Electric Heat Strip

For homeowners

Electric heat strips are resistance heating elements used either as backup heat in a heat pump system or as primary heat in an all-electric home without a heat pump. Same physics as a toaster — a long length of nichrome wire heated by passing electricity through it. Air blows across the hot wires and gets warmed up.

Three things make this practical for HVAC use:

The sequencer controls when the strip turns on. It’s a small thermal delay relay — when 24V from the thermostat hits its coil, a tiny heater warms a bimetal disk inside the sequencer, which after 20–30 seconds snaps a contact closed and energizes the strip. The delay is intentional: it staggers multiple strips so they don’t all hit the breaker at once, and it gives the blower a moment to come up to speed before the heat element gets hot.

The high-limit switch is the safety. If airflow fails (blower failure, packed filter, blocked register), the element will overheat. The limit switch is a thermal cutout that opens at a set temperature, killing the strip before it damages anything.

Cost. At 1:1 efficiency, strips deliver one watt of heat for every watt of electricity. A heat pump delivers two to three watts of heat per watt of electricity. This is why strips are emergency/backup in heat pump systems — they’re expensive to run.

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For technicians

The element itself is a length of nichrome resistance wire (typically 80% nickel, 20% chromium) suspended in a metal mounting frame by ceramic standoffs that insulate the wire from the frame. The wire is wound into a serpentine pattern to fit a long length into a short physical space, and the U-bends at each end are where the wire is most thermally stressed.

A typical residential strip element is 5 kW per stage at 240V — drawing approximately 20.8 amps when energized. Systems are sold in multiples: 5, 10, 15, 20 kW packages that combine multiple 5 kW stages. A 15 kW package has three independent elements with three sequencers.

Why nichrome. It has high electrical resistivity (so a usable length of wire produces meaningful heat at line voltage), maintains its resistance across a wide temperature range (so power output is stable as it heats up), resists oxidation at high temperatures (the chromium content forms a protective oxide layer at the surface), and has a usable working temperature up to ~2200°F. The wire glows orange-red in normal operation at around 1500–1800°F.

The sequencer mechanics. A sequencer is a thermal time-delay relay, not a magnetic contactor. Inside the housing:

• A small electric heater wraps around a bimetal disk
• The bimetal disk is mechanically linked to a snap-action contact
• 24V applied across the coil heats the bimetal slowly
• After 20–30 seconds, the bimetal warps enough to snap the contact closed
• Closed contact carries line voltage to the heating element
• When 24V is removed, the bimetal cools and the contact opens after about 60–90 seconds

This intentional slowness has two purposes:

Staggered startup. A 15 kW system has three sequencers, factory-timed at slightly different delays — say 25 seconds, 35 seconds, 45 seconds. The strips come on one at a time, not simultaneously. This keeps the inrush current within the breaker’s instantaneous trip threshold and avoids dropping line voltage dramatically when the unit fires. You can hear this in operation: click… click… click… in 10-second intervals as each stage energizes.

Blower head-start. The blower needs a moment to come up to speed and start moving air before the strip heats up. If the strip energized instantly with no airflow, the limit switch would trip within seconds and the strip would never deliver heat. The sequencer delay gives air a chance to start flowing first.

The high-limit switch is a normally-closed bimetal thermal disk wired in series with the heating element. Set to open at a specific temperature — typically 130–180°F depending on the application. When the temperature at the limit switch reaches that threshold, the bimetal warps enough to break the contact. Element de-energizes. Once temperature drops back below the reset threshold (typically 30–40°F below the open temperature), the switch closes again automatically.

Most high-limit switches are auto-resetting. Some installations include a secondary one-shot thermal fuse — a fuse that physically destroys itself if temperature exceeds a higher threshold, requiring manual replacement to restore operation. The one-shot is a backup against the auto-reset failing in the closed position.

Energy cost reality. A 10 kW heat strip running at full output draws 41.6 amps at 240V — that’s 10 kWh of electricity per hour of operation. At average US electricity rates of $0.16/kWh, that’s $1.60 per hour. A 24-hour cold day with the strips running half the time costs about $19 in heating. Over a month of cold weather, several hundred dollars.

A 3-ton heat pump running at COP 3.0 (typical for moderate weather) delivers 36,000 BTU/hr of heat while consuming about 3.5 kW of electricity. The same 36,000 BTU/hr from strips would consume 10.5 kW. The strips cost three times as much to run for the same heat output.

This is why heat pump control schemes carefully balance strip usage:

• Heat pump runs first, alone, for normal cold weather
• Strips only engage when the heat pump can’t maintain setpoint (auxiliary heat)
• Strips engage automatically during defrost cycles (when the heat pump is briefly running in reverse to melt frost off the outdoor coil)
• Some thermostats have an “emergency heat” setting that locks out the heat pump and runs strips exclusively — useful when the heat pump has failed, but never optimal otherwise

The W2 / auxiliary heat call. Most heat pump thermostats output a separate signal — W2 or aux — when they decide strips are needed in addition to the heat pump. The control board in the air handler responds by energizing the appropriate sequencer. A two-stage system might bring on one strip stage at the first W2 call, a second strip stage at a more aggressive W2 demand.

Failure modes:

Nichrome wire failure. Repeated thermal cycling work-hardens the wire over years until it cracks. The break almost always happens at one of the U-bends, where thermal stress is concentrated. Open element reads infinite resistance across its terminals. No heat output from that stage; other stages may still function. Replace the whole element. Field repair of nichrome wire (twisting strands back together) is theoretically possible but useless — the joint cooks itself open within hours.

Sequencer failure. Coil burns out: relay never closes, strip never energizes. Coil shorts: tripped breaker or transformer overload. Contacts welded closed: strip stays on after thermostat satisfies, blower keeps running on continuous fan, supply air comes out very hot. Replace.

Limit switch nuisance trips. If the limit opens repeatedly during normal operation, the root cause is airflow restriction — dirty filter, blower problem, blocked supply registers, undersized return. Don’t replace the limit switch; fix the airflow.

Limit switch failure stuck closed. Dangerous mode. The strip can overheat unchecked. The one-shot thermal fuse is the secondary protection, but if both fail, the strip can melt through its housing and start a fire. Annual maintenance should include a functional test of the high-limit by mechanically blocking airflow briefly and verifying the limit opens.

Diagnostic test. With power off, megger from each element terminal to the frame ground — should be infinite or very high resistance. Low resistance means insulation breakdown (nichrome shorting to the frame), and the element needs replacement. Across each element pair, measure resistance — should be in the range of 5–25 ohms for a typical residential strip (240V / 20A = 12 ohms). Open reading or hugely off-spec means failed element.