A functioning furnace is essential for maintaining a comfortable indoor temperature, especially during the coldest winter months. When your furnace fails to kick on, it can lead to discomfort and even potential safety risks. Common signs that your furnace isn’t working include no warm air blowing, strange noises, or the system not responding to thermostat settings.

This article aims to help you identify potential causes and solutions for why your furnace isn’t kicking on, ensuring you can address the issue efficiently and safely.

How Your Furnace Works

When your furnace receives a signal from the thermostat sensors indicating that the indoor air temperature has dropped below the heat settings, it triggers the control board to start the heating cycle. The furnace ignites the burners, drawing in cold air from your home, and warms it through the heat exchanger. 

The heated air is then circulated through the ductwork and out of the vents into each room, keeping your home warm and comfortable. Whether it’s powered by natural gas, propane, electricity, or another fuel source, this process ensures you have warm air circulating through your space.

Furnace Not Kicking On When Temp Drops

When the temp drops, there could be several issues causing your furnace to stop turning on. Some common tasks like checking the HVAC system might be needed to help prevent further damage. If you’re uncertain about the situation, it’s always wise to call a professional who can assess the issue and recommend a solution based on their experience.

1. Your Thermostat Might Be the Problem

If your furnace isn’t turning on, the issue might lie with the thermostat. Sometimes, something is amiss with the thermostat, causing it to fail in telling the furnace to turn on or off properly. A faulty thermostat can easily prevent your furnace from doing its job, making it essential to check for any settings, miswiring, or malfunction that might be affecting its operation. A simple inspection of the thermostat can save you from unnecessary repairs.

Solution:

Start with the Thermostat

First, ensure that the thermostat is set up properly. Sometimes, the settings might be off. Check if the heat is actually switched on. If it’s set to cooling, the furnace won’t come on, even though it seems like it should. 

Also, ensure that the temperature is higher than the indoor temperature, or else the furnace won’t trigger. To test it, you can increase the temperature by a few degrees and see if that gets your furnace running.

Change the Batteries

It’s important to replace the batteries in your thermostat regularly. A dead battery could cause the thermostat to lose its settings, resulting in the furnace not responding. I recommend making it a habit to change the batteries at the start of every season both heating and cooling. This simple step can help you avoid unexpected interruptions.

Check the Day, Time, and Settings

It might sound trivial, but check that the correct day and time are set. Sometimes, incorrect a.m. and p.m. settings could cause the furnace to run at the wrong time, even if the thermostat appears to be fine. 

Verify that everything is displayed correctly. Also, check if the thermostat is running a programmed schedule. If it’s not, you can manually bypass the program by punching in the desired temperature and pressing “hold”. This will allow the furnace to run without waiting for the pre-set schedule.

Inspect the Thermostat for Dust or Damage

Another issue that’s easy to overlook is dust or debris behind your thermostat. Remove the thermostat from the wall and carefully check for any dust accumulations. These might affect the sensors, preventing the thermostat from properly detecting the temperature and turning on the furnace. 

Use a soft brush or paintbrush to clean the sensors gently. While you’re at it, inspect the wires for any visible damage. If you notice any, you can repair them using electrical tape.

2. Gas Supply Issues

If your furnace isn’t turning on, the problem might be with the gas supply. Sometimes, issues can occur if the gas to the furnace is interrupted. This could be due to a gas line work in the area or an unpaid bill affecting the supply. 

Also, the furnace gas valve might be off, or the furnace supply line could be disconnected. If your furnace runs on propane, check if the supply tank is not low. These simple checks can quickly solve the problem and get your furnace running again.

Solution:

Check the Gas Supply

Make sure the gas supply to your furnace is on. Without a proper gas supply, the furnace won’t be able to generate heat. Start by inspecting your other gas appliances like the stove to ensure they’re working properly. If they aren’t, it could be a sign that the gas supply is interrupted. You may need to call the utility company to confirm that your bills have been paid and that the gas supply hasn’t been interrupted.

Inspect the Furnace Supply Valve

The furnace supply valve controls the flow of gas to the furnace, and if it’s closed, your furnace won’t start. Locate the gas pipe leading to the furnace and find the valve. This valve often has a red or yellow lever or knob. Ensure that the lever or knob is in a parallel position with the gas pipe. If it’s perpendicular to the pipe, it means the valve is closed, and you should turn the handle to open it.

