No Heat
Furnace won't light, lights and shuts off, or blows cold.
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 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 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.
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.
Heat Pump Auxiliary Heat vs. Emergency Heat: When Each Runs and Why Aux Runs Too Much
The difference between auxiliary heat and emergency heat on a heat pump, what's normal, and how to diagnose a heat pump that runs on aux heat constantly or never brings it on.
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.
How to Test a Blower Motor: PSC, Constant-Torque ECM and Variable-Speed ECM
How to tell which kind of blower motor you have and test it the right way, from capacitor and amp checks on PSC motors to control signals on constant-torque and variable-speed ECMs.
How to Test an HVAC Transformer: 24-Volt Checks, VA Rating and Shorts
How to test the 24-volt control transformer in a furnace or air handler, how to size a replacement by VA, and how to find the short that killed the last one.
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.
Thermostat Blank or No Power: Finding the Missing 24 Volts
How to troubleshoot a blank thermostat: batteries, the C wire, the furnace door switch, a blown board fuse, a tripped float switch, the transformer and power to the air handler.
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 High Limit Switch Tripping: Causes and How to Fix It
Why a furnace's high limit opens, how to confirm it with temperature rise and static pressure, and when the switch itself is the problem.
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 Pressure Switch Troubleshooting: Open, Stuck Closed and Dropping Out
Diagnose furnace pressure switch codes by measuring the inducer's draft with a manometer and checking vent, intake, hose, condensate trap and switch.
Furnace Flame Rollout Switch Tripped: What It Means and What to Check
What a tripped flame rollout switch means, why you should not just reset it, and how to check the vent, burners and heat exchanger.