When Does a Heat Pump Switch to Emergency Heat? Aux Heat Temps, Costs, and Fixes

Cold snaps can make any home wonder: when does a heat pump switch to emergency heat? This guide explains how heat pumps decide to use auxiliary or emergency heat, the temperatures that trigger it, what it costs, and how to keep backup heat use in check. It covers standard and cold-climate systems, smart thermostat settings, and practical troubleshooting tips for U.S. homes.

What “Emergency Heat” Means On A Heat Pump

Heat pumps provide efficient heating by moving heat from outdoors to indoors. When outdoor temperatures drop or the system needs help, backup heat steps in. Two terms matter: auxiliary heat and emergency heat.

Auxiliary heat (AUX) is automatic backup, typically electric resistance heat strips or a gas furnace in dual‑fuel systems. It assists when the heat pump cannot meet the thermostat setpoint quickly or during defrost cycles.

Emergency heat (EM HEAT) is a manual thermostat mode that disables the outdoor unit and runs only the backup heat source. It is intended for system failures, severe icing, or when the heat pump is locked out by design.

The phrase “when does a heat pump switch to emergency heat” is often used loosely. Technically, systems switch to auxiliary heat automatically, while emergency heat is user‑selected.

When A Heat Pump Switches To Auxiliary Or Emergency Heat

Automatic Auxiliary Heat Triggers

Most thermostats and controls engage auxiliary heat when specific conditions occur. These are common triggers that answer, in practice, when a heat pump “switches” to backup heat.

• Temperature difference (ΔT) threshold: If indoor temperature is 2–3°F or more below the setpoint, AUX may stage on to recover faster.
• Runtime limit: If the heat pump runs continuously for 10–30 minutes without closing the setpoint gap, AUX engages to help.
• Outdoor temperature lockout: Some systems enable AUX below a set outdoor temperature (commonly 25–40°F) configured by the installer or thermostat.
• Defrost cycles: During coil defrost, the system reverses to cooling; AUX energizes to prevent blowing cold air inside.
• Capacity shortfall: If delivered supply air temperature is low or airflow is restricted, controls may stage AUX to maintain comfort.

These controls vary by brand and model. Modern cold‑climate heat pumps reduce AUX use with variable speeds and advanced algorithms.

Manual Emergency Heat Situations

Emergency heat should be selected manually only when running the outdoor unit could cause harm or it cannot operate. Typical scenarios include:

• Outdoor unit failure: Tripped breaker, compressor fault, outdoor fan failure, or error codes.
• Severe icing: The entire outdoor coil is encased in ice and defrost does not clear it after multiple cycles.
• Refrigerant or sensor issue: Supply air is cool and the system never gains ground, with unusual noises or frost patterns.
• Intentional lockout: Dual‑fuel setups that lock out the heat pump at very low temperatures to use a gas furnace.

When emergency heat is on, expect much higher electricity or fuel use. It is best as a temporary measure until a technician resolves the root cause.

Typical Temperatures And The Balance Point

The “balance point” is the outdoor temperature at which heat pump output equals the home’s heat loss. Above it, the heat pump meets demand alone; below it, backup heat helps.

For many standard air‑source heat pumps, the balance point falls between 30–45°F, depending on home insulation, airflow, and equipment size. Better envelopes or larger systems push that lower; leaky homes push it higher.

Cold‑climate heat pumps (ccASHP) maintain useful capacity down to -5 to -13°F and often keep COP above 2 near 0°F. That significantly reduces auxiliary heat runtime in northern states.

Because configurations vary, there is no single temperature where a heat pump “switches” to emergency heat. Instead, auxiliary heat stages in as needed, and emergency heat is a manual override.

Key point: A properly sized and commissioned cold‑climate heat pump can heat most U.S. homes without frequent AUX above 10–15°F, while standard units often need AUX below 25–35°F.

How Thermostats Decide: Stages, Timers, And Lockouts

Thermostats coordinate staging between the heat pump and backup heat. The terms used vary by brand, but the logic is similar.

