Electric furnace / air handler with heat strips: field service manual

Covers: Electric furnaces and air handlers with resistance heat strips, sequencers or relays, and limits/thermal links

Single sourceBrands: Goodman, Carrier, Trane, Rheem, Lennox, York, Nordyne/Intertherm3 components

Heat comes from resistance elements switched by sequencers or relays. These run on 240 V and high current: every live test is caution work. Low heat usually means one element or its limit/link has opened.

Safety first

240 V and high current: turn off every breaker feeding the unit and verify dead before touching heater wiring.
Live amp-draw tests are caution: use a clamp meter, keep hands clear.
Never bypass limits or thermal links.

How it works

The thermostat W call energizes the sequencer heaters or relay coils with 24 V.

Sequencers close their contacts in stages, bringing elements on one after another; the blower is brought on with the first stage.

Each element is protected by a limit and often a one-time thermal link in series.

Components & tests

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Official manufacturer documentation

Always confirm values against the tech sheet for the exact model.

OnCall Tech is a reference tool for trained technicians and can make mistakes. Always disconnect power and confirm with the manufacturer's documentation before you work. © 2026 OnCall Tech. Original content; not a reproduction of any manufacturer document.Generic part photos (not of this brand): Clothes dryer with its vent duct in a small laundry closet by Introspectivedsgn, Pexels License

Diagnostic guides for this appliance

Component test · Furnace

How to test a gas furnace draft pressure switch (electrical and draft checks only)

The pressure switch proves the inducer is pulling enough draft before ignition is allowed. Test it with a meter (24 V across it) and a manometer against the set point printed on the switch. Gas valve, gas pressure and burner work are not covered: refer those to a licensed gas technician.

5 steps ~30 minadvanced

Guide · Furnace

Furnace runs no heat (blower only or nothing)

Start with the board LED code, then work the call path: thermostat, 24 V, safeties, inducer, ignition, flame proving. Most no-heat calls end at a filter, a flame sensor, a pressure switch issue or gas being off.

5 steps ~45 minmoderate

Guide · Furnace

Furnace short cycling (starts and stops quickly)

Short cycling is usually a safety opening mid-cycle (limit from low airflow, flame sensor dropping out, pressure switch opening) or a thermostat issue. Read the code and time the cycle.

4 steps ~45 minmoderate

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Carrier Performance 96 / Comfort 96 gas furnaces (59TP6, 59SP6, 59SC6): quick sheet

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