Winter
Thermostat Settings for a Heat Pump Winter
Heat pump thermostat settings work best when they stay steady: pick a comfortable temperature in Heat mode, skip big overnight setbacks that call strip heat, and leave aux and compressor lockouts to the installer.
What setting works best for a heat pump in winter?
The best heat pump thermostat setting is one comfortable number in Heat mode, held day and night. ENERGY STAR's heat pump tips say that, unlike a furnace or boiler, a heat pump does not save energy when you turn it down while away or asleep. The same page says to avoid Auto mode, so the system does not flip between heating and cooling on a mild afternoon followed by a 25°F night.
On a 20°F January morning in Lexington, where the 1991–2020 normal January low is 25.4°F (NOAA normals), a heat pump may run for long stretches. That is normal.
Emergency heat is not a cold-weather setting. Goodman's homeowner FAQ says there is no need to switch on emergency or auxiliary heat just because it is below freezing; aux comes on by itself when the heat pump needs help. The aux and emergency heat page covers when the EM position does belong.
Why can a big setback cost more?
A big setback can cost more because the thermostat may call for electric strip heat to bring the house back up, and BASC calls resistance backup an inefficient and typically expensive form of heat. A DOE Building America report runs electric resistance heat at a COP of 1.0: one unit of heat for each unit of electricity. By ENERGY STAR's figure, the heat pump itself delivers as much as three times the heat energy it draws in electricity.
DOE's Energy Saver Guide (2022) says plainly not to set back a heat pump manually if it brings on backup electric-resistance heating. The same guide's general advice of turning the thermostat down 7–10°F for 8 hours carries a heat pump caveat: don't do it without a smart or programmable thermostat designed for heat pumps.
Large manual setbacks can cost more because the thermostat may call for strip heat to recover. If you set back at all, keep it small and use a thermostat with heat-pump recovery logic; with a basic thermostat, a steady setting is the safe choice.
Large manual setbacks can cost more because the thermostat may call for strip heat to recover.
What does a heat pump thermostat do differently?
A thermostat made for heat pumps manages two heat sources and tries to keep the cheap one working. DOE's Federal Energy Management Program says these thermostats ramp the temperature up slowly to avoid activating the electric resistance elements, and it calls for them on federal purchases.
Several things show on the display. Honeywell's support page on Aux Ht and Em Ht says "Aux Heat On" appears when backup heat runs, and that aux and the compressor may run together. Trane notes AUX can also light up during a defrost cycle, which is routine from the mid 40s down to the high teens.
Some thermostats stage aux heat automatically. ecobee's threshold article describes an Aux Savings Optimization option that decides when to bring on auxiliary heat based on the gap between the setpoint and the indoor temperature. That logic is the reason a small setback can be tolerable with the right thermostat and costly with a basic one.
If you are shopping for a replacement, supply houses list thermostats by type and stage count (PlumbingSupplyAndMore, shares owners with Heat Pump Weather); check that a model supports heat pumps, and dual fuel if you have a furnace. Wiring changes beyond the maker's homeowner instructions are a job for the installer.
What are aux and compressor lockout settings?
Aux and compressor lockouts are outdoor-temperature limits that tell the system when each heat source may run. ecobee's threshold settings article defines "Aux Heat Max Outdoor Temperature" as the setting that keeps auxiliary heat from running above a chosen outdoor temperature, and "Compressor Min Outdoor Temperature" as the lowest outdoor temperature at which the compressor operates.
These limits connect to the balance point, the outdoor temperature below which the heat pump alone can no longer meet the house's heating load. For standard single-speed equipment sized to the summer cooling load, one ORNL report often finds it between the upper 20s and low 30s; variable-speed and cold-climate models can go lower. That is a description, not a target to dial in.
ecobee says the thresholds can only be changed on the thermostat itself, under Installation Settings, not in the app. The article adds a warning: if you are unfamiliar with your equipment's capabilities, contact ecobee Support before making any changes. The article gives no default values, so none are quoted here.
Which settings should only the installer change?
Anything under an installation or installer menu belongs to the person who set up the equipment. That covers aux and compressor lockouts, staging, equipment type, and the switchover point on a dual fuel system, which Carrier says a technician adjusts from the local climate and energy rates.
Those values depend on the equipment and the house. A lockout set too warm can leave the compressor idle on a 30°F night while strip heat carries the load; one set too cold can leave rooms chilly during a cold snap. Ask the installer what each threshold is set to and why, and write the answers down.
Your part is the everyday layer: mode, setpoint, schedule, fan, and checking the filter monthly. Thermostat wiring, the breaker panel and the outdoor disconnect's insides are not homeowner territory. If you're unsure, call a licensed technician. Ask the installer to show you which menus to leave alone.
| Setting | Who changes it | What it does |
|---|---|---|
| Mode (Heat, not Auto) | You | Keeps the system from switching to cooling on a mild day |
| Setpoint and schedule | You | One steady number; small setbacks only with heat-pump recovery logic |
| Em Ht position | You, temporarily | Compressor off, backup heat only, for a failed outdoor unit |
| Aux Heat Max Outdoor Temperature | Installer | Blocks strip heat above a chosen outdoor temperature |
| Compressor Min Outdoor Temperature | Installer | Lowest outdoor temperature the compressor runs |
| Dual fuel switchover | Installer | Outdoor temperature where the furnace takes over |
Loose ends
Smaller questions readers ask about this subject, answered briefly.
Should a heat pump thermostat be set to Auto in winter?
ENERGY STAR advises against Auto mode and suggests Heat in winter. Auto can switch the system into cooling on an unseasonably warm afternoon, then back to heating at night.
Is it bad to turn the heat pump down at night?
A small setback is fine with a thermostat designed for heat pumps, per DOE's 2022 guide. With a basic thermostat, a big setback can bring on strip heat during recovery, which can erase any saving.
Why does my thermostat show Aux on a mild morning?
Trane says AUX can appear during a defrost cycle, and Honeywell says aux runs whenever the compressor needs help. Brief aux periods are normal; aux that seems to run most of the time on mild days is worth asking a technician about.
Can a smart thermostat lower my heating bill with a heat pump?
DOE recommends a smart or programmable thermostat with functions suitable for a heat pump. No source read for this site gives a dollar or percent figure for heat pump homes.
Where are the lockout settings on my thermostat?
On ecobee models they sit under Main Menu, Settings, Installation Settings, Thresholds. Other brands use similar installer menus; ask the installer before changing anything there.
Should the fan run On all winter?
Auto is the usual choice, so the fan runs when heat is being delivered. In summer the Building America Solution Center specifically recommends Auto over On, as the summer settings feature explains.
Does a new thermostat need to match my heat pump?
Yes. The model must support heat pumps, the number of heating stages you have, and dual fuel if a furnace is the backup. Connecting it to the system is a job for the installer if the maker's homeowner guide does not cover your setup.