Gas Heat & Furnaces

Furnace sequence of operation, ignition, gas valves, flame sensing, heat exchangers, combustion and CO, manifold pressure and venting categories.

5 guides

• Combustion, CO, and Cracked Heat Exchangers
  What complete vs incomplete combustion looks like, what drives CO up, how to read a combustion analyzer, the realistic numbers for O2/CO/flue temp, and how to actually find a cracked heat exchanger instead of guessing.
• Flame Sensing and Microamps: Clean It or Replace It
  How flame rectification works, how to read flame-sense microamps with your meter, the numbers that separate a healthy rod from a dying one, and when cleaning fixes it versus when you replace the rod or chase a bad ground.
• Furnace Ignition Systems: Hot Surface vs Spark vs Standing Pilot
  The three furnace ignition methods a tech runs into — hot surface igniter, spark (intermittent pilot or direct), and standing pilot — how each one lights the gas, how to test it, and the failure modes specific to each.
• Gas Furnace Sequence of Operation, Start to Finish
  The full step-by-step sequence a modern single-stage gas furnace runs through on a call for heat — thermostat call, inducer, pressure switch proof, igniter warm-up, gas valve, flame proving, blower on, and shutdown — and where it stops if something is wrong.
• Gas Valve Operation and Measuring Manifold Pressure
  How a modern redundant gas valve works, how to measure inlet and manifold pressure correctly, the target pressures for natural gas and LP, and how to adjust and verify firing rate without overfiring the furnace.

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