Gas Furnaces
Ignition, flame sensing, board codes and short cycling.
Furnace Blowing Cold Air: A Diagnostic Order That Finds It Fast
Why a gas furnace blows cold or lukewarm air, in the order to check it: thermostat fan setting, lockouts, flame proving, limits, gas supply, and duct leaks.
How to Check Furnace Gas Pressure: Inlet and Manifold with a Manometer
Step by step for measuring furnace inlet and manifold gas pressure with a manometer, typical ranges for natural gas and propane, how to adjust, and how to clock the meter.
Furnace Temperature Rise: How to Measure It and What the Reading Means
How to measure furnace temperature rise correctly, where the normal range comes from, how to estimate airflow from it, and what a high or low rise tells you.
How to Test a Furnace Hot Surface Igniter (and Why They Keep Failing)
How to test a hot surface igniter by watching the sequence, checking voltage and measuring resistance, the difference between silicon carbide and silicon nitride igniters, and what shortens their life.
Millivolt Wall Heaters and Floor Furnaces: Thermopile and Gas Valve Troubleshooting
How millivolt (powerpile) systems on wall heaters, floor furnaces and fireplaces work, how to test the thermopile, and how to find the voltage drop that keeps the main burner from lighting.
Standing Pilot Won't Stay Lit: Thermocouple Testing on Older Furnaces and Boilers
How a standing pilot and thermocouple work on older furnaces, boilers and water heaters, how to test the thermocouple with open-circuit and closed-circuit millivolt readings, and what else makes a pilot go out.
Furnace Flame Sensor: Microamp Readings, Cleaning and When to Replace
How flame rectification works, how to measure flame sensor microamps, what readings are normal, and how to fix weak flame signal.
Furnace Control Board Fuse Blown (3A): Finding the 24V Short
Why a furnace control's low-voltage fuse blows and how to find the short in thermostat, outdoor unit or accessory wiring before replacing it.
Furnace Ignition Lockout: Failed Ignition Troubleshooting Step by Step
Diagnose a furnace that tries to light and locks out by following the ignition sequence: inducer, pressure switch, igniter, gas valve, flame sense.
Furnace Blinking Light Codes: How to Read Them on Any Brand
How furnace control boards report faults with LED flashes, how to read the code correctly, and what the common code categories point to.
Furnace Short Cycling: 7 Causes and How to Pin Down Which One
Diagnose a gas furnace that lights and shuts off too soon, from flame sensing and limit trips to oversizing and thermostat settings.
Belt-Drive Blowers: Belt Tension, Pulley Alignment, Bearings and Setting Airflow
How to service a belt-drive blower on older furnaces, air handlers and rooftop units, including belt tension and alignment, bearings, adjustable pulleys and how they change airflow.
Furnace Inducer Motor: Testing, Noises and When It's Really the Pressure Switch
How a furnace draft inducer works, how to test one that won't start, runs noisy or runs but won't prove, and how to tell an inducer problem from a pressure switch or venting problem.
Tools, charts and checklists
- Furnace error codes by brand
- Furnace airflow from temperature rise
- Furnace maintenance checklist
- Combustion analysis log