Pilot light is Lit But Furnace Won’t Kick On

This is a fairly common problem. You’re trying to turn on the furnace, but it won’t kick on. Interestingly, the pilot light is ON and looks healthy. So, why won’t the burners come on? What’s stopping the pilot from igniting the burner, or why else aren’t you getting heat? We look at common causes and how to fix each below.

How Does a Furnace Work Step by Step?

Let’s begin with a quick overview of how the furnace works until the burners are lit, and the home is getting heated, as this can help you pinpoint potential issues with your furnace. We’ll assume that your home’s electricity supply system is working normally and the circuit breaker is ON. Additionally, we’ll assume that your home is getting gas.

If so, the heating process begins when the thermostat calls for it. This can happen immediately you switch on the furnace using the onboard ON/OFF button. Alternatively, some homeowners prefer to use the auto-timer so that the furnace automatically comes on when indoor temperatures drop below the thermostat setting.

Anyway, as long as the furnace is switched ON, heating begins when indoor temperatures drop below your setting. The thermostat will immediately request the control board to initiate the heating process. At this point, the furnace will engage the draft inducer motor to clear all exhaust gases lingering in the heat exchanger or exhaust pipes. This step sets the stage for the heating process.

If the draft inducer motor functions properly, it will generate a negative pressure as it pulls out exhaust gases. This negative pressure closes a critical relay called the pressure switch, signaling the control board to engage the ignition system. Different furnaces have different ignition systems.

Traditional units have standing pilot systems with constant flame, while modern systems have either intermittent pilot (spark) or hot surface ignition. Hot surface systems are the latest ignition technology. Standing pilot systems ignite the burners without further ado as long as the pilot light is on and steady.

However, in the other two systems, the pilot is OFF when the furnace is OFF, meaning the furnace must first ignite the pilot light, which, in turn, light the burners. The burners cannot come on if the pilot light doesn’t ignite. Once the pilot is lit, the furnace opens the main gas valve, allowing fuel to flow to the burner(s). Then the pilot lights the burner, and the unit starts producing heat.

Furnaces work by drawing heat from your home (or outside the home), heating it, and sending the now-warm air into your rooms. Return air (cold air entering the furnace) enters via the return plenum and goes through a filter that traps airborne impurities, such as dirt and dust. Then a blower motor located just behind the filter forces the cold air across the heat exchanger, where it absorbs (mixes with) heat from the heat exchanger.

The blowing action also helps push the now-warm air out of the furnace via the supply air plenum and into your home’s ductwork. Then the warm air exits via the supply vents in the various rooms throughout your home. Meanwhile, the combustion gases exit the furnace via the exhaust pipe.

The heating cycle typically lasts 10 to 15 minutes before the furnace cycles off. However, it may last longer depending on the unit’s heat output rate and other factors such as appliance age. A flame sensor monitors the burner (and, in some furnaces, pilot) flame throughout the heating cycle and instantly opens a switch to discontinue heating if it loses sight of the flame.

What Does it Mean If the Pilot is Lit, But the Furnace Won’t Kick On?

Typically, something is wrong with the furnace if the pilot light is on, but the furnace won’t kick on. Maybe the furnace isn’t getting gas, or perhaps it’s getting gas, but the burners are blocked. Or perhaps one of the relay circuits is open. The good news is that most of the possible issues are easy to troubleshoot and often solvable through DIY.

What are the Reasons Why a Furnace Won’t Kick ON and How Can You Fix Them?

 Let’s now look at a few common reasons why the furnace may refuse to kick on with the pilot ON ad how to fix each issue.

Thermostat issues 

The first thing to consider is that maybe your thermostat isn’t doing its work properly or not doing it at all. As we saw initially, the heating process begins at the thermostat. The thermostat senses when temperatures fall below your setting (typically 68°F to 72°F) and requests heating immediately.

However, what if the thermostat is broken? Or, what if it’s wrongly set? In both cases, it may cause the furnace to stay OFF even if the pilot light is ON. This typically happens in standing pilot systems where the pilot system ignites after the thermostat calls for heating.

Causes and Solutions 

  • The thermostat is broken/dead

If the thermostat is dead or malfunctioning, it cannot sense when temperatures drop below the desired level. Or, if it does, it may not call for heating. The solution is to repair the broken thermostat and replace dead ones.

