Heat Pump Heat Strips: How Auxiliary Heat Works, Costs, Sizing, and Smart Control

Heat pump heat strips provide electric backup heat when outdoor temperatures dip or during defrost cycles. Used properly, they keep homes comfortable without blowing the energy budget. This guide explains what auxiliary heat strips are, when they turn on, how to size them, what they cost to run, and how to control them for comfort and efficiency in U.S. homes.

What Are Heat Pump Heat Strips?

Heat pump heat strips are electric resistance heating elements installed in the air handler or ductwork. They act as auxiliary or emergency heat, supplementing the heat pump when extra capacity is needed or when the compressor is unavailable.

Unlike a heat pump, which moves heat using refrigerant, heat strips generate heat directly by passing electricity through coils. They are simple, reliable, and deliver predictable output measured in kilowatts (kW).

How Heat Strips Work

When the thermostat calls for more heat than the heat pump can provide, the control board energizes staged heat strips. Air passes over the energized coils and warms before entering the supply ducts. Many systems use sequencers to bring strips online in steps to avoid large inrush currents.

Output is linear: 1 kW ≈ 3,412 BTU/h. A 10 kW kit adds roughly 34,000 BTU/h, similar to a small furnace. Because resistance heat has a coefficient of performance (COP) near 1, every kWh consumed becomes heat.

Auxiliary Versus Emergency Heat

Auxiliary heat (Aux) turns on automatically with the compressor whenever extra heat is needed. Emergency heat (EM Heat) is a manual mode that forces strips on and locks the compressor off. EM Heat is for compressor failures or maintenance, not routine cold weather.

On many thermostats, seeing “Aux Heat” means the strips are assisting. Seeing “EM Heat” means the system is intentionally bypassing the heat pump and running only the electric heat strips.

When Do Heat Strips Turn On?

Heat pump heat strips engage under several normal operating scenarios. Proper controls minimize their runtime while maintaining comfort.

• Outdoor balance point: Below a certain temperature, the heat pump’s capacity drops below the home’s heat loss. Strips make up the difference.
• Defrost cycles: In heating mode, the outdoor coil can frost. During defrost, the system briefly runs in cooling mode and may energize strips to temper supply air.
• Large temperature recoveries: After deep setbacks or when doors/windows are open, strips may assist to meet a large setpoint change quickly.
• Compressor lockout or fault: If the compressor is locked out due to low temperature, pressure protection, or failure, EM Heat maintains heating.

Modern variable-speed heat pumps with inverter compressors have lower balance points and need heat strips less often. Some cold-climate models keep strong capacity below 0°F, reducing strip size requirements.

Sizing Heat Strips: kW, Tons, And Climate

Correct heat strip sizing balances comfort, electrical capacity, and energy cost. Oversized kits increase operating cost and can cause frequent high-limit trips; undersized strips may not maintain setpoint during cold snaps or defrost.

Rules Of Thumb And Load-Based Sizing

Best practice is to size by heating load (Manual J or equivalent) and the heat pump’s capacity at design temperature. Strips should cover the shortfall plus a margin for defrost and recovery.

• Approximate rule: 2–3 kW per ton of heat pump in mild climates; 3–5 kW per ton in colder regions.
• 1 kW ≈ 3,412 BTU/h. If the load shortfall is 18,000 BTU/h at design, a ~6 kW strip likely suffices.
• Use staged kits (e.g., 5 kW + 5 kW) to modulate and limit demand charges.

Airflow matters. Temperature rise (ΔT) from strips is governed by ΔT ≈ (kW × 3,412) ÷ (1.08 × CFM). Adequate CFM prevents overheating and limits high-limit trips.

Suggested Heat Strip Ranges

These ranges are general guidance. Always confirm with load calculations and manufacturer specifications for the air handler and duct capacity.

Energy Use, Costs, And Efficiency

Resistance heat is simple and reliable but energy intensive. Understanding usage and cost helps optimize settings and reduce bills.

COP, HSPF2, And When Strips Make Sense

Heat pumps often deliver 2–3 units of heat per unit of electricity (COP 2–3) in mild cold. At lower outdoor temperatures, COP declines. Heat strips have COP ≈ 1. If the heat pump can meet most of the load, it is cheaper per BTU than strips.

Seasonal efficiency is rated by HSPF2 (heating) and SEER2 (cooling). While strips are not rated by HSPF2, systems that maintain higher capacity at low ambient temperatures reduce auxiliary heat run time and seasonal costs.

Hourly Operating Cost Examples

At a residential electricity rate of $0.12–$0.20/kWh, the cost of running heat strips is:

The same hour of heating from a heat pump at COP 2.5 costs less than half as much per BTU. Minimizing strip runtime is the biggest lever to cut winter electric bills.

