Auxiliary Heat: What AUX Means on Your Thermostat, What Triggers It, and What It Costs in Power
- What Is Auxiliary Heat? The Meaning of AUX on a Heat Pump Thermostat
- What Triggers Auxiliary Heat, and How Much Power Do Heat Strips Use?
- Winter Outage Planning: Keep Basic Loads On or Keep the Heat Pump Running
- Auxiliary Heat vs Emergency Heat: Same Heat Source, Different Switch
- Is Auxiliary Heat Expensive? How to Keep It From Running More Than It Needs To
- When Auxiliary Heat Stays On: Signs Your Heat Pump Needs Service
- Conclusion
- FAQs
It's a frosty January morning, and your thermostat suddenly reads "AUX" or "Aux Heat On." Is something broken? Probably not. Auxiliary heat is a normal part of how a heat pump works when the air outside gets cold. Here, you'll learn what triggers it, how much power heat strips draw compared with the heat pump, and how to tell normal AUX from a real problem. And since a winter outage turns aux heat into a backup power question, we'll walk through that decision too.
What Is Auxiliary Heat? The Meaning of AUX on a Heat Pump Thermostat
So what is auxiliary heat? It's a second heat source that turns on by itself when the heat pump can't keep up. That's usually electric strips in the air handler, or a gas or oil furnace. Carrier's aux heat guide describes it as backup heat that's built right into a heat pump setup.
Searches like "what does auxiliary heat mean," "whats auxiliary heat," "what do auxiliary heat mean," "what does aux heat mean," "what is aux heat," and "what do aux heat mean" share one answer. Electric strips may assist the heat pump, while a dual-fuel furnace typically takes over according to the system controls. Check whether you have strips or a furnace, since that changes power draw and outage needs.
Why a Heat Pump Needs Backup Heat
Your heat pump doesn't create warmth; it carries it indoors from outside. That's why ENERGY STAR's heat pump guidance says it delivers up to three times more heat than the power it uses. But colder air holds less heat to move. A Bonneville Power Administration lockout guide notes that output falls as the temperature drops.
What the Words on Your Thermostat Mean
Each label tells you what's running. Here's a quick key.
Heat (no AUX): Only the heat pump compressor and fans run.
AUX, Aux Heat On, or Aux Ht: Backup heat is on automatically. Strips run with the heat pump; a dual-fuel system usually switches to the furnace.
Defrost (not shown on every model): The heat pump briefly reverses to melt frost, and backup may run.
EM Heat, Em Ht, or Emergency: Only backup heat runs and the compressor is off. You turn this on manually.
A steady setpoint keeps AUX quiet, and other heat pump thermostat settings follow from it. Next: what triggers AUX.
What Triggers Auxiliary Heat, and How Much Power Do Heat Strips Use?
AUX comes on for a few predictable reasons. Still, Carrier notes that no one outdoor temperature triggers it on all of its models. Power is easier to pin down.
The Four Common Triggers
Most AUX moments have one of four causes. Here's how each works.
Cold air below the balance point. Past the point where heat pump output matches heat loss, backup fills the gap.
Defrost. Trane's aux heat guide lists defrost mode, when backup keeps vent air warm.
A big jump on the thermostat. Trane says raising the heat by more than 3 to 4 degrees at once can bring on AUX.
Recovery after a setback. Turning a heat pump down at night saves no energy, per ENERGY STAR. NYSERDA's heat pump operating tips suggest raising it a few degrees at a time.
Who Decides the Trigger Temperature
Installers can often set a lockout. For its PTCS program, BPA calls for a strip lockout at 35°F or below, which keeps strip use low and saves energy.
That's a regional program rule, not a national one. Ask your installer what your system uses.
How Much Power Auxiliary Heat Uses
Strips draw roughly their kilowatt rating. If you searched "heater auxiliary heat" to learn yours, here's how Trane's 5TEM7 air handler data lists its kits.
