Large stainless refrigerator set flush between kitchen cabinets
6 components covered

Field manual · Furnace

80% gas forced-air furnace: field service manual

Covers: Non-condensing (about 80% AFUE) induced-draft gas furnaces with hot surface ignition and an integrated control board; metal flue vent

Single sourceBrands: Carrier, Bryant, Payne, Goodman, Amana, Daikin, Trane, American Standard, Lennox, Rheem, Ruud, York, Coleman, Heil, Tempstar, Comfortmaker6 components

The most common residential furnace in the US. Recognise it by a single metal flue pipe, no condensate drain, an inducer on top of the burner box, and a board with a diagnostic LED. Most faults are found by reading the LED and following the heat sequence.

Safety first

  • If you smell gas: do not operate switches or the furnace, leave the building and call the gas utility from outside.
  • Make sure the home has a working carbon monoxide (CO) alarm; if it is sounding or anyone has headache/nausea, get everyone out and call 911 / the gas utility.
  • Turn off power at the furnace service switch or breaker before opening the burner or blower compartment; the door switch alone is not a lockout.
  • Gas-side diagnosis or repair (gas valve, gas pressure, burners, heat exchanger, venting) is for a licensed gas/HVAC technician only.
  • Never jumper or bypass a limit, rollout or pressure switch.

How it works

Thermostat closes R to W. The board checks its safeties (limit and rollout circuit closed, pressure switch open).

The inducer starts, the pressure switch closes once draft is proven, and the board runs a short pre-purge.

The hot surface igniter heats up, then the board energizes the gas valve with 24 V and starts the flame-proving timer.

The flame sensor proves flame by flame rectification (a small DC microamp signal). If flame is not proven in time, the valve closes and the board retries, then locks out.

After a delay, the blower starts. When the call ends, the gas valve closes, the inducer post-purges and the blower runs an off-delay.

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): Large stainless refrigerator set flush between kitchen cabinets by Alex Qian, 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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3 components2 common failuresCarrier, Bryant, Goodman, Amana, Daikin, Trane, American Standard, Lennox, Rheem, Ruud, York, Heil, Tempstar
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Electric furnace / air handler with heat strips: field service manual

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

Carrier condensing gas furnaces: Performance 96 two-stage 59TP6 (variable 25-speed blower), Performance 96 single-stage 59SP6, and Comfort 96 single-stage 59SC6 (multi-speed blower); earlier 59SC5 / 59TP5 generation is similar. Bryant sells equivalent platforms.

7 components3 common failuresCarrier, Bryant
80% gas forced-air furnace: field service manual · OnCall Tech