Defrost And Short Bursts

During a defrost cycle, the system may run heat strips for only a few minutes to temper supply air. Short, intermittent use has minor impact on total costs. Long continuous Aux Heat operation during mild weather is a red flag to investigate.

Controls And Thermostat Settings

Good controls deliver comfort without overusing heat pump heat strips. Many issues stem from thermostat settings rather than equipment failures.

Use A Heat Pump Thermostat

Install a thermostat designed for heat pumps with auxiliary heat staging and outdoor temperature sensor support. Smart thermostats can adaptively stage strips and learn recovery times to avoid unnecessary Aux calls.

Setbacks And Recovery

Large setbacks often trigger Aux Heat during morning recovery. For heat pumps, use small setbacks of 2–3°F, or hold a steady setpoint. If using time-of-use rates, start recovery early so the compressor, not strips, does most of the work.

Outdoor Lockouts And Balance Points

Configure an aux heat lockout above a chosen outdoor temperature (e.g., 35–40°F) when the heat pump can meet the load alone. Some systems also set a compressor lockout at very low temperatures to protect the unit and rely on strips or dual-fuel backup.

Staging, Droop, And Cycle Rates

Enable multi-stage Aux Heat with time and temperature thresholds. For example, add the first 5 kW if indoor temperature is more than 1–2°F below setpoint for several minutes; add more only if needed. This reduces peak demand and avoids overshooting.

Electrical And Safety Considerations

Heat strips draw significant current and must be installed with proper circuits, controls, and safeties. Most kits integrate with the air handler and include sequencers and high-limit protection.

Voltage, Amps, And Breakers

At 240 V, current ≈ kW × 4.17. Because electric heat is considered a continuous load, the National Electrical Code (NEC) requires branch circuits sized at 125% of the continuous load.

• 5 kW: ≈ 20.8 A load → 30 A breaker
• 10 kW: ≈ 41.7 A load → 60 A breaker
• 15 kW: ≈ 62.5 A load → 80 A–90 A breaker (often split into two circuits)
• 20 kW: ≈ 83.3 A load → 100 A–110 A breaker (usually split)

Manufacturers often split large heat strip kits into multiple 5 kW stages, each with its own breaker and wiring. Always follow the air handler’s rating plate for maximum overcurrent protection and conductor sizes.

Airflow And Temperature Rise

Verify blower CFM matches the added heat. A quick check: ΔT ≈ (kW × 3,412) ÷ (1.08 × CFM). Example: 10 kW with 1,200 CFM yields ≈ 26°F rise. If airflow is too low, high-limit switches may trip, and elements can overheat.

Safety Features

Modern heat strip assemblies include manual reset and automatic limit switches, sequencers/relays, and proper thermal insulation. Properly installed, they are safe; a brief “warm dust” smell may occur on first seasonal operation.

Maintenance And Troubleshooting

Routine checks keep heat pump heat strips reliable and efficient. Many problems are inexpensive to address if caught early.

Maintenance Tips

• Filters and airflow: Replace or clean filters regularly. Blocked airflow causes high-limit trips and cold rooms.
• Electrical connections: Inspect strip kit lugs and breakers annually for tightness and discoloration.
• Control testing: In service mode, stage strips on and verify amp draw with a clamp meter matches nameplate.
• Thermostat settings: Confirm heat pump mode, Aux staging, and lockouts after any thermostat replacement.

Common Symptoms And Fixes

• Aux Heat runs constantly in mild weather: Check thermostat configuration (heat pump vs conventional), outdoor sensor, and balance point settings. Investigate low refrigerant charge or undersized equipment.
• No Aux Heat when very cold: Possible blown fuse link in strip assembly, failed sequencer/relay, tripped manual limit, or breaker issue.
• Burnt odor beyond initial startup: Shut off heat and inspect for debris on elements or wiring damage. Persistent odor warrants professional service.
• Frequent breaker trips: Over-amping due to wrong strip size, shared circuits, or failing elements. Verify circuit sizing and staging.

Tip: If “EM Heat” appears without being selected, the system may have locked the compressor out due to a fault. Schedule service to diagnose outdoor unit issues and avoid extended resistance heat use.

Cold-Climate Options And Alternatives

Advances in compressor technology are reducing reliance on heat pump heat strips, especially in colder parts of the U.S. Consider these strategies when installing or upgrading.

Cold-Climate Inverter Heat Pumps

Cold-climate air-source heat pumps (ccASHP) maintain strong capacity and good COP at low ambient temperatures. Many systems operate to -5°F or lower while delivering COPs above 1.5, cutting Aux Heat hours dramatically.

Look for models listed by regional initiatives like the Northeast Energy Efficiency Partnerships (NEEP) and verify low-ambient capacity in the AHRI certificate.