Trane heater kit | Rated size | Heater amps at 240V | Energy used in one full hour |
EH05-SVZ-S / -M | 5 kW | 20.8 A | 5 kWh |
EH08-SVZ-S / -M | 8 kW | 33.3 A | 8 kWh |
EH10-SVZ-M / EH10-MPA-LB | 10 kW | 41.7 A | 10 kWh |
EH15-MPASLB | 15 kW | 31.2 A on each of two circuits | 15 kWh |
EH17-MPASLB | 17.5 kW | 36.5 A on each of two circuits | 17.5 kWh |
Trane's 3-ton heat pump data lists 14.1 A for the A5HP5036A1000A compressor and 0.64 A for its fan at 208/230V. Trane calls the compressor figure a branch circuit selection current for sizing wire and breakers, not a power reading.
A 10 kW kit uses 10 kWh an hour at 240V. Carrier rates strips at 100% efficiency and a heat pump at up to 300%, so a heat pump can match that heat with as little as a third of the power. Strips work like furnace elements, so electric furnace wattage and cost math applies.
Those figures matter most in an outage. Then you pick basics or the heat pump.
Winter Outage Planning: Keep Basic Loads On or Keep the Heat Pump Running
In an outage, the whole system stops. Should your backup keep 120V basics running, or a compatible 240V heat pump circuit? EcoFlow TITAN Classic and EcoFlow TITAN Plus fit those two goals, and neither is meant to run aux heat.
Count the Three Loads Separately
Each part draws its own power. The Trane figures below identify electrical requirements, not the measured power of a matched heat pump system.
Load | Published electrical data | Why it matters |
Compressor and outdoor fan | Compressor RLA 14.1 A; outdoor fan full-load current 0.64 A at 208/230V | Compressor LRA is 83.2 A. A backup-power assessment must consider starting requirements separately from running power. |
Indoor blower (Trane 5TEM7C04) | Blower full-load current 3.3 A at 208/230V | This current rating is not a measured wattage; obtain input-power data for the intended operating conditions. |
Electric aux heat | 5 to 17.5 kW | The biggest load by far. |
Do not add these current ratings and label the result as operating watts. An electrician should assess the compatible heat pump, indoor blower, starting demand, and any enabled heat strips against the backup system's limits. A 5 kW strip kit running continuously for an hour alone uses 5 kWh, before blower demand and conversion losses.
EcoFlow TITAN Classic for 120V Basics
For most homes, basics come first. EcoFlow TITAN Classic handles 120V loads like the fridge, lights, and Wi-Fi. It can run a gas furnace's 120V blower if an electrician approves. It can't run a heat pump or any 240V load.
Say your fridge uses 630 kWh a year (1,726Wh a day). After 15% conversion loss (5,000Wh × 0.85 = 4,250Wh), the base battery runs it about 2.5 days.
EcoFlow TITAN Plus for a Compatible Heat Pump Circuit
If the heat pump matters more, look at EcoFlow TITAN Plus. It suits homes where the heat pump and other loads fit its output. An electrician's load calculation checks the nameplate volts, running amps, and starting current.
Estimate runtime only after establishing the actual combined power of the compressor, fans, controls, and any enabled backup heat. Runtime in hours is usable battery energy in kWh divided by average load in kW; account for cycling and conversion losses. Knowing how 120V and 240V circuits differ helps identify the required backup equipment.
People often mix up AUX and EM heat. One runs itself, and the other waits for you.
Auxiliary Heat vs Emergency Heat: Same Heat Source, Different Switch
AUX and EM heat may use the same backup strips or furnace. AUX is called automatically; EM is selected manually and turns the heat pump compressor off. During AUX, electric strips can assist a running heat pump, while a dual-fuel system typically switches between heat sources.