Examine the Gas Valve Inside the Furnace Cabinet

If everything looks good outside, check inside the furnace cabinet. There is another gas valve that controls the flow of gas to the furnace itself. Follow the black metal gas pipe into the furnace to locate this valve. It’s often part of a small box with a switch or dial that says on, off, and pilot. Double-check that the switch or dial is turned to the “on” position. If your furnace has a pilot light, this step is crucial for proper ignition.

Check the Fuel in the Propane Tank (for Propane Furnaces)

If you have a propane furnace, make sure the fuel in the propane tank is sufficient. Without enough fuel, the furnace won’t operate. If your propane furnace isn’t kicking on, checking the propane tank level is an easy way to troubleshoot the issue.

3. The Circuit Breaker May Have Tripped

Sometimes, your furnace may not kick on due to circuit breakers being tripped. This can happen because of a short-circuit, an electrical surge, or even a fault in the system. If a defective fuse causes the issue, the breaker might have been turned off and you may have forgotten to turn it back on. Always check if the breaker is in the correct position before calling for repairs.

Solution:

Locate the breaker panel

Your breaker panel is usually found in utility rooms, laundry rooms, or basements. It should be near the area where your furnace is installed.

Check the switch position

Look for the switch labeled “furnace,” “FAU,” or air handler. The switch might be in the off position, or if it’s tripped, it could be stuck in the center position.

Flip the switch 

If the switch is in the off position, simply flip it to the on position. If the breaker was tripped, the switch will not stay in the on position. In that case, flip the switch to the off position first and then back to the on position to reset it.

Check if the furnace turns on 

After resetting the breaker, check if your furnace starts up. If it does, then the issue is resolved. However, if the breaker trips again, it could indicate a deeper electrical problem.

Call a professional 

If the breaker continues to trip or the switch stays in the center position, it’s a good idea to call a professional to diagnose and fix the issue. This could be due to faulty wiring, an overloaded circuit, or a malfunctioning air handler or forced-air unit.

4. Defective Circuit Control Board

If your furnace isn’t turning on, it could be due to a defective circuit control board. This board receives the signal from the thermostat, and if it’s malfunctioning, the heating cycle won’t start. In some cases, an overheated fan motor or circuit board might also cause your furnace to smell like it’s burning, signaling a bigger issue. If this happens, turn off the system and call an HVAC service to prevent further damage.

Solution:

Check the Voltage Across the Control Board

Using a multimeter, check the voltage at the control board. It’s usually found in the compartment near the blower housing. Look for the “W” and “C” terminals. A reading of 24 volts means the board is working. If there’s no voltage, the control board might be defective. Be careful to avoid electric shock while testing.

Also, check the diagnostic indicator light on your furnace. If it’s on or blinking, the furnace is getting power.

Verify Furnace is Receiving Power

Make sure your furnace is receiving power. Use a multimeter to check the voltage at the thermostat’s “W” and “C” terminals. If there’s no voltage here, there might be a power supply issue or a faulty control board. Again, check the diagnostic indicator light for clues.

5. Your Flame Sensor is Dirty or Your Pilot Light Went Out

If your furnace isn’t starting, it could be due to a dirty furnace flame sensor or a pilot light issue. The flame sensor ensures the gas valve opens only when flames are detected. If it’s coated with residue, it can’t detect the flames, causing the gas valve to stay shut and preventing the furnace from starting its heating cycle. 

Alternatively, if the pilot light goes out, the furnace won’t ignite the burners. Older furnaces rely on a constantly burning flame, while newer ones use electronic ignitions. Regular maintenance can help avoid these common issues.

Solution:

Check the Flame Sensor

Start by turning off the power to your furnace using the switch or circuit breaker. Remove the access panel and locate the flame sensor, which is a thin metal rod near the burners. Unscrew it with a quarter-inch hex screw and disconnect the wires. Clean the sensor with steel wool or fine sandpaper. Avoid touching the ceramic housing. Reattach the sensor and restore power.

Inspect and Relight the Pilot Light

Next, check if the pilot light is visible through a small window on the furnace’s base. If it’s out, open the access panel and relight it following the instructions. Once the blue flame is back, your furnace should start working.

6. Ignitor Problems That Stop Your Furnace

Most furnaces today don’t use a pilot to stay lit. Instead, they rely on ignitors to fire up the burners. If your ignitor is faulty, your furnace won’t turn on. A hot surface ignitor is a small metal piece that gets red-hot to start the heating process. Some systems use intermittent pilots or direct spark ignitions, which produce sparks to