If your home is heated by a heat pump, your thermostat manages two heat sources, not one. There is the heat pump itself, the efficient part that does most of the work, and a backup that exists for the moments the heat pump cannot keep up or is out of service. The way those two interact has three distinct behaviors, and homeowners mix them up constantly. Getting them straight is the difference between a normal winter bill and a quietly expensive one.
I am an EPA 608 certified tech, and most of the heat-pump calls I take in winter are not really about the cold. They are about a system running on the wrong source with nobody noticing until the bill arrives. So here is how the three behaviors actually work.
Auxiliary heat is the automatic helper
Auxiliary heat comes on by itself. When you bump the setpoint up four or five degrees at once, or on an unusually cold morning, the thermostat decides the heat pump alone will take too long. It fires the backup source, gas furnace or electric strips, alongside the heat pump. Once the room temperature reaches the setpoint, it drops the backup and lets the heat pump carry on solo.
The key point is that the heat pump never stops during aux. The backup is a temporary boost, not a replacement. A little aux on a cold January morning is normal. A lot of aux, day after day, is a symptom.
Emergency heat is the manual lockout
Emergency heat is something you turn on by hand. It tells the system to ignore the heat pump entirely and heat only on the backup. On a dual-fuel setup that means the gas furnace does all the work. On an all-electric system the resistance strips do.
You reach for emergency heat when the heat pump is broken. The outdoor unit is iced solid, the fan is not spinning, the compressor is struggling, or the air coming out is just not warm. It keeps the house comfortable while you wait for service. That is the whole purpose. It is not a cold-weather mode, and it is not a comfort upgrade. Leave it on past the repair and you are paying the expensive way for no reason.
Balance-point lockout decides who runs when
The third behavior is the one most homeowners never see in the menu. Thermostats wired for a heat pump usually have a balance point or two lockout temperatures. One locks out the backup above a certain outdoor temperature, so the strips cannot run on a mild day. The other can lock out the compressor below a set temperature, handing the load to backup when the heat pump efficiency drops too far to be worth it.
Set these correctly and the system uses the heat pump for nearly the whole season. Set them wrong, which I find often on retrofits where the installer left the defaults in place, and the thermostat calls backup hundreds of hours it never needed to. I check these settings on every heat-pump diagnostic.
Why the backup always costs more
The economics are simple. Electric resistance strips run at a coefficient of performance near 1. One unit of electricity in, one unit of heat out. A heat pump moves heat rather than creating it, so at our winter temperatures it returns roughly 2.5 to 3.5 units of heat for every unit of electricity. That means an hour on strip backup costs two to three times what the same hour on the heat pump costs.
A dual-fuel system with a gas furnace as backup is cheaper per BTU than strips, but the heat pump is still the efficient source most of the time. Either way, the math says to minimize backup runtime, not to lean on it for comfort.
Bay Area design temperatures rarely justify backup
This matters here because our climate is mild. The Tri-Valley winter design temperature sits around 30 to 34 degrees F, and we almost never spend extended hours below that. A correctly sized variable-speed heat pump holds usable capacity well into the teens. So in San Ramon, Danville, or Pleasanton, a properly sized unit carries the full load on nearly every winter night with no backup at all.
If your system is leaning on aux or emergency heat in our climate, the cause is usually one of three things. The unit is undersized for the house, the thermostat lockout settings are wrong, or the heat pump has a fault and the backup is masking it.
A few things to check yourself first
Before calling, run through these. They do not require tools and sometimes resolve the issue outright.
- Filter. A clogged filter starves airflow and can trigger backup operation. Pull it and replace if it has not been changed in the last few months.
- Thermostat mode. Confirm the mode is set to Heat, not Cool or Emergency Heat by accident. Also check that the fan is on Auto, not On (continuous fan pulls in unconditioned air and lowers supply temperature).
- Breaker and disconnect. If the outdoor unit is not running at all, check the breaker in your panel and the lockout disconnect at the unit. If the breaker has tripped, reset it once. If it trips again immediately, stop and call, that is a sign of a wiring or motor fault.
- Outdoor unit. Look at it. Is it running? Is there a thick cap of ice on the coil? Some frost during defrost cycles is normal. A solid ice block is not.
If those all check out and the system is still falling back on backup heat or blowing lukewarm air, you have a fault that the controls cannot clear on their own.
When constant backup means a real fault
A heat pump that keeps putting you on backup, or that runs while the house stays cold, almost always has a specific, fixable fault underneath it. The usual suspects are a low refrigerant charge, a reversing valve stuck in cooling, a defrost board that is letting the outdoor coil ice over, or a failed outdoor fan motor. None of these heal on their own, and every one of them is far cheaper to fix than a full winter of backup heat.
Diagnosing and fixing any of these involves refrigerant handling, line-voltage electrical work, or both. Refrigerant recovery requires EPA certification and specialized equipment. Reversing valve replacement means opening the refrigerant circuit. Defrost board diagnosis means working around live voltage inside the unit. This is the line where homeowner troubleshooting stops and a licensed tech takes over.
One note on planning, since it comes up often. The federal 25C heat-pump tax credit expired December 31, 2025, so do not size or time a project around it. If there are local incentives in play, we confirm what is actually paying at estimate time rather than quoting numbers that may have changed.
Call us
If your aux or emergency indicator is on more than occasionally, that is the time to call. We diagnose the actual fault, correct the thermostat lockout settings, and get the heat pump carrying the load again so the meter stops running on the expensive side of your system. You get a written quote before we start, and we aim for same or next-day service.
Reach us through adriumservice.com or call to book. The sooner the fault is identified, the less of the season you spend paying strip or furnace rates for heat your heat pump should be delivering for a fraction of the cost.
For deeper work, load calculations, balance-point engineering, or full system replacement, that is the specialty of our HVAC division, Bay Area HVAC Service. Factory-trained Daikin and Mitsubishi work is where the heavy engineering lives if your project goes beyond a repair.