave it to a tech.

Related questions

Written by Andrew Kuznetsov. Andrew is the founder and owner of Bay Area HVAC Service (ADRIUM Service Solutions). He holds a California Contractor License (CSLB #1136642), EPA 608 certification, and completed factory training at the Daikin/Goodman plant in Houston in 2025. He writes from direct field experience, not marketing copy.

Further reading

• Mitsubishi Hyper-Heat (H2i): How It Keeps Putting Out Heat When It Gets Cold in the Bay Area , Most people think heat pumps quit when it gets cold. Mitsubishi's Hyper-Heat (H2i) line is built to keep working as temperatures drop, which matters more here than folks expect, especially on cold mornings and up in the East Bay hills.
• Mitsubishi Mini-Split Blinking a Fault Light? What It Means and What to Do , A blinking light on your Mitsubishi indoor head is the system telling you something is wrong. Here is what the blink pattern means, the checks you can safely do yourself, and when to bring in a factory-trained tech.
• Why Your Mitsubishi Mini-Split Is Leaking Water Inside (and How to Fix It) , A Mitsubishi mini-split dripping water inside almost always comes down to condensate that can't drain. Here's what causes it, the safe checks you can do yourself, and when to call a tech.