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.
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.
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.
Getting it diagnosed
If your aux or emergency indicator is on more than rarely, that is the time to call. We diagnose the actual fault, correct the thermostat lockouts, and get the heat pump carrying the load again so the meter stops running on the expensive side of your system. Our diagnostic fee is $75, and we waive it when the repair proceeds. Every job comes with a written quote before we start.
The deep heat-pump design work, the load calculations, balance-point tuning, and full system replacement, is the specialty of our dedicated HVAC division, Bay Area HVAC Service. I run that side as factory-trained Daikin and Mitsubishi work, and it is where the heaviest engineering lives if your project goes beyond a repair.