What it is

An electric furnace heats air by running house current through resistance wire — the same principle as a toaster, scaled up. There's no flame, no combustion, no flue, no gas. That makes it the simplest heating equipment to diagnose: it's almost entirely an electrical problem when it fails.

In practice "electric furnace" and "air handler with strip heat" are the same animal viewed two ways. A straight electric furnace is a cabinet with a blower and electric heat elements. An air handler is the indoor unit of a heat pump or AC system, and when it carries electric heat elements those strips serve as backup/emergency heat (heat pump) or the only heat (cooling-only with electric heat). Either way the heating hardware is identical: resistance elements, a blower, and the controls that stage them.

How it's built

  • Cabinet and blower. A squirrel-cage blower (PSC or ECM) moves return air across the elements and out to the supply. On a heat-pump air handler this same blower also moves air across the indoor coil for cooling and heat-pump heating.
  • Heat strip elements. Coiled nichrome resistance wire strung on ceramic insulators, mounted in the airstream downstream of the coil. Sized in kilowatts — common residential banks are 5, 10, 15, or 20 KW, built up from individual 5 KW elements.
  • Element controls — sequencers or relays/contactors. Each element (or pair) is switched on by either a sequencer (a thermal time-delay relay that closes slowly, a few seconds after it's energized) or a definite-purpose contactor/relay (closes instantly). Sequencers stage the elements on one at a time over several seconds so you don't slam the whole load on at once.
  • Limits and fusible links. Each element bank has a primary high-limit (auto-reset) and usually a secondary one-shot fusible link / thermal cutoff that blows permanently if the element overheats badly (typically from airflow loss). The limit is the reset-able safety; the fusible link is the last-ditch one-time fuse.
  • Disconnect/whip and breaker. Electric heat pulls big amps, so it has heavy feeders, often a dedicated breaker (or two), and frequently field-installed fuses on the heat kit.

How it's wired and staged

Electric heat is a current monster — a 10 KW strip on 240V pulls about 42 amps by itself. That's why it's fed with large conductors and broken into stages.

  • Single-stage thermostat (W): energizes the heat kit; the sequencers bring elements on in steps.
  • Heat-pump application (W/Aux/Em): the strips are backup. On a normal heat call the heat pump runs; the strips only come on when the thermostat calls auxiliary (heat pump can't keep up / low outdoor temp) or emergency heat (heat pump locked out, strips do all the work). The thermostat and/or an outdoor thermostat decide when to release the strips.
  • Blower interlock: the blower MUST run whenever elements are energized. The fan circuit is interlocked so you can't cook the elements in still air. Lose the blower and the limits/fusible links are all that stand between you and a fire — which is exactly why they're there.

In the field — sequence of operation

On a call for electric heat: thermostat closes the heat circuit → blower energizes → the first sequencer/contactor closes and brings on element 1 → a few seconds later the next stage closes, and so on up the bank → elements heat the passing air → on satisfy, the controls open in reverse and the blower runs a short off-delay to dump residual heat. With a heat pump in the picture, the strips only enter on an aux/emergency call as above.

Normal values & targets

  • Current per element: roughly 5 KW per ~21 A at 240V. So a full 10 KW bank ≈ 42 A, 15 KW ≈ 63 A, 20 KW ≈ 83 A, drawn in stages. Verify each element's amp draw with a clamp — a dead element reads zero on its leg.
  • Temperature rise: the rating-plate rise band still rules, commonly somewhere in the 20–50°F range for electric heat depending on KW and CFM. Too high = airflow problem; too low = an element isn't firing.
  • Voltage: typically 240V (sometimes 208V commercial). Element wattage scales with the square of voltage, so an element run on 208 instead of 240 puts out noticeably less heat — that's normal, just know the supply.
  • Sequencer timing: stages should close a few seconds apart over maybe 10–60 seconds total; all stages should eventually be closed on a full call in cold weather.

Common faults & what they mean

  • No heat at all, blower runs. Heat call isn't reaching the elements — bad sequencer/contactor coil, open limit, blown fusible link, tripped breaker on the heat circuit, or a control wiring fault.
  • Weak heat / low temperature rise. One or more elements not firing. Clamp each leg: a 10 KW unit only pulling 21 A has one 5 KW element dead. Open element, open fusible link, or a stage that isn't closing.
  • Breaker trips on heat call. Shorted element to ground/case, a damaged feeder, or a sequencer/contactor welded closed slamming full load. Megger or ohm the elements to ground.
  • Blown fusible link / thermal cutoff (won't reset). These blow from overheating — almost always an airflow loss (dirty filter, failed blower, closed dampers) or a stuck-closed element control with the blower off. Replace the link AND find why it cooked, or the new one blows too.
  • Heat-pump system runs strips constantly. Aux/emergency staying on means the heat pump isn't heating (low charge, bad reversing valve, defrost stuck) or the outdoor/indoor thermostat logic is releasing strips too early. The strips are masking the real fault and spiking the electric bill.

Tech tips & gotchas

  • Clamp every leg. Electric heat diagnosis is mostly "which element is pulling its amps and which isn't." A clamp meter on each element conductor tells you instantly. Expected per element ≈ 21 A at 240V.
  • A blown fusible link is a symptom, not the disease. It blew because something overheated. Replace it and find the airflow loss or stuck control, or you're coming back.
  • Strips running on a heat pump in mild weather is a flag, not normal. If the customer's bill is high and the strips run a lot above ~35–40°F outdoor, the heat pump side needs investigating, not just the strips.
  • Respect the amperage. A 20 KW kit is over 80 amps of resistance load. Verify the breaker, feeders, and any heat-kit fuses are correct and tight — loose lugs on this much current burn up.
  • Sequencers fail slow; contactors fail welded. A sequencer that won't close = no stage. A contactor welded closed = a stage that won't shut off (and can overheat with the blower off). Diagnose accordingly.

Safety / code notes

  • Electric heat conductors, overcurrent protection, and disconnects follow NEC requirements; the equipment's listed MCA (minimum circuit ampacity) and MOCP (maximum overcurrent protection) on the data plate govern wire and breaker sizing — honor them.
  • The blower-element interlock is a safety, not a convenience. Never wire elements to energize without the blower.
  • Don't bypass a fusible link or jumper a limit to "get heat" — those prevent an overheated element from starting a fire.
  • A disconnecting means within sight of the unit is required per NEC §440/§424 provisions for the equipment; verify it's present and that you lock out before working on the heat kit.