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newSchoolWorks.com › VoTechWorks › Building & Construction › HVAC/R › Heat Pumps: Principles, Installation, and Service › Lesson 8
BLD-HVAC07-04 · Lesson 8 · 12 clock hours · demo

Auxiliary and Emergency Heat: Electric Strip Heaters and Gas Backup Systems

Objective. The student will measure the voltage, amperage, and resistance of an electric heat strip, verify correct sequencer operation, and explain when the thermostat switches to Emergency Heat mode.

Why Heat Pumps Need Backup Heat

A heat pump extracts heat from outdoor air and moves it indoors. As outdoor temperatures drop, the heat pump's capacity falls. Below a design balance point—often somewhere in the 30s °F depending on the system—the heat pump alone cannot keep the building comfortable. Backup heat fills that gap. Two common backup types are electric resistance strip heaters (the most common in residential and light-commercial split systems) and gas furnace sections used in dual-fuel or hybrid systems.

Electric Heat Strip Construction and Operation

An electric heat strip assembly sits inside the air handler, downstream of the indoor coil. It contains one or more nichrome resistance elements that glow red-hot when current flows through them. Each element is rated in kilowatts (kW) at a specific voltage—commonly 240 V in residential applications. The data plate mounted on the air handler lists total kW capacity, voltage, minimum circuit ampacity (MCA), and maximum overcurrent protection (MOP).

Because all elements switching on simultaneously would cause a large inrush current spike, sequencers stagger the energizing of each element. A sequencer is a thermally operated relay: when its coil receives low-voltage control current, a bimetal strip heats and closes a set of high-voltage contacts after a brief delay—typically 30 to 90 seconds. Additional sequencers then close in turn, bringing elements on-line one at a time. The result is a gradual rise in amp draw rather than a single large surge, which protects the electrical system and reduces demand charges.

Control Circuit Path for Auxiliary Heat

The thermostat's W2 terminal sends a 24 V signal when the system calls for auxiliary heat. That signal travels to the sequencer coil. After its thermal delay, the sequencer contacts close and supply 240 V to the heat element. A separate set of contacts on the same sequencer may power a second sequencer, creating a daisy-chain that staggers additional elements. High-limit safety switches—bimetal devices that open on overtemperature—are wired in series with each element circuit. If airflow is blocked and the element overheats, the limit opens and de-energizes the element until it cools.

Emergency Heat Mode

On a heat pump thermostat, Emergency Heat (EM Heat) is a manually selected mode. When it is engaged:

Emergency Heat is used when the heat pump itself is inoperable—for example, during a refrigerant leak, compressor failure, or when the outdoor unit is being serviced. It is not a winter efficiency setting; running it when the heat pump is functional wastes energy.

Gas Backup in Dual-Fuel Systems

A dual-fuel (hybrid) system pairs a heat pump with a gas furnace. The gas furnace acts as the backup and takes over below the balance point or when the utility rate structure makes gas more economical. Control logic—either in a communicating thermostat or a separate control board—monitors outdoor temperature and switches heating sources accordingly. Servicing gas backup components requires following gas-related codes and procedures in addition to the electrical safety steps covered here.

Measuring Voltage, Amperage, and Resistance Safely

Always apply lockout/tagout (LOTO) before opening an air handler panel. Follow your facility's written LOTO procedure: identify all energy sources (typically a dedicated circuit breaker), apply a lockout hasp and padlock, test that voltage is absent with a meter, then proceed.

Reading the Data Plate for Expected Resistance

Ohm's Law and the power formula connect what is printed on the data plate to the resistance you will measure:

A measured value within ±10% of the calculated value indicates a serviceable element. A reading of OL means the element is open and must be replaced.

Key terms8

SequencerA thermally operated relay inside an electric heat strip assembly that staggers the energizing of individual heating elements to prevent large simultaneous inrush currents.
Auxiliary heatSupplemental heat—electric strip or gas—that operates alongside or in place of the heat pump when outdoor temperatures drop below the system's balance point.
Emergency Heat (EM Heat)A manually selected thermostat mode that locks out the heat pump compressor and runs only the backup heat source; used when the heat pump itself is inoperable.
W2 terminalThe thermostat terminal that carries the 24 V control signal for second-stage or auxiliary heating; it triggers the sequencer coil in electric heat systems.
High-limit switchA bimetal safety device wired in series with a heat element that opens the circuit if the element temperature exceeds a safe threshold, protecting against overheating caused by blocked airflow.
Minimum Circuit Ampacity (MCA)The minimum current-carrying capacity required of the supply wiring to an electric heat strip, calculated from the continuous load and listed on the data plate.
Dual-fuel (hybrid) systemA heating system that combines a heat pump with a gas furnace, switching between sources based on outdoor temperature or energy cost to maximize efficiency.
Lockout/Tagout (LOTO)An OSHA-required procedure for isolating and de-energizing equipment before service work, using physical locks and warning tags to prevent accidental re-energization.

Worked examples4

1. A heat strip data plate reads 10 kW at 240 V. Calculate the expected resistance of the element.
  1. Identify the formula: R = V² ÷ P
  2. Square the voltage: 240² = 57,600
  3. Divide by power in watts: 57,600 ÷ 10,000 = 5.76
  4. State the result with units: R ≈ 5.76 Ω
Answer: The expected resistance of the 10 kW, 240 V element is approximately 5.76 Ω.
2. You measure the same 10 kW element with a DMM and get 5.4 Ω. Is this element serviceable, or should it be flagged?
  1. Calculate the acceptable range: ±10% of 5.76 Ω
  2. Lower limit: 5.76 × 0.90 = 5.18 Ω
  3. Upper limit: 5.76 × 1.10 = 6.34 Ω
  4. Compare the measured value: 5.4 Ω falls between 5.18 Ω and 6.34 Ω
Answer: 5.4 Ω is within ±10% of the expected 5.76 Ω, so the element is serviceable.
3. A 5 kW element at 240 V is operating normally. Calculate the expected current draw for that single element.
  1. Use the power formula: P = V × I, rearranged to I = P ÷ V
  2. Convert kW to watts: 5 kW = 5,000 W
  3. Divide: 5,000 ÷ 240 = 20.83
  4. State the result: I ≈ 20.8 A
Answer: A 5 kW element at 240 V draws approximately 20.8 A.
4. An air handler has three 5 kW elements that sequencer-stage on one at a time. What is the amp draw after the first stage energizes, after the second, and at full load?
  1. Single element current: 5,000 ÷ 240 ≈ 20.8 A
  2. After stage 1 (one element): 1 × 20.8 = 20.8 A
  3. After stage 2 (two elements): 2 × 20.8 = 41.6 A
  4. After stage 3 (all three elements): 3 × 20.8 = 62.5 A
Answer: Stage 1: ~20.8 A; Stage 2: ~41.6 A; Stage 3 (full load): ~62.5 A.

The math in this lesson1

Learners compute element resistance from 10 kW at 240 V and current draw per staged element.
A-CED.4
Brush up in the math halls at newSchoolWorks.com:
In class: Instructor demonstrates safe access to an air handler auxiliary heat section using lockout/tagout; students observe the sequencer energizing individual heat elements in stages and record amp draw at each stage with a clamp-on ammeter. · Students use a digital multimeter to test element continuity and measure resistance of each heat strip element, comparing readings to the data plate rating and flagging any open elements. · Diagram exercise: students draw the control circuit from the W2 thermostat terminal through the sequencer coil to the heat element contactor, then trace what changes electrically when the thermostat is set to Emergency Heat.
Materials: Air handler with electric heat strip (lab unit), lockout/tagout kit, clamp-on ammeter, digital multimeter, electric heat strip data plate, sequencer, wiring diagram, lab sheets
Check: Instructor checklist: LOTO applied correctly before access; resistance measurement within ±10% of data plate value; control circuit diagram complete and accurate; pass mark: all three items passed.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

HVAC Excellence – Electrical Systems
1. What is the primary purpose of a sequencer in an electric heat strip assembly?
NATE – Safety and LOTO
2. Before measuring the resistance of a heat strip element with a DMM, the technician MUST:
NCCER HVAC – Electrical Theory
3. A data plate lists a heat element as 7.5 kW at 240 V. Using R = V² ÷ P, what is the expected resistance?
NATE – Electrical Measurement
4. A technician clamps an ammeter around both conductors of a 240 V heat element circuit and reads zero amps. What is the most likely explanation?
NATE – Heat Pump Systems
5. When a heat pump thermostat is manually set to Emergency Heat, which of the following correctly describes system operation?
HVAC Excellence – Electrical Diagnostics
6. A measured element resistance of 12.8 Ω is compared to a calculated expected value of 11.5 Ω. The acceptable tolerance is ±10%. Should the element be replaced?
NCCER HVAC – Controls and Wiring
7. Which thermostat terminal carries the 24 V control signal that triggers the sequencer coil for auxiliary electric heat?
NATE – Safety Controls
8. A high-limit switch in an electric heat strip assembly opens under which condition?
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