Sequencer
The time delay device that staggers electric heat elements so they do not all energize at once.
Safety
What it is
A sequencer is a heat activated switch used in electric furnaces and heat pump air handlers. A small heater inside warms a bimetal element, which closes a set of line voltage contacts after a delay of several seconds to a minute.
Multiple sequencers, or a single sequencer with several sets of contacts closing at different times, bring heat strips on in stages.
Why it matters
Electric heat draws very large current. Energizing every element at the same instant would produce a heavy inrush, cause visible light flicker and stress the service. Staging also lets the blower get moving before the elements heat up.
- Sequencers close slowly and open slowly, so heat lingers briefly after the call ends.
- The blower is often driven by one set of sequencer contacts, which is why the fan may keep running after the thermostat is satisfied.
- Because they are thermal, they cannot switch quickly and are not suited to rapid staging.
How it works
- 1The thermostat closes the auxiliary or emergency heat circuit and energizes the sequencer heater.
- 2The bimetal warms and the first set of contacts closes, energizing the first element and often the blower.
- 3Additional contacts or additional sequencers close in turn, bringing on further elements.
- 4When the call ends the heater de-energizes, the bimetal cools, and the contacts open in reverse order.
How to diagnose it
Confirm 24 volts at the sequencer coil terminals during a heat call
No control voltage means the fault is in the thermostat or board, not the sequencer
Wait the full delay before judging the result
Sequencers are intentionally slow and can take up to a minute
Clamp each element circuit
An element that never draws current has an open sequencer contact, an open element or an open limit
Measure voltage across closed line contacts
Significant voltage means pitted, high resistance contacts
Check that contacts open after the call ends
Stuck contacts leave heat energized and can trip the limit repeatedly
High current circuits
Tools involved
- Clamp ammeter
- Multimeter
- Wiring diagram showing element and sequencer staging
Practice & Mastery
- Sequencer Troubleshooting Scenarios2
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