Dual-Fuel (Hybrid) Heat

In gas-served homes, a dual-fuel system pairs a heat pump with a gas furnace. The thermostat uses the heat pump above a switchover temperature and the furnace below it. This can provide lower operating cost than electric strips in cold climates while retaining heat pump efficiency in shoulder seasons.

Weatherization And Envelope Upgrades

Air sealing, insulation, and window improvements reduce heating loads, letting the heat pump carry more of the load and shrinking required strip size. Utility programs often offer rebates for envelope upgrades and heat pumps.

Installation Best Practices

Good design ensures comfort, safety, and efficiency. Heat strips are part of a system, not an afterthought.

Ducts, CFM, And Temperature Rise

Confirm ductwork supports the airflow needed for both the heat pump and strips. Use the ΔT equation to verify the supply temperature rise stays within manufacturer limits, commonly 20–35°F for comfort and safety.

• Size blower speed to maintain target CFM in heat mode.
• Balance zones to prevent low-flow branches when strips stage.
• Consider supply air sensors to limit staging when ΔT is high.

Staged Strip Kits And Demand Management

Choose staged kits (e.g., 2 × 5 kW) to match load and limit peak current. Utility demand charges and peak events can make a single large strip stage costly. Smart thermostats or utility-connected controllers can temporarily limit strips during price spikes.

Commissioning Checklist

• Verify strip kW options in the air handler match design.
• Confirm breaker sizes, wire gauges, and disconnects per NEC and nameplate.
• Program thermostat Aux thresholds, lockouts, and staging delays.
• Test defrost operation and observe whether strips temper supply air appropriately.
• Measure supply ΔT and element amp draws under load.

How To Reduce Heat Strip Runtime

Reducing Aux Heat hours saves money without sacrificing comfort. These actions have outsized impacts in typical U.S. homes.

• Optimize thermostat settings: Use small setbacks, enable outdoor Aux lockout, and extend recovery times.
• Seal and insulate: Lower the balance point by cutting drafts and adding insulation, especially in attics and rim joists.
• Maintain filters and coils: Better airflow improves heat pump capacity, delaying Aux calls.
• Upgrade to inverter heat pumps: Variable-speed systems sustain higher capacity in cold weather.
• Right-size strips: Avoid oversizing; use staged kits matched to calculated shortfall.

Key Calculations At A Glance

Frequently Asked Questions

Are Heat Pump Heat Strips Necessary?

In most U.S. climates, yes. They provide reliable backup during extreme cold and during defrost. In very mild climates or with oversized cold-climate heat pumps, strips may rarely run and can be smaller.

Should EM Heat Ever Be Used Manually?

Only if the outdoor unit is malfunctioning or being serviced. Routine cold weather should be handled by Aux Heat with the compressor running. Do not use EM Heat to speed up warm-up; it is costly.

How Can Someone Tell If Heat Strips Are Running?

Many thermostats display “Aux Heat.” Electric meter speed may jump noticeably. A clamp meter on the air handler feed will show increased amps when strips energize. Some smart thermostats log Aux runtime in their apps.

Why Does The House Feel Cooler During Defrost?

During defrost, the heat pump briefly reverses and cools the indoor coil. Strips usually temper the air, but you may still notice slightly cooler supply air for a few minutes. This is normal.

Is It Safe To Oversize Heat Strips “Just In Case”?

Oversizing increases peak demand, operating cost, and the chance of tripping supply air limits. Instead, use proper load calculations and staged elements. Let controls add only what is needed.

Do Heat Strips Affect SEER2 Or HSPF2 Ratings?

The electric elements themselves are not part of SEER2 or HSPF2 ratings, but how often they run affects seasonal energy use. Systems that maintain heat pump capacity at low temperatures will rely less on strips and deliver better seasonal performance.

What If The Breaker Trips When Aux Heat Turns On?

Possible causes include incorrect breaker size, loose wiring, failed element, or too many stages energizing at once. A technician should verify circuit sizing, sequencing, and element condition against the manufacturer’s data.

Can Utility Programs Help Manage Strip Use?

Yes. Some utilities offer smart thermostat rebates, demand response programs, or time-of-use rates. Participating devices can limit Aux staging during peak hours while maintaining comfort.

Further Resources

For deeper guidance on heat pumps and efficient electric heating, explore:

• U.S. DOE Energy Saver: Heat Pump Systems
• AHRI Directory for equipment performance data
• NEEP High-Efficiency Heat Pumps and cold-climate specifications

Bottom line: Heat pump heat strips are invaluable for comfort and reliability. With correct sizing, smart controls, and a well-tuned heat pump, they provide backup heat when needed while keeping winter energy costs in check.