• Staging by ΔT: If the room falls several degrees below setpoint, Stage 2 (AUX) engages.
• Staging by time: If the heat pump cannot close the gap within a programmed time, Stage 2 starts.
• Outdoor lockouts: Thermostats with outdoor sensors can enable or lock out the heat pump or AUX below or above set temperatures.
• Adaptive algorithms: Smart thermostats “learn” how quickly a home heats and minimize AUX use while meeting comfort targets.

Installers often program these during commissioning. Homeowners can fine‑tune to reduce costs while maintaining comfort, especially with cold‑climate systems.

Signs Backup Heat Is Running (And How To Check)

Knowing when auxiliary or emergency heat is active helps control energy bills. Common indicators include:

• Thermostat display: “AUX Heat,” “Stage 2,” or “EM Heat” icons or text illuminate during operation.
• Warmer supply air: Electric strips or gas heat produce hotter air than the heat pump alone.
• Electric use spike: Resistance heat draws 5–15 kW; the electric meter or monitor will jump during AUX.
• Outdoor unit status: During EM Heat, the outdoor unit is off. During AUX, it typically still runs.
• Breaker labeling: Air handler heat strip breakers are often separate and large (e.g., 60–100A combined).

To verify, observe the thermostat, listen for the outdoor unit, and check your energy monitor. Smart thermostats often log staging in their app histories.

Cost, Efficiency, And Carbon Impact

Electric resistance heat has a Coefficient of Performance (COP) of 1.0. Heat pumps usually deliver 2–4 units of heat per unit of electricity, depending on outdoor temperature and equipment.

At a national average residential rate of roughly $0.17/kWh, 1 MMBtu of heat costs about $49–$50 with resistance heat (293 kWh × $0.17). A heat pump with COP 3 delivers the same heat for roughly $16–$17.

Key takeaway: Running emergency heat can cost 2–3× more than running the heat pump, so it should be used only when necessary.

Carbon impact varies by grid mix. Heat pumps generally reduce emissions compared to electric resistance and often compared to fuel oil or propane, especially on cleaner grids. Backup gas furnaces in dual‑fuel systems can be economical below a specific temperature but may increase emissions compared with cold‑climate heat pumps.

Regional Expectations Across The U.S.

Climate influences when auxiliary heat runs and whether emergency heat is needed. These patterns are typical but depend on home and equipment.

• Southeast and Gulf Coast: Standard heat pumps often meet demand without AUX except on rare freezes. Defrost events are occasional.
• Mid‑Atlantic and Southern Midwest: AUX may engage on nights below 30°F. Cold‑climate units reduce AUX dramatically.
• Northeast and Upper Midwest: Cold‑climate heat pumps handle most winter days; AUX shows up during single digits or severe wind chills. Dual‑fuel is common.
• Mountain West and High Plains: Large swings and wind loads increase AUX. Proper sizing and airflow are critical.
• Pacific Northwest: Mild winters favor heat pump efficiency; AUX is limited to cold snaps and defrost.

Best Practices To Minimize Emergency Heat Use

Small changes can substantially reduce auxiliary and emergency heat runtime while maintaining comfort.

• Weatherize first: Seal air leaks, add insulation, and upgrade windows where cost‑effective. Lower heat loss reduces AUX calls.
• Use modest setpoints: Avoid large nightly setbacks in very cold weather that can trigger AUX during morning recovery.
• Optimize thermostat settings: Increase AUX lockout temperatures only if comfort requires. Enable adaptive recovery and outdoor sensors.
• Keep airflow healthy: Replace filters regularly, keep vents open, and ensure ducts are sealed and balanced.
• Protect the outdoor unit: Clear snow and debris, ensure proper drainage, and maintain 12–24 inches of clearance.
• Schedule maintenance: Annual professional checks for refrigerant charge, defrost sensors, and electrical connections improve performance.

Troubleshooting Checklist Before Switching To Emergency Heat

Before manually choosing EM Heat, run through this quick list to avoid unnecessary costs.

