If your furnace igniter glows red or orange but the burners never light, the furnace is making it partway through the startup sequence but something is preventing ignition.
A glowing igniter tells you that the furnace is receiving a call for heat and that at least part of the ignition system is working. But the furnace still needs the proper gas flow, burner operation and safety signals before the burners can ignite.
Several problems can cause the igniter to glow without producing a flame.
The gas valve may not be opening
After the igniter heats up, the furnace control board should signal the gas valve to open. If the gas valve does not open, the burners will not receive fuel and cannot ignite. The problem could involve the gas valve itself, the wiring to the valve or another control issue that is preventing the valve from being energized.
Gas valve problems require professional diagnosis.
There may be a gas supply problem
The furnace may also fail to ignite if there is no gas reaching the burner assembly.
Possible causes include:
A closed gas shutoff valve
A problem with the home’s gas supply
Low gas pressure
A problem with the furnace gas valve
Another interruption in fuel delivery
If other gas appliances in the home are also not working, there may be a broader gas supply issue. If you smell gas, leave the area and contact the appropriate gas utility or emergency service from a safe location.
The igniter may glow but still be too weak
A hot surface igniter can sometimes glow without becoming hot enough to reliably ignite the gas. Igniters become more fragile as they age and can develop small cracks or electrical resistance problems. The igniter may appear to be working because it glows, but it may not be reaching the temperature needed for dependable ignition. A technician can test the igniter rather than assuming it is good based on appearance alone.
Dirty burners can interfere with ignition
The burner assembly has to allow gas to flow and ignite properly. Rust, dirt, debris or other contamination can interfere with burner operation. In some cases, gas may reach the burners but fail to ignite where it should. Burner problems can also affect how flame travels from one burner to another after ignition begins. Because burners are part of the combustion system, they should be inspected and cleaned properly rather than poked, scraped or adjusted at random.
The pressure switch may not be proving properly
Before the furnace allows normal ignition, it verifies that the venting system is operating correctly. The inducer motor creates draft through the furnace, and the pressure switch helps confirm that the furnace can safely vent combustion gases.If the pressure switch opens or fails to prove at the wrong time, the control board may interrupt the ignition sequence.
Possible causes can include:
A blocked vent or intake pipe
A weak inducer motor
Damaged pressure-switch tubing
A clogged condensate drain
A faulty pressure switch
Other venting problems
Depending on the furnace design, the igniter may still glow during part of this sequence before the startup is interrupted.
A clogged condensate drain can stop ignition
High-efficiency furnaces produce condensation during operation. If the condensate drain system becomes blocked, water can back up and interfere with pressure-switch operation or other safety controls. The furnace may begin the ignition sequence but stop before the burners light. This is one reason a glowing igniter does not automatically mean the problem is located at the igniter itself.
The control board may not be sending power to the gas valve
The furnace control board manages the entire startup sequence. Once the igniter reaches the proper temperature, the board should send power to the gas valve. If that signal is not being sent, the burners will not receive gas. The cause could be the control board itself, but it could also be another safety condition preventing the board from allowing ignition.
Proper electrical testing is needed to tell the difference.
Listen to what happens after the igniter glows
The sequence of events can provide useful clues.
A typical startup may sound like this:
The inducer motor starts
The pressure switch proves
The igniter begins glowing
The gas valve opens
The burners ignite
The blower starts after the furnace warms up
If the igniter glows and then simply shuts off without burner ignition, the problem may involve gas flow, the gas valve, a safety control or the control board. If you hear a click from the gas valve but still get no flame, that is also useful diagnostic information for the technician.
Could a dirty flame sensor cause this?
Usually, a dirty flame sensor causes a slightly different symptom. The burners typically ignite first, then shut off after a few seconds because the furnace cannot verify the flame. If the burners never light at all, the problem is more likely to involve the igniter, gas supply, gas valve, burner assembly or another part of the startup sequence.
Will the furnace keep trying?
Many furnaces will make several ignition attempts before stopping. If the burners fail to light repeatedly, the furnace may eventually go into lockout. This is a safety feature designed to prevent the system from endlessly attempting ignition. If the furnace goes into lockout, resetting it may temporarily allow another startup attempt, but it will not fix the underlying problem.
When should you call for furnace repair?
You should have the furnace inspected if:
The igniter glows but the burners never light
The furnace repeatedly attempts ignition
The furnace goes into lockout
You hear the gas valve click but get no flame
The igniter works only intermittently
The furnace has stopped heating the home
You smell gas
The problem returns after the furnace is reset
A technician can test the igniter, gas valve, gas supply, burners, safety controls and control board to determine exactly where the ignition sequence is failing.
Need help with a furnace that will not ignite?
If your furnace igniter glows but the burners will not light, the problem may involve the igniter, gas valve, gas supply, burner assembly, pressure switch or control system.
Roberts Brothers Comfort can inspect the furnace startup sequence, identify why the burners are not lighting and make the necessary repairs. We provide expert furnace repair service throughout Ohio’s Miami Valley region including Casstown, Covington, Fletcher, Piqua, Rosewood, Saint Paris, Sidney, Tipp City, Troy, Urbana, West Milton and all the surrounding areas.
