Furnaces require regular attention to keep running properly. If you have ever awoken to a cold house because the furnace stopped working, you realize quickly just how essential they are.
Why does my furnace flame go out when the blower comes on? The most common causes for a furnace flame going out include:
- A dirty or malfunctioning sensor
- Defective ignition board
- Insufficient gas pressure
- Faulty limit switch
- Faulty thermostat
- Wiring issues
Boiler systems are another common central heating system that operates in the same basic way so the issues and solutions are pretty much the same.
This article will detail how a furnace or boiler works, the main culprits that cause the burner to shut off, and how to create a regular maintenance schedule to hopefully stop problems arising again.
How Does a Furnace or Boiler Work?
Central gas furnaces, either natural gas or propane burning, are the most common heating source in the U.S., used in 50.3 million homes. Boilers are the second most common heating source used in 6.8 million homes (source).
Furnaces and boilers should last for 15-20 years if maintained properly.
Furnaces and boilers both provide heat by burning a mixture of fuel and air within the combustion chamber of a heat exchanger. The heat is transferred to a medium outside of the exchanger. From there, some deviation happens depending on the system. Let’s compare:
|Central Gas Furnace||Gas Boiler|
|Heat Medium||Air||Water or Steam|
|Method of Transfer||Large fan forces warm air through duct system and out registers in rooms||Pump forces warmed water or steam to circulate through piping system and radiators|
The combustion process in either a furnace or boiler produces exhaust gases, such as oxygen, carbon dioxide, water vapor, and carbon monoxide, that are vented outside through a vent pipe, either exiting the roof or side wall.
Furnaces and boilers can have 3 basic ignition systems as well, usually depending on the age of the unit. Here’s a side by side comparison:
|Standing Pilot||1920s-1980s||Pilot remains lit all of the time||Very low- 50-65% AFUE rating*||Higher chance of fire or gas leak, fewer if any safety switches|
|Intermittent Pilot||1980s-1990s mostly||Electronic components open gas valve and ignite pilot as needed||High- Often over 90% AFUE rating||More safety and control switches and lower risk of fire from combustion tank|
|Hot Surface Ignition||1990s-present||Silicon Carbide surface heats up and directly fires the burners||Very high- well over 90% AFUE rating||Lots of controls and safety switches, low fire risk|
Why is My Furnace or Boiler Burner Shutting Off?
Now that you understand the basics of furnace or boiler operation, let’s look at all the things that could be going wrong to cause your furnace or boiler burner to shut off.
Often the burner will come on for a short period of time and then shut off, only to restart the system a little later. This is called cycling and is a common symptom of one of the following problems.
Dirty Flame Sensor
By far the most common issue, a dirty flame sensor will cause the gas valve to shut off as a safety precaution. Without it shutting off, gas could penetrate your home, causing a very dangerous build-up of toxic and flammable gas. Thankfully it does its job well and is easy to fix. To clean, simply:
- Remove the flame sensor wand. Sometimes it is easily accessible, but you may need to unscrew a panel to reach it.
- Clean the flame sensor by rubbing it with steel wool or an emery cloth. You should notice it becoming shinier.
- Rub any remaining residue with a clean cloth.
- Reinsert the flame sensor into position.
This short YouTube video does an excellent job of visually explaining the dirty sensor issue:
Broken Flame Sensor
Occasionally the flame sensor will break. Check this by removing the flame sensor as above and inspecting it for cracks in the porcelain. If there is noticeable damage, replace as per manufacturer’s instructions.
Defective Ignition Board
The ignition board is the computer board that controls the gas, spark, and ignition systems of intermittent pilot or hot surface ignition systems. Occasionally, the board can become damaged. To check:
- First, try turning the furnace power switch on and off. The board, like most computer systems, may just need a restart.
- If this doesn’t work, turn off the power and gas supply to the furnace. Without touching it, inspect the heating element for cracks or damage and the wiring for any frays or possible malfunction.
- If you notice anything, call a technician for repairs.
Insufficient Gas Pressure
If not enough gas is supplying the furnace or boiler, it will not continue running. Check that gas valves are properly opened.
Warning: If you smell gas or suspect a leak, immediately leave the house and call your utility company. A gas leak can not only affect the functioning of your furnace but create a potential health and safety hazard.
Overheating of the system can happen from many sources, all of which will cause the burner to shut down automatically. The most common causes are:
- Dirty air filters: Air filters should be changed at least every 90 days. Check yours for clogging and debris and change as necessary.
- Blocked or closed air supply vents: Check that all of the vents are open throughout the home. Too many closed vents can cause heat to build up in the system.
- Exhaust flue/pipes blocked: Check for debris in the exhaust vent, such as birds’ nests, leaves, and excessive dirt.
- Dirty blower wheel: If the filters are left dirty too long, they can affect the blower wheel by causing dirt to build up on it. This of course will limit its ability to function. Call for servicing.
Faulty Limit Switch
The limit switch is meant to keep the furnace from overheating from all the above reasons and more. It can wear out, failing in an open position and turning off the heating element.
The blower continues to run, however.
Replace the switch if it has failed.
The thermostat controls the functioning of the furnace by telling it when to turn on and off. A few things can happen with a thermostat:
- The batteries may need replacing.
- The wiring may have short circuits or ground fault issues. Check the breaker and wring system.
- The thermostat may be placed in an area that does not accurately reflect heating needs. If the thermostat is located in a very sunny location or near a strong heat source, move it to a new position.
It is possible the wiring or ground wire connections for the furnace have damage. Especially if the circuit breaker is tripping, check all exposed wiring for pest damage, pinching, and wear. You may need to call an electrician if wiring issues are suspected.
Creating a Maintenance Schedule to Keep the Burner Working
The best way to ensure the burner and blower will keep running throughout the season is to perform regular maintenance on the system. Dirt and water can lead to loss of efficiency, failures, and safety problems.
Always check your owner’s manual for specific instructions for your unit, but the following general maintenance recommendations apply to basically any system. (source)
- Clean the flame sensor regularly throughout the heating season.
- Replace air filters at least every 90 days, but check them monthly.
- Inspect the pilot light and burner flame color and height. The flame should be sharp and mostly blue. If not, clean the burners or contact a technician to adjust the amount of air entering the system.
- Check burners for greasy dirt of soot build-up. Gas should burn cleanly; if you notice these problems contact a technician for servicing.
- Clean the draft hood, the mechanism that controls exhaust gas, with a vacuum, and inspect the exhaust stack for damage or corrosion.
- Inspect the exhaust flue for debris and blockages.
- Inspect all of the electrical system for signs of wear and damage. Electrical problems pose a serious safety hazard, so be sure to repair, or have repaired, any deficient wiring.
- Clean the furnace room. Area dirt, lint, and hair can cause the unit to become clogged. By keeping that area clean, you can cut down air supply issues. Also make sure no combustible materials are stored near the furnace.
- Clean and lubricate blower fan and motor as per manufacturer’s instructions.
- Inspect the blower fan belt for wear and correct tension. Adjust as necessary.
If the burner on your unit continuously goes out when the blower comes on, most likely some general maintenance and cleaning are needed for the unit.
Some replacement parts may be necessary, but by first systematically cleaning and maintaining the flame sensor, air filters, vents, and other components, you are likely to extend the efficiency and life of your central heating system.
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