Furnaces
3 components covered

Field manual · Furnace

90%+ condensing gas furnace: field service manual

Covers: High-efficiency condensing furnaces (about 90-98% AFUE) with a secondary heat exchanger, PVC venting and a condensate trap; single-stage, two-stage and modulating

Single sourceBrands: Carrier, Bryant, Goodman, Amana, Daikin, Trane, American Standard, Lennox, Rheem, Ruud, York, Heil, Tempstar3 components

Same basic sequence as an 80% furnace, plus a secondary heat exchanger that condenses water from the flue gas. Recognise it by two PVC pipes (or one PVC vent), a condensate trap and drain. The trap and drain are the extra failure point.

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

The sequence of operation matches the 80% unit: safeties checked, inducer, pressure switch proves, igniter, gas valve, flame proving, blower delay.

Flue gas passes through a secondary heat exchanger where water condenses; it drains through the collector box and trap.

If the trap or drain backs up, water restricts the flue path and the pressure switch opens.

Two-stage and modulating units use more than one pressure switch and may calibrate the switch at startup.

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.

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

Other furnaces manuals

Manual · Furnace

80% gas forced-air furnace: field service manual

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

6 components3 common failuresCarrier, Bryant, Payne, Goodman, Amana, Daikin, Trane, American Standard, Lennox, Rheem, Ruud, York, Coleman, Heil, Tempstar, Comfortmaker
Manual · Furnace

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

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

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Manual · Furnace

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
90%+ condensing gas furnace: field service manual · OnCall Tech