Gas furnace: no heat
Work down this path in order. Each step confirms one part of the heating sequence before moving on, so you are always testing the next thing that has to happen rather than guessing.
Safety
Live voltage and combustion are both present during these checks. Never bypass, jumper or defeat a safety control to make the furnace run. Requirements vary by jurisdiction, equipment type, manufacturer instructions and applicable codes — always verify current local code, regulations, permit requirements and the manufacturer installation instructions before performing work.
Step 1
Is there a call for heat at the furnace control board?
Measure 24V between W and C at the board with the thermostat calling. A thermostat display alone does not prove the signal arrives.
Use a meter rated for the circuit. Do not jumper R to W as a first step — confirm the signal first.
Common causes at this step
- Thermostat not calling
- Broken or shorted thermostat wire
- Open low-voltage fuse
- Failed control transformer
Background: Thermostats and Control Wiring
Local Code Notice
HVAC installation, venting, combustion air, ventilation air, gas piping, clearances, termination locations, electrical requirements, permits, and equipment requirements vary by jurisdiction and equipment. Always verify the applicable local codes, adopted code edition, regulations, permits, and manufacturer installation instructions before installation or service.