The best winter setting for a heat pump is a steady one: start around 68°F while you are home and awake, keep any nighttime setback to two or three degrees, and leave the fan on Auto. Big setbacks and big jumps on the thermostat are what pull a heat pump onto its electric backup heat, which costs far more to run. In Leander and across Central Texas, where many homes heat with a heat pump through a short heating season, getting these few settings right matters more than any single temperature.
What temperature should I set my heat pump to in winter?
Carrier recommends starting at 68°F in winter while you are home and awake, then adjusting by 1 or 2 degrees at a time until the house feels right. A heat pump holds a set temperature well; it is the swings that cause trouble, so pick a number you can live with and leave it there.
Expect the air from the vents to feel cooler than a furnace's even when everything is working. Carrier puts heat pump supply air at roughly 85 to 100°F in heating, against about 120 to 140°F from a gas furnace. Air in that range is below skin temperature, so the room warms steadily rather than in blasts.
Should I turn my heat pump down at night?
Only a little. Carrier's heat pump guidance is that lowering the heat by 10 degrees at night can waste money, because recovering that drop the next morning often relies on the auxiliary heat strips, and it recommends mild setbacks of only two to three degrees when sleeping or away.
The U.S. Department of Energy's general advice is to turn the thermostat down 7 to 10°F for 8 hours a day in the heating season, but its Energy Saver guide adds a caveat for heat pumps: do not do this without a smart or programmable thermostat designed for heat pumps. A thermostat like that can bring the house back up slowly on the heat pump instead of calling the strips.
What is the difference between auxiliary heat and emergency heat?
Auxiliary heat turns on by itself; emergency heat is a switch you flip. Carrier describes auxiliary heat as a secondary heat source, usually electric resistance strips inside the air handler, that comes on automatically when the heat pump cannot reach the set temperature on its own. Emergency heat shuts the heat pump off and runs the backup heat alone.
| Setting | How it turns on | What runs | When it is normal |
|---|---|---|---|
| Auxiliary heat (AUX) | Automatically | Heat pump plus backup strips | Cold snaps, defrost cycles, recovering from a big setpoint jump |
| Emergency heat (EM) | You switch it on | Backup strips only | Only when the heat pump has broken down |
The cost gap is the reason it matters. Carrier explains that electric auxiliary heat is 100% efficient while a heat pump can be 300% efficient, so every hour the strips carry the load costs more than the same hour on the heat pump. Carrier says to use emergency heat only when the heat pump has failed, and to call an HVAC professional rather than leave it there as a substitute.
Why does my thermostat show AUX?
A thermostat showing AUX means the backup heat is running, and Carrier calls that normal in very cold weather or after the setpoint is raised by several degrees at once. Carrier notes there is no single outdoor temperature at which every heat pump brings on auxiliary heat. It depends on the point where the system's capacity meets the home's heating demand.
The setpoint jump is the one you control. Carrier says raising the thermostat by more than 2 to 3 degrees at once can call the auxiliary heat to close the gap quickly. On a heat pump thermostat that stages heat, Carrier explains the strips come on when the room falls two to three degrees below the setpoint and shut off again as it gets close. Raise the temperature a degree or two at a time and the heat pump can usually get there alone.
Defrost is the other normal cause. Carrier notes that supplemental heat can run during a defrost cycle so the vents do not blow cold while the outdoor unit clears its frost.
Should the fan be on Auto or On in winter?
Leave the fan on Auto. Carrier names a fan set to On as the most common reason a furnace seems to blow cold air, because the blower keeps moving room-temperature air between heating cycles. A heat pump's air is already mild, so air circulating between cycles on a heat pump feels even cooler at the vent.
Does my thermostat need to be a heat pump thermostat?
Yes. Carrier explains that a heat pump thermostat switches the reversing valve through an O or B terminal and manages the auxiliary heat stages, and that a standard thermostat lacks those terminals and can leave the auxiliary heat running constantly. A smart thermostat works on a heat pump only if it explicitly supports heat pumps and has the O/B and auxiliary terminals; Carrier warns an incompatible one can cause malfunctions and damage the compressor.
Two features are worth looking for on a heat pump. Carrier describes adaptive recovery, which learns how long the house takes to warm and starts the heat pump early instead of leaning on the strips, and an auxiliary lockout temperature, which keeps backup heat off when it is mild outside. Most smart heat pump thermostats also need a C-wire for steady 24-volt power, which Carrier says may mean an adapter kit or a professional install. We fit and set up heat pump thermostats as part of our thermostat installation work.
Check the schedule after the clocks change this fall. Carrier notes a programmable schedule can shift after daylight saving time or a power outage.
When does AUX running mean something is wrong?
Backup heat running on a mild day is a warning sign. Carrier says AUX running when it is above 40°F outside may point to a dirty air filter, a refrigerant leak, a failing sensor, or a thermostat raised too quickly, and that dirty filters or low refrigerant can push a system onto backup heat even in mild weather.
The filter is the part you can check. Carrier recommends checking a heat pump filter every 60 to 90 days. If the filter is clean, the setpoint is steady and AUX still runs on mild days, the cause is inside the system, and refrigerant and sensor faults belong to a licensed technician under heating repair.
Getting it set up right
Comfort Pros is a Carrier factory authorized dealer serving Leander, Cedar Park, Georgetown and Liberty Hill, licensed under TACL# TACLB116912E. If your heat pump keeps running on backup heat, or you are not sure your thermostat is wired for a heat pump, call us and we will check the staging, the thermostat setup and the system before the first cold snap does.
A fall maintenance visit is also a good time to have the whole system looked over before the coldest nights arrive.