  • Wrong thermostat settings

The wrong setting, such as accidentally setting the thermostat to 60°F when you meant to set it to 70°F, means the burner won’t engage even if temperatures fall below 68°F – which is pretty cold. The solution here is to correct the setting. The DOE recommends setting your furnace thermostat to 68°F-72°F.

Draft inducer system issues

If the thermostat is working well and correctly set, the next place to check is the draft inducer system, i.e., the combination of components that remove exhaust gases from the furnace. The draft inducer system essentially comprises the draft inducer motor and pressure switch. The motor removes exhaust gases from the furnace and, in so doing, creates negative pressure inside the closed motor housing.

This housing connects to the pressure switch via a relay such that the relay is pulled to close the switch if there’s sufficient negative pressure and let loose and open if there’s little or no negative pressure. An open pressure switch means that the furnace circuitry is complete. Thus, heating cannot go on even if the pilot light is ON, no matter how often the thermostat requests heating.

Causes and Solutions

  • The pressure switch is stuck open

The pressure switch can get stuck in the open position for many reasons, including dirt buildup and electrical malfunctions. Thoroughly cleaning the switch can help do away with the dirt buildup, while an electrical fault or short-circuit may necessitate fresh wiring

  • The draft inducer motor is dead or defective

Maybe some of the wires are damaged, the motor is dead, or the fan blades are broken. Rewiring and a few repairs might be enough to reverse the damage. However, you may also need to replace the draft fan.

Gas supply issues 

Next up, check whether your burners are getting gas. This applies to traditional furnaces with standing pilots and modern units with intermittent and hot surface ignition systems. First, there’s a chance that your burners may not get gas if the gas line is blocked somewhere between the outdoor meter and the burners on your furnace.

Such blockage can prevent gas from reaching the burner. Alternatively, the burners themselves could be blocked. Above all, you may also have a gas leak. A leak on the gas lines can reduce gas pressure, making it difficult or even impossible to keep a flame on your burners.

Causes and Solutions 

  • Blocked/leaky gas lines

 Blocked gas lines can result from several issues, including improper maintenance and unclean gas. Meanwhile, leakages typically result from holes or cracks on the line. Both issues require professional intervention. Turn off the gas valve, shut off the furnace, and call a professional.

  • Blocked burners

Blocked burners are often the result of dirt buildup due to poor maintenance. To fix the problem, shut down the furnace, turn off the gas supply, and take off the burner heads. Then soak the burner in soapy water for 30 minutes. After that, scrub the heads and caps with a non-abrasive scrub pad and use a paper clip to unblock the port openings.

Air filter issues 

The air filter is arguably the most sensitive part of the furnace. It takes just a minor filter issue for the furnace to stop operating. The furnace may be clogged if the unit doesn’t kick on even when the pilot light is ON. All the air that eventually ends in your rooms as warmth goes through your filter first.

Therefore, a clogged filter means not much air will go through. This usually results in overheating inside the furnace as the heat from your burners would have nowhere to go. Such overheating can cause the limit switch to open, thus keeping the furnace OFF until you fix the underlying issue.

Alternatively, your filter may be too efficient. A high MERV furnace filter makes it difficult for air to pass through because of the tiny holes.

Causes and Solutions

  • Clogged filters

Filter clogging is a direct consequence of poor maintenance habits. Ensure to clean your furnace filter at least once a week and replace it every 2-3 weeks or as the manufacturer says (check the owner’s manual). You should be able to catch further filter issues during regular maintenance.

  • Overly efficient filter

If your filter is too efficient, it means you purchased the wrong filter. Although ASHRAE says that MERV 13 is the best filter rating, you must first ensure that your furnace can support the high rating. Otherwise, stick to MERV 9-13 filters.

FAQs

  • What Happens if the Pilot Goes Out on a Furnace?

It depends on the type of pilot and when it goes out. An intermittent or hot surface pilot going out after the burners are lit is not a problem. However, the pilot going off before the burner ignites (for all pilot types) usually means heating is impossible.

  • How Do You Check the Pilot Light on a Furnace?

Remove the furnace’s front panel and check the burner compartment. The pilot light will be lit if it’s a standing pilot system. If it isn’t, you may have a problem. Either you have a non-standing pilot system, or the pilot light is out for a reason.

Summary 

You have a problem on your hands if your pilot light is ON but the furnace won’t kick ON. Either the thermostat has failed, the draft inducer system is compromised, your filters are filthy, or you have a gas supply problem. The good news is that you can easily troubleshoot these issues and resolve them, often without professional help.

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