What it is

Electric resistance heat — the "heat strips" in an air handler or the backup heat in a heat pump — pulls a LOT of current. A bank of strips might draw 20, 40, even 60+ amps total. If all of them slammed on at the same instant, that inrush would stress the wiring, the breaker, and the building service, and it'd dim the lights every cycle. A sequencer solves that by bringing the strips on in stages, a few seconds apart, and taking them off the same way. It also typically delays the indoor blower so the strips warm up first and keeps the blower running briefly after to purge the heat.

How it works

The classic sequencer is a thermally-actuated device: a small heater element inside the sequencer warms a bimetal disc, and as the bimetal heats up it closes one or more sets of contacts — but slowly, over several seconds. That built-in delay is the whole point. The control circuit energizes the sequencer's heater on a heat call; a few seconds later its first contacts close and bring on the first strip (and often the blower); a few more seconds and the next set closes for the next strip; and so on. On shutdown, the heater de-energizes and the bimetal cools, dropping the strips back off in stages.

So a sequencer is essentially a time-delay relay built with a heater and a bimetal, used to stagger heavy resistive loads. Some have multiple sets of contacts in one body (each closing at a slightly different time); a unit may use several sequencers to stage a larger strip bank.

Sequencer vs. relay vs. contactor:

  • A heat relay / strip contactor switches a strip on immediately when energized — no built-in delay. Some modern air handlers use relays or a control board with timed outputs instead of thermal sequencers to do the staging in firmware.
  • A sequencer has the deliberate thermal delay baked in — slower make and break, which is exactly what limits inrush.
  • Either way, the goal is the same: bring big resistive loads on in steps, not all at once.

The blower is usually tied to the heat staging too — you don't want strips glowing with no airflow (they'd overheat and trip their limits). A sequencer commonly closes a blower contact as part of its first stage, or the board runs the blower on the heat profile.

In the field

Testing a sequencer:

  1. Watch the staging. Call for heat (or aux/em heat on a heat pump). Clamp the strip circuits or watch amperage climb in steps — you should see current come up in stages over several seconds, not all at once. If it all comes on instantly, or never fully comes on, the sequencer's staging is off.
  2. Check the coil/heater side. The sequencer's heater gets 24V (or line, depending on type) on a call. Confirm it's energized. No power to the sequencer heater = it'll never close its contacts.
  3. Check the contacts. After the delay, the load contacts should close (continuity, and ~0V across the closed contacts while carrying current). A contact that never closes = that stage of heat never comes on. A contact welded closed = that strip stays on even after the call ends (overheating risk).
  4. Confirm each strip actually draws current. A strip with an open element (or a blown strip fuse/limit) won't draw even if the sequencer closes — so verify amperage on each leg, not just that the sequencer clicked.

Symptoms map cleanly to staging: if a 3-strip unit only brings on 2 strips' worth of amps, one stage isn't closing (sequencer contact, that strip's element, or its fuse/limit).

Normal values & targets

  • Stage delay: strips come on/off roughly several seconds apart (often ~10–90 seconds per stage depending on the sequencer) — staggered, not simultaneous.
  • Strip current: each element draws heavily; a common 5 kW strip at 240V pulls roughly ~20A (5,000 ÷ 240 ≈ 20.8A). Multiple strips stack up fast — total can be 40–60A+.
  • Sequencer heater: energized by the control call (24V on many, line on others — match the part).
  • Across closed contacts under load: ~0V (passing power). ~full voltage across them while calling = contact not closing.
  • Blower: comes on with (or just before) the first heat stage and runs on an off-delay after.

Common faults & what they mean

  • Not all strips come on / weak heat — one or more stages not closing: a sequencer contact stuck open, an open strip element, or a blown strip fuse/limit. Clamp each leg to find which stage is missing.
  • Strips come on but no blower (or blower trips the strips' limits) — blower not staging with the heat; the strips overheat and open their thermal limits. Check the blower output/relay and the sequencer's blower contact.
  • Strips stay on after the call ends — welded sequencer contact or stuck relay. This is a real overheat hazard; the elements glow with the call satisfied. Replace the sequencer.
  • Breaker trips on heat call — possible simultaneous inrush (sequencer not staging, contacts all closing at once / a relay where a sequencer should be), a shorted element, or genuinely undersized circuit. Verify staging and element condition.
  • Heat lags badly before coming on — long sequencer delay is normal to a point; an abnormally long or no-show stage points at a weak sequencer heater or failing bimetal.

Tech tips & gotchas

  • Sequencers are slow on purpose. A several-second delay before strips energize (and the staggered staging) is normal and good — it's protecting the service from inrush. Don't mistake the delay for a fault.
  • Clamp each strip leg. "The sequencer clicked" doesn't mean heat — confirm actual amperage on each element. A closed contact feeding an open element (or a blown strip fuse/limit) makes no heat. Each element typically has its own fuse and high-limit; check those when a stage is missing.
  • A welded sequencer is dangerous — strips that won't shut off keep making heat with no call. If you find strips energized after the call clears, replace the sequencer; don't just reset it.
  • Know whether you're looking at a thermal sequencer or a board/relay. Newer air handlers stage in firmware with relays; there may be no thermal sequencer at all. Read the diagram before hunting for a part that isn't there.

Safety / code notes

  • Electric heat strips draw large currents — conductor sizing, breaker/fuse protection, and the disconnect must be correct per the applicable electrical code for the connected kW load. Don't add strips beyond what the circuit is rated for.
  • The strips' thermal limits are safety devices — never bypass them. A tripped strip limit usually means an airflow problem; fix the airflow.
  • A sequencer/relay that won't drop the strips on loss of call is a hazard — strips energized without airflow or without a call can overheat. Replace failed switching devices rather than resetting.
  • Verify the blower energizes with strip heat; energizing resistance heat without airflow is unsafe and will trip limits.