Furnace won't start

No response at all when there is a call for heat: no inducer, no ignition sequence, no blower.

Heating

high

Step-by-step: Gas furnace: no heat

Work through an ordered diagnostic path for this symptom.

Diagnosed here

Gas Furnace: No Heat
Troubleshooting
The furnace is calling for heat but the burners never light, or they light and then shut down.

Possible cause

Thermostats and Control Wiring
Component
What each control wire does, how thermostat types differ, and how to troubleshoot a control circuit.
Control Transformer
Component
The component that creates the 24 volt control circuit, how it is sized, and why its fuse keeps blowing.

Affected equipment

Gas Furnace
Equipment
How a modern forced air gas furnace proves each condition before it allows the next step.

Background

Gas Furnace Sequence of Operation
Concept
Step by step through a call for heat on a modern induced draft furnace, and what each step proves.

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.