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Sequencers

Sequencers control when electric heat strips come on and go off. Instead of slamming every element onto the line at once, a sequencer brings heaters on in stages, a few seconds apart, and shuts them off the same way. That staging keeps inrush current down, protects breakers and wiring and lets the blower get moving before full heat arrives. If a sequencer fails, you get either no heat, partial heat or heat strips that stay on when they should not.

This category covers electric heat sequencers for air handlers, electric furnaces and heat pump backup heat. You will find SPST and DPST contact configurations, single and multiple stage designs, a range of on and off timing values and 24-volt coils that match standard thermostat control circuits. Supco and other universal lines offer sequencers that cover many OEM part numbers when matched by contacts, timing and ratings.

Sequencers fail from heat, age and the steady current they carry. Contacts pit, heaters inside the sequencer burn open and terminals loosen from thermal cycling. On many older air handlers, a failed sequencer is one of the first suspects when a heat strip will not come on.

How Sequencers Work

Bimetal heater design

Most electric heat sequencers are thermal devices. When the thermostat calls for heat, 24 volts goes to a small internal heater inside the sequencer. That heater warms a bimetal element. After a set delay, the bimetal moves and closes the first set of contacts, energizing the first heat strip. Additional contacts close in sequence. When the call ends, the heater cools and the contacts open in sequence after their off delay.

Why the timing matters

Staged timing reduces the electrical surge and gives the blower time to start before significant heat builds up. The off delay lets the blower carry away residual heat. On many systems, one set of sequencer contacts is also used to bring on the blower.

SPST and DPST contacts

A SPST (single pole, single throw) sequencer has one set of contacts per stage, used to switch one leg of a heater. A DPST (double pole, single throw) sequencer switches two circuits or both legs of a 240-volt heater. Multi-stage sequencers combine several sets of contacts with staggered on and off times to stage several heaters, and sometimes include a set of low-current contacts for a blower relay or another sequencer.

Sequencers in heat pump systems

On a heat pump air handler, the electric strips provide auxiliary heat when the heat pump cannot keep up, backup heat during defrost and emergency heat if the outdoor unit is down. The thermostat or control board sends 24 volts to the sequencer on those calls. A failed sequencer may go unnoticed in mild weather and only show up on the first cold night or during defrost, when the supply air suddenly turns cool.

24V coil

Most residential sequencers use a 24 VAC heater circuit that is controlled directly by the thermostat W terminal or by the air handler's control board. Some commercial models use other control voltages.

Signs a Sequencer Is Failing

  • No electric heat with 24 volts present at the sequencer heater terminals and line voltage at the contacts.
  • Only part of the heat comes on. One stage closes but others do not.
  • Heat strips stay on after the thermostat is satisfied, which suggests welded contacts.
  • Blower does not start with the heat on systems where the sequencer controls the blower.
  • Breakers trip at startup because all stages are closing at once.
  • Burnt terminals, melted wires or discolored contacts at the sequencer.
  • Open heater when checked across the 24V terminals with power off.

Check amp draw on each heater leg with a clamp meter during a heat call. A heater leg with no current may point to the sequencer, a heater element or a limit. Test each part before replacing.

How to Choose the Right Replacement

Match the contact configuration

Count the stages and contacts on the original. A sequencer controlling two heater elements on 240 volts may require DPST contacts or multiple SPST stages. Note any auxiliary contacts used for the blower or for another sequencer.

Match the timing

Sequencers are rated with on and off delay times, typically in seconds. Each stage may have different values. Match the original timing or follow the equipment manufacturer's specification. Faster on times can increase surge; slower off times affect blower operation.

Match the coil voltage

Most residential sequencers use a 24 VAC heater circuit. Confirm before ordering.

Match the contact rating

Electric heaters draw heavy current. The sequencer's contacts must be rated for at least the amps and voltage of the heaters they switch. Check the heater kW rating and the sequencer's rating.

Match the mounting

Confirm mounting hole spacing, terminal arrangement and size. A universal sequencer may include adapters.

Cross-reference the OEM number

The air handler model and OEM sequencer number are the most reliable guide. Product pages include cross-reference data and specifications.

ConfigurationWhat it switchesTypical useKey specs to match
Single stage SPSTOne leg of one heaterSmall electric heat, single elementOn and off delay, contact rating, 24V coil
Single stage DPSTBoth legs of one heater240V heaters needing full disconnectOn and off delay, two-pole rating
Two stageTwo heaters, staggeredResidential air handlers, heat pump backupTiming per stage, ratings, auxiliary contacts
Multi-stageSeveral heaters plus blower or pilot contactsLarger air handlers and electric furnacesStage count, timing, ratings, pilot contacts

Installation and Safety Tips

  • Turn off all power to the air handler and verify with a meter. Many air handlers have more than one supply circuit for the heat strips. Check every leg.
  • Photograph and label wires before removing the old sequencer. Contact positions and pilot contacts must go back exactly right.
  • Inspect wiring and terminals. Replace any heat-damaged wires and terminals with properly rated parts. A loose terminal will burn up a new sequencer.
  • Tighten connections firmly but do not crack the sequencer housing.
  • Check limits and heater elements while the panel is open. A failed limit or element can mimic a bad sequencer.
  • Test the full sequence. Make a heat call and confirm each stage energizes in order, measure amp draw on each heater and confirm the blower runs. End the call and confirm stages drop out.
  • Work on electric heat requires care. These circuits carry high current on 240 volts and should be serviced by a qualified technician.

Part Details & Shipping

HVAC365 carries Supco sequencers because Supco has been serving the HVAC trade since 1945 and builds universal sequencers that match many OEM applications. Product pages list contact configuration, timing, coil voltage and cross-reference data so you can confirm the match, and these items ship in 3-5 business days.

Frequently Asked Questions

Can I replace a sequencer with a relay?

Not directly. A relay closes immediately, so all heaters would come on at once, increasing surge and possibly tripping breakers. Some modern equipment uses relays with a control board providing the timing, but a replacement should match what the equipment was designed for.

What is the difference between SPST and DPST sequencers?

SPST switches one leg of a circuit. DPST switches two circuits at the same time, such as both legs of a 240V heater.

Why does only one of my heat strips work?

A sequencer stage, a heater element or a limit may be open. Check voltage and amp draw for each heater circuit to find which part has failed.

Why do my heat strips stay on after the thermostat stops calling?

Welded sequencer contacts are a common cause. Turn off power and replace the sequencer. Check for the cause, such as loose terminals or overcurrent.

How long should a sequencer take to close?

It depends on the model. On and off delays are listed in seconds on the sequencer and in product specifications. Match the original timing.

Should I replace a sequencer that tests good but looks burnt?

Discolored terminals and heat-stained housings point to loose connections or contacts running hot. Even if it still works, a sequencer in that condition is near the end of its life, and the wiring connected to it should be inspected and repaired at the same time.

Do sequencers use 24 volts?

Most residential sequencers use a 24 VAC heater circuit controlled by the thermostat or air handler board. Check the label to confirm.

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