1. Confirm setpoint and mode: Heat set correctly, not “Cool” or “Off.” Fan to “Auto,” not “On.”
2. Wait a cycle: In very cold weather, allow 20–30 minutes. The heat pump may close the gap without AUX.
3. Check outdoor unit: Running normally? Light frost is normal; a solid ice block is not.
4. Inspect filters and vents: Replace clogged filters and open blocked registers to restore airflow.
5. Thermostat staging: If “AUX” shows but the home warms steadily, EM Heat is not needed.
6. Breaker check: Ensure outdoor unit breakers are on. If they trip repeatedly, call a professional and use EM Heat temporarily.
7. Defrost timing: During defrost, wait 5–15 minutes. AUX may run, and normal heat resumes afterward.

If the outdoor unit is silent during a cold period with a large temperature drop, or it is encased in ice, switch to Emergency Heat and call for service.

Special Cases: Dual‑Fuel And Variable‑Speed Systems

Dual‑fuel (hybrid) systems pair a heat pump with a gas furnace. Instead of electric strips, the furnace acts as backup.

Controls in dual‑fuel setups often use an economic balance point. Below a chosen outdoor temperature, the furnace heats because it is cheaper or maintains comfort better. The heat pump may be locked out.

Emergency heat on dual‑fuel typically means running the furnace only. Homeowners should verify which heat source is active via thermostat labels or installer notes.

Variable‑speed and inverter heat pumps deliver more heat at lower temperatures while keeping COP higher. They reduce AUX use by ramping capacity instead of jumping to resistance strips. Proper sizing, line‑set lengths, and charge are vital to achieve these benefits.

Defrost Cycles And Why Backup Heat Helps

In humid, cold conditions, the outdoor coil can frost. The system reverses to cooling briefly to melt the ice—a defrost cycle.

During defrost, the indoor coil cools. To avoid uncomfortable drafts, controls often stage auxiliary heat to temper supply air. Defrost typically lasts 5–15 minutes and may occur every 30–90 minutes depending on conditions.

Frequent, long defrosts may signal low airflow, low refrigerant charge, or a failing sensor. Maintaining clean coils and correct charge reduces defrost energy penalties.

How Installers Configure Lockouts And Staging

Installer setup strongly influences when a heat pump appears to “switch” to backup heat. Key parameters include:

• Compressor lockout temp: Below this, the heat pump disables and backup heat runs. Rare for cold‑climate models unless dual‑fuel.
• Aux heat lockout temp: Above this, AUX is disabled to save energy. Useful with efficient systems and good envelopes.
• Staging thresholds: ΔT and runtime rules determine when AUX adds heat.
• Defrost strategy: Timed or demand‑based defrost alters how often AUX is needed.

Ask the contractor for the programmed temperatures and logic, and request adjustments if AUX is frequent in mild weather. A small change can reduce bills without sacrificing comfort.

Common Myths, Clarified

• Myth: Heat pumps automatically switch to emergency heat at 32°F. Reality: There is no universal temperature; AUX engages as needed, and EM Heat is manual.
• Myth: Emergency heat is “better” heat for comfort. Reality: It is typically more expensive and should be temporary.
• Myth: Never run a heat pump below freezing. Reality: Modern units work well well below freezing, especially cold‑climate models.
• Myth: Turning the thermostat way up warms the home faster. Reality: It often just triggers AUX sooner without speeding the heat pump.

Estimating Backup Heat Costs In A Real Home

Suppose a home has a 10 kW electric AUX heat strip. At $0.17/kWh, one hour of AUX costs about $1.70. If AUX runs six hours during a cold snap, that is $10.20 in a single day just for backup heat, plus the heat pump’s consumption.

If the same home’s heat pump averages 3 kW while heating with a COP of 2.5, that hour costs $0.51 and delivers more heat per kWh. The difference adds up quickly over a week of cold weather.

For dual‑fuel, compare the furnace cost per MMBtu to the heat pump’s effective cost. Many utilities publish fuel cost calculators to find the economic balance point.

How To Set Smart Thermostats To Reduce AUX

Smart thermostats can minimize auxiliary use while maintaining comfort. These tips help fine‑tune settings:

• ecobee: Set “Heat Pump Min Outdoor Temp” low enough for your unit (e.g., 5–15°F for ccASHP). Increase “Heat Pump to Aux Temp” so AUX does not engage for small setbacks.
• Nest: Choose “Max Savings” or a middle Heat Pump Balance. Set “Aux Heat Lockout” near 20–30°F for standard units or lower for ccASHP, based on comfort.
• Honeywell: Use an outdoor sensor. Adjust ΔT staging from 2°F to 3°F to delay AUX if acceptable. Enable intelligent recovery to preheat gradually.

