HS3P1S – HEAT SEQUENCER (HS24A345, DPST-SPST)

If your electric furnace is not heating evenly, only partially heating, or not turning on heating elements in sequence, the heat sequencer may be faulty. This sequencer stages electric heating elements so they don’t all kick on at once and overload the circuit.

Specs: 24V coil, 1 DPST + 1 SPST relay configuration (series), 25A capacity

Replaces: Q105, HS3P1S, HS24A345, NS105, SSEA34-5, 621678, 621678R, B12565-53, B1256553

Fits: Nordyne/Intertherm/Miller electric furnaces; universal fit for systems requiring DPST+SPST sequencer configuration

In stock at Open To Public HVAC Parts in Dallas. We test parts at the counter before you buy. Call or text (214) 340-9421.

Specs

• TypeElectric heat strip sequencer
• Pole ConfigurationDPST-SPST
• Coil Voltage24V AC
• Delay30-90 seconds between stages (typical)
• OEM CompatibilityUniversal

Cross-reference numbers

This part replaces the following OEM and aftermarket numbers:

• Q105
• HS3P1S
• HS24A345

Not sure if your number matches? Call (214) 340-9421 or bring the old part in — bench-testing is free.

Fits these models

• Universal aftermarket heat sequencer — staggers electric heat strip energization on residential and light-commercial electric heat kits
• Common applications: Rheem RXBH / RXMS / RXST, Goodman HKS, First Co H1830, Carrier KFCEH, Trane heat kits
• Match pole configuration (SPST/DPST/DPDT/TPST) to your existing sequencer; cross-reference part number with your heat kit's wiring diagram

How does a heat sequencer work?

A small bimetallic heater inside the sequencer gets 24V on a heat call from the thermostat. The heat slowly warms a bimetal disc that closes successive contact sets over 30-90 seconds — energizing each heat strip in sequence to prevent a simultaneous current spike that would trip the breaker.

How do I test a heat sequencer?

Apply 24V to the coil terminals (no load). Wait 30-90 seconds — you should hear individual contacts click closed in sequence. Use a multimeter for continuity across each load terminal pair. If one stage never closes, replace.

SPST vs DPST vs DPDT vs TPST — what's the difference?

- SPST = single pole, single throw (1 contact set, switches 1 strip) - DPST = double pole, single throw (2 contacts, switches 2 strips simultaneously) - DPDT = double pole, double throw (2 contacts with NO + NC positions) - TPST = triple pole, single throw (3 contacts, switches 3 strips simultaneously)

Why won't my heat strips come on?

Test the sequencer first — bad sequencer = no signal to the strips. Also check: thermostat call on W terminal, 24V at sequencer coil during heat call, line voltage on strip side, intact heater elements (continuity test each strip).