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No heat

What is going on

Thermostat is calling for heat but the furnace does not start or does not light.

Likely causes

  • Thermostat setting, batteries or wiring
  • Power switch, breaker or blown low-voltage fuse
  • Gas supply off or low pressure
  • Dirty flame sensor or failed igniter
  • Blocked vent, intake or condensate trap tripping the pressure switch
  • Limit or rollout switch open

Check in this order

  1. Confirm the thermostat is in Heat and set above room temperature; check batteries
  2. Check the furnace power switch, breaker and the 3 or 5 A fuse on the control board
  3. Read the LED flash code on the control board and look it up under Error Codes
  4. Confirm the gas valve is on and check inlet gas pressure
  5. Watch the ignition sequence inducer, pressure switch, igniter glow, flame, flame proving
  6. Clean the flame sensor and check flame microamps if the burner lights then shuts off
  7. Clear the condensate trap and check vent and intake terminations for blockage
  8. If a rollout switch has tripped, inspect the heat exchanger and venting before resetting

Applies to: Gas furnaces

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Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

For licensed technicians. Gas, electrical and refrigerant work must be done by a qualified, licensed person. Shut off power and gas before servicing, never bypass a safety control, and follow the manufacturer’s manual for the exact model; it and local codes always take priority. HVACopedia is not affiliated with any manufacturer; brand and model names only identify the equipment.