Always confirm with installer recommendations and equipment specifications. Over‑aggressive lockouts can reduce comfort during extreme cold.

Safety And Equipment Protection

It is safe to run emergency heat, but costs will rise significantly. Protect the equipment by avoiding outdoor unit operation when:

• Fan is not spinning in freezing rain or snow: Ice can build rapidly and strain components.
• Coil is fully encased in ice: The compressor should not run until cleared.
• Electrical odors or unusual noises suggest a failing component.

In these cases, switch to EM Heat, power down the outdoor unit at the disconnect if needed, and call a licensed HVAC technician.

Frequently Asked Questions

Does Emergency Heat Come On At A Set Temperature?

No. Emergency heat is a manual mode. Auxiliary heat may engage at or below configured temperatures or when the heat pump cannot meet demand.

Should Emergency Heat Be Turned On Before A Cold Front?

Generally no. Let the heat pump run and allow auxiliary heat to assist automatically if needed. Use EM Heat only if the outdoor unit is damaged, iced solid, or locked out intentionally.

Why Does The Thermostat Say “AUX” During Defrost?

During defrost, the system reverses to melt frost. AUX energizes to avoid cold indoor air. This is normal and brief.

Is It Cheaper To Use A Space Heater Instead Of AUX?

Electric space heaters are also resistance heat (COP ~1). They may heat a small occupied area cheaply if the central setpoint is lowered, but they do not reduce whole‑home energy use compared to the heat pump.

How Low Can Cold‑Climate Heat Pumps Run Without AUX?

Many ccASHP models maintain capacity into single digits and operate down to -13°F or lower, depending on brand and sizing. Check manufacturer performance tables for exact data.

Can Large Night Setbacks Save Money In Winter?

Large setbacks in very cold weather often trigger AUX during morning recovery. Smaller setbacks or steady setpoints typically save more with heat pumps.

What If AUX Runs Constantly Above 35–40°F?

That may signal incorrect thermostat settings, poor airflow, undersizing, or a refrigerant issue. Have a technician check staging, charge, and ductwork.

Glossary Of Common Terms

• Auxiliary Heat (AUX): Automatic backup heat that assists the heat pump.
• Emergency Heat (EM HEAT): Manual mode that disables the outdoor unit and runs backup only.
• Balance Point: Outdoor temperature where heat output equals home heat loss.
• Defrost Cycle: Reversing operation to melt frost on the outdoor coil.
• COP (Coefficient of Performance): Ratio of heat output to electrical input.
• HSPF2: Seasonal efficiency metric for heat pumps under new test procedures.

Actionable Steps To Answer “When Does A Heat Pump Switch To Emergency Heat?”

Because emergency heat is manual, the practical question is when auxiliary heat engages. Follow these steps to manage both effectively:

1. Know your system: Identify whether backup is electric strips or gas furnace.
2. Review thermostat settings: Set reasonable AUX and compressor lockouts with outdoor sensors.
3. Observe behavior: Watch for “AUX” indications during cold evenings and defrost cycles.
4. Optimize comfort vs savings: Use smaller setbacks and adaptive recovery to limit AUX.
5. Use EM Heat only when needed: Failure, severe icing, or lockout—then call for service.

Helpful Resources

For deeper technical guidance and efficiency programs, consult:

• U.S. Department of Energy: Heat Pumps
• ENERGY STAR: Air‑Source Heat Pumps
• NEEP: Cold‑Climate Air‑Source Heat Pumps

Key Takeaways At A Glance

• Emergency heat is manual; auxiliary heat is automatic.
• No single temperature triggers EM Heat. AUX often engages below 25–35°F for standard units, lower for cold‑climate models.
• Costs jump 2–3× when running resistance emergency heat.
• Smart thermostat settings and weatherization can sharply reduce AUX runtime.
• Use EM Heat only for failures, severe icing, or intentional lockouts; then schedule service.