Troubleshooting Guide for Gas Furnaces and Heaters
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Troubleshooting Guide for Gas Furnaces and Heaters

A comprehensive guide for troubleshooting your gas-fired furnace or heater.

It is always frustrating when your gas furnace or heat fails to start up or run long enough to keep your home warm. Many modern furnaces have multiple safeties installed to shut down the unit upon the slightest miscue. Usually the solution involves no more than pressing a button. This guide should aid the homeowner in correcting a wide range of faults and conditions that prevent your furnace or heater from functioning properly.

Furnace Diagram

Troubleshooting Guide

Furnace will not run

1. No power: Check for blown fuses or tripped circuit breakers at main entrance panel, at separate entrance panel, and on or in furnace; restore circuit.

2. Switch is off: Turn on separate power switch on or near furnace; usually red emergency shutoff light switch.

3. Motor overload: Most motors have a thermal overload, but not all motors will have a reset button. If yours does, let the unit cool down for at least 30 minutes; press reset button on motor. Repeat if necessary.

4. Pilot light out: Follow the directions for your unit and relight the pilot. If you have a hot surface ignitor (HSI) you will probably need to replace it.

5. No gas: Make sure gas valve to furnace is fully open. Sometimes homeowners shut the gas off to the unit in the summer and forget to open the valve before the first call for heat. If the problem persists, shut off the unit and contact an HVAC technician or your utility company.

6. Check other power or safety switches: Be sure all safety interlock switches such as the one on access panels are depressed and that the doors are securely shut. Replace safety switch if the unit fails to run after corrected.

7. Condensing units: High efficiency units have an inlet and outlet for combustion air. Check to see that there is no water inside the unit or exhaust pipe which will trip the pressure switch. Water can block the flow of air and cause the unit to shut down. Drain the water and look for sediment clogging the drainage tubing or outlet.

Unit does not provide enough heat

1. Thermostat set too low: Raise thermostat setting 5°.

2. Filter dirty: Replace or clean filter.

3. Blower dirty: Remove and clean blower assembly.

4. Registers closed or blocked: Verify that all registers are open; make sure they are not blocked

by furniture or that the dampers are closed.

Supply register with dampers closed

5. System not balanced: It can be difficult to balance the ductwork system without knowledge of the sizing and fittings. It would be best to open each balancing damper to 50% and go room to room to provide adequate airflow.

6. Blower belt loose or broken: Replace belt.

7. Burner dirty: Clean burner if you are familiar with the unit, if not, hire a professional.

Pilot will not light

1. Pilot opening blocked: Clean thermocouple tip and pilot opening. Newer units may have a flame sensor that has failed or is dirty.

2. No gas flow: Make sure pilot light button is fully depressed; make sure gas valve to furnace is fully open. Older units may have a small valve on a separate line for standing pilots.

Pilot will not stay lit

1. Loose or faulty thermocouple: Tighten thermocouple nut slightly with a small adjustable wrench; if the pilot will still not remain lit, replace the thermocouple.

2. Pilot flame set too low: The flame of the pilot light needs to heat the flame sensor. Adjust the flame so that it is about 2 inches long. Follow the gas tubing back from the pilot to the gas valve and look for an adjustment screw that controls the pilot flame. Turning it counter-clockwise increases the flame size; clockwise reduces it.

3. Electronic ignition will not work: Call a professional to replace the ignition system. If you have a replacement HSI or spark ignitor, replace it.

Furnace cycles off and on repeatedly

1. Filter dirty: Replace or clean filter.

2. Motor or blower needs lubrication: If the motor and blower have a oil ports, add oil. The oil port is usually sealed with a small set screw. One is by the shaft coming out of the motor and the other is on the opposite end.

3. Air proving switch is bad: Locate the air proving, air pressure, or blower fan switch on the control diagram or owner’s manual for your unit and replace it if you can. Call a professional for service if necessary.

Air proving switch

How to Test and Adjust your Fan Limit Switch -

Blower does not shut off

1. Fan control set to ON: Go to the thermostat and set the FAN switch to AUTO.

2. Limit switch setting wrong: Reset limit switch for stop-start cycling.

3. Limit control needs adjustment: Should be handled by a professional HVAC technician.

Furnace Noises

1. Access panels loose: Reset and fasten access panels correctly.

2. Blower belt worn or damaged: Replace worn or damaged belts. If the belts are new and squeal, spray drive belts with belt dressing. Squealing will eventually subside.

3. Blower belt is too loose or too tight: Loosen the motor mount and adjust the tension on the belt until there is ½ to ¾ inch of slack.

4. Motor and/or blower needs lubrication: Fill oil ports with oil.

5. Burner is dirty: Clean the burner assembly if possible or hire a professional.

6. Blower makes a clicking or banging noise: Look for loose objects or filter hitting the fan inside the unit.

Always use common sense when working on electrical or gas-fired equipment. Never bypass any safety devices installed in your furnace or heater as fire or serious injury could result.

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Comments (5)

Another great maintenance guide for my house book...voted

Daniel, this is superior work. Another fine piece.

Super article for the DIY.

Great work.

Daniel, you should consider doing an E-book on home repairs for the home handyman and marketing it on line. Personally, I think that you would have a best seller.