Auxiliary heat (AUX) | Emergency heat (EM) | |
How it starts | Automatically | Manually, by you |
Heat pump compressor | Usually running with electric strips; may stop when a dual-fuel furnace takes over | Off |
Heat source | Strips or furnace | Same strips or furnace |
When it's meant to run | Cold snaps, defrost, setpoint jumps | Only when the heat pump has failed |
Power use with strips | Strips plus heat pump | Strips plus blower only |
When to Use Emergency Heat
Carrier advises switching to EM heat only when the heat pump has actually failed or broken down. With strips, EM puts all your heat on the costly resistance side. If the outdoor unit is iced over or damaged, call a technician and use EM only until the repair.
Next comes cost. A few habits keep aux heat in check.
Is Auxiliary Heat Expensive? How to Keep It From Running More Than It Needs To
Trane notes that strips are less efficient and burn more energy than the heat pump. A bit of AUX on cold days is fine, and these habits trim the extra.
Hold one steady setting. ENERGY STAR advises against frequent changes; Trane suggests about 68 degrees.
If you set back, recover slowly. NYSERDA's tip is to raise it in small steps.
Use a heat pump thermostat. Let your installer set lockouts.
Keep airflow clear. Carrier links dirty filters to mild-weather AUX, so change filters and clear snow.
Know your dual-fuel setup. A furnace may cost more or less, per Trane.
Don't disable backup heat, though. NYSERDA says it should run enough in extreme cold to keep pipes from freezing.
Cost is one reason to watch AUX. Long runs in mild weather are another.
When Auxiliary Heat Stays On: Signs Your Heat Pump Needs Service
Hours of AUX in mild weather isn't normal. Trane says to look into AUX that runs over an hour or two above 40 degrees. Carrier links mild-weather AUX to a clogged filter, a refrigerant leak, or a faulty sensor.
Quick Checks Before Booking a Repair
Check the filter, thermostat mode, recent setpoint changes, and ice on the outdoor unit. Call a technician if ice will not clear, the system short cycles, or a fault code appears. An outdoor unit stopping while a dual-fuel furnace takes over can be normal. If your system uses electric strip backup and the outdoor unit unexpectedly stops, have a technician check it against the manufacturer's operating sequence.
When Aux Heat Won't Come On
If aux heat never comes on, have a pro check the strips or breaker. Ice storms cause the reverse, since freezing rain damage to HVAC can leave AUX running nonstop until it's repaired.
Conclusion
Auxiliary heat is your heat pump's automatic backup, usually 5 to 17.5 kW strips on Trane's air handlers or a furnace. It kicks in for cold, defrost, setpoint jumps, or a fault. Hold a steady setpoint, keep the filter and outdoor unit clear, keep EM heat for a failed unit, and get service when AUX lingers for hours on days above 40°F. For outages, compare EcoFlow TITAN Classic for 120V basics with EcoFlow TITAN Plus for a compatible heat pump circuit.
FAQs
What does aux heat mean on a thermostat?
Your thermostat has automatically called for backup heat to reach the setpoint. Electric strips may run alongside the heat pump. In a dual-fuel system, the controls typically switch between the heat pump and furnace according to operating conditions and settings.
Is it bad if auxiliary heat comes on?
No. Short bursts are normal in cold weather, during defrost, or after a big setpoint jump. Trane suggests investigating if AUX stays on more than an hour or two above 40°F.
What is the difference between auxiliary heat and emergency heat?
They may use the same backup heat source. AUX is automatic: electric strips can assist the heat pump, while a dual-fuel furnace typically takes over from it. EM is manually selected and shuts the compressor off; use it according to the manufacturer's instructions when the heat pump fails.
How much power does auxiliary heat use?
Resistance strips draw roughly their kilowatt rating at the specified voltage. A 10 kW kit running continuously at its rated output for one hour uses 10 kWh, excluding the blower. Heat pump energy use varies with the equipment and conditions; use actual input watts or measured kWh rather than deriving it from branch-circuit current ratings.
Can a home battery run auxiliary heat during an outage?
Don't plan on it. A 5 kW kit can drain a 5kWh battery in an hour, and bigger kits exceed its output. EcoFlow TITAN Classic covers 120V basics. EcoFlow TITAN Plus fits only a heat pump circuit a load calculation supports.
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