Electric Heat Sequencers

What is an Electric Heat Sequencer?

Electric heat sequencers are time-delay relays used anywhere electric resistance heat needs to be brought on (and off) in stages to avoid big inrush current.

What the Sequencer Does

• Stages the heat strips on delays (e.g., first strip after ~10–30 s, next strip(s) after additional delays).
• Prevents inrush spikes and nuisance breaker trips by not energizing all strips at once.
• Often brings the blower on with a delay and keeps it running briefly after heat drops out to purge residual heat.

Where It Sits

Mounted on the electric heat kit/element rack. Each sequencer has a 24-V coil and one or more line-voltage contact sets feeding individual strip heaters (and sometimes a fan circuit).

Typical Control Sequence (Heat Call)

Thermostat call

• W1 → 24 VAC sent to the first sequencer coil (usually through the unit’s limit/rollout safety string).
• On heat pumps, AUX/W2 from the stat or a defrost board can energize additional sequencer coils.

First stage ON delay

• After the built-in delay, contacts close → first heat strip (e.g., 5 kW) energizes.
• Some sequencers also close a blower contact (line-voltage fan lead or a low-voltage feed to the fan relay).

Additional stages

• If demand persists (W2/W3 or internal staging), other sequencer coils energize → remaining strips come on one by one with their own delays.

Call ends → OFF delays

• 24 V is removed; coils cool; contacts open after a short delay (so strips don’t all drop at once).
• Blower may run a bit longer via the sequencer’s fan contact to clear heat and avoid limit trips.

For example, the White-Rodgers 24A34-1, is a single-stack time-delay sequencer (24 V control coil; one switch) that will bring one heat circuit on after a short delay, then drop it out after a delay.

When Does a Heat Sequencer Go Bad or Fail?

They “go bad” when cumulative heat and arcing wear out the heater/bi-metal and contacts—often after many seasons or in systems that short-cycle or overheat.

Common Failure Conditions

• Contact wear/arc-welding: High inrush from heat strips pits contacts; they stick closed (heat won’t shut off) or burn and stay open (no heat).
• Heater/bi-metal fatigue: The internal 24 V heater or bi-metal loses calibration or opens → no “stage-up,” erratic timing, or very long delays.
• Overheat & cycling abuse: Dirty filters/low airflow trip limits; repeated hot/cold cycles cook the sequencer and spade terminals.
• Electrical issues: Low/high control voltage, loose push-on terminals, or backfed circuits overheat contacts.
• Moisture/corrosion & vibration: Especially in RTUs/attics; leads to intermittent operation.

Symptoms

• Heat strips never come on (24 V is present at the sequencer coil but no amp draw after 30–90 s).
• Heat won’t shut off (stuck contacts; strips stay energized after the call ends).
• Wrong staging (all strips slam on at once → lights dim/breaker trips; or only some stages ever energize).
• Limit trips or scorched/crumbly terminals at the sequencer.
• Timing drift (delays far outside nameplate).

Quick Checks

1. Visual & tug test (power off): look for heat-darkened plastic, loose terminals, brittle insulation.
2. Coil/heat element test: isolate the 24 V terminals; ohm them. Open/infinite Ω = bad. (Exact Ω varies by model.)
3. Live test: call for heat, verify 24 V at coil; within spec delay the contacts should close. Use a clamp meter on each strip to confirm staging.
4. Voltage drop across closed contacts: should be ~0–0.3 V. Higher = pitted/high-resistance contact → replace.
5. Rule out look-alikes: open limits, failed heating element, bad transformer/thermostat wiring, or a tripped breaker.

Replacement Tips

• Match coil voltage, # of stages/poles, contact amp rating, and timing.
• If one section in a multi-stack is bad, replacing the entire sequencer and any cooked terminals is usually faster and more reliable.
• After replacement, verify airflow and filter condition to prevent repeat overheating.