Electric Heat Strip Diagnosis and Wiring
Electric heat strips (also called electric resistance heating elements or heat kits) are installed in air handlers and heat pump systems to provide supplemental or emergency heat. They operate at 240V AC, draw 15–60 amps per stage, and use sequencers (time-delay relays) to stagger element startup and prevent a single huge current surge. On Carrier, Trane, Lennox, Rheem, and Goodman air handlers, each stage of heat has its own sequencer, heating element, and high-limit switch. Any of these components failing disables that stage of heat — or all heat, depending on wiring.
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Common Symptoms
- Electric air handler running but air is cold or barely warm
- Heat pump auxiliary heat not working
- Emergency heat mode blows cool air
- Only partial heating — one stage works, others don't
- Breaker trips when heat mode is activated
- Sequencer clicks but element doesn't heat up
Most Likely Causes
- 1
Failed Sequencer (Most Common)
The heat sequencer is a bimetal time-delay relay that closes contacts to energize the heating element after a 30–60 second delay. When the sequencer fails open (contacts never close), that stage of heat is disabled even with 24V applied to the sequencer coil. Test by measuring 240V across the sequencer output terminals with the sequencer energized — no voltage means the contacts are not closing.
- 2
Tripped High-Limit Switch
Each heat strip stage has a high-limit thermal cutout that opens if the element overheats — usually due to blocked airflow. The limit switch may be auto-reset (closes after cooling) or manual-reset (requires pressing a button to reset). Locate the limit switch on the element assembly and press the reset button if present.
- 3
Open Heating Element
The Nichrome resistance element can break internally, creating an open circuit. With power off, disconnect the element wires and test resistance with a multimeter — a good element reads 5–50 ohms depending on wattage. OL (open) = failed element, needs replacement.
- 4
Missing L1 or L2 Voltage
Heat strips require both L1 and L2 (240V) to operate. A blown fuse in the air handler disconnect, a tripped breaker, or a loose line voltage connection can remove one leg of power. With only 120V on one leg, the elements don't heat. Test L1-to-L2 at the sequencer output — should read 240V when energized.
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Quick DIY Checks
Electric heat strips operate at 240V AC and draw very high current (15–60 amps per stage). Always shut off the dedicated 240V breaker AND the air handler disconnect before working inside the electric heat section. Verify power is off with a multimeter before touching any wiring.
Never operate electric heat strips without adequate airflow. Running elements with a blocked or low-airflow blower rapidly overheats the elements, trips limit switches, and can cause element failure or fire. Confirm the blower is running at full speed before testing heat operation.
- 1Check the breaker panel — heat strips typically have a dedicated 240V breaker (30–60 amp). If it's tripped, reset it. If it trips again immediately, there is a short circuit or grounded element in the air handler. Do not continue to reset without finding the cause.
- 2Access the air handler's electric heat section — usually a removable panel below or beside the blower section. Locate the heating elements, sequencers, and limit switches. With power off, look for any visibly burned wires, melted insulation, or discoloration.
- 3Locate the high-limit switches on the element assembly — small disc devices with two wires and often a reset button. At room temperature, test each limit switch with a multimeter in continuity mode. Good: continuity. Open: failed or tripped. If tripped: press the reset button (if manual-reset type) and retest.
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Try Pro — $7.99/mo- 4Test each heating element: disconnect the wires and measure resistance across the element terminals. Expected range: a 5 kW element on 240V draws about 20.8 amps and has a resistance of about 11.5 ohms. A 10 kW element: about 5.8 ohms. OL = open element, needs replacement. Very low resistance (1–2 ohms) with a tripping breaker = shorted element.
- 5Test the sequencers: with the system calling for heat, measure 24V AC across the sequencer control terminals (M1 and M2 or equivalent) — should read 24V when the thermostat calls for W2 (second stage heat). Then measure 240V across the sequencer output terminals (the high-voltage contacts). If you have 24V in but no 240V out after 60 seconds, the sequencer contacts are not closing — failed sequencer.
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Repair vs Replace
Electric heat strip component replacement is straightforward and inexpensive. Sequencers and limit switches are $15–$40. Even a full element replacement is $40–$120. Full air handler replacement is rarely necessary due to heat strip issues alone.
Est. Repair Cost
$15–$40 (limit switch or sequencer); $40–$120 (replacement heating element)
Est. Replacement Cost
$800–$2,000 for full electric air handler replacement
Recommended Tools & Parts
- Buy on Amazon →
Heat Sequencer (24V control, 240V load contacts)
Time-delay relay for staggering electric heat strip startup. 24V control coil, 240V load contacts rated 25–30A. Match the terminal configuration to your existing sequencer.
$15–$35
- Buy on Amazon →
Electric Heating Element (match kW and voltage)
Replacement Nichrome resistance heating element for electric air handlers. Match kilowatt rating and voltage (240V) to the original. Common ratings: 5 kW, 7.5 kW, 10 kW.
$40–$100
- Buy on Amazon →
High-Limit Thermal Cutout (match temperature rating)
Safety thermal cutout for electric heat strips. Match the temperature rating and mounting style to the original. Auto-reset and manual-reset types available.
$10–$25
Links are Amazon affiliate links (tag: fixitfastai-20). Prices are estimates.
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Frequently Asked Questions
- Why does my heat pump emergency heat blow cold air?
- Emergency heat mode bypasses the outdoor unit and runs only the electric heat strips in the air handler. If it blows cold air, check: (1) the dedicated electric heat breaker — reset if tripped, (2) limit switches on the heat strips — press manual reset buttons if tripped, (3) sequencer function — test 24V control input and 240V output as described above, (4) heating element resistance — open element reads OL on a multimeter.
- How do I know which stage of electric heat has failed?
- Most air handlers with multiple heat stages heat partially when one stage fails. If the air is barely warm instead of fully warm, one stage is working and one isn't. Each stage has its own sequencer, limit switch, and element. Test each sequencer's 240V output independently with the system calling for heat. The stage with no 240V output has the failed component.
- Why does my electric heat breaker keep tripping?
- A repeatedly tripping heat breaker usually means a shorted or grounded heating element, a failed sequencer with welded contacts (keeps the element on continuously), or a undersized breaker for the load. Test each element for resistance — very low resistance (near 0 ohms) indicates a short. Also confirm the breaker amperage matches the heat kit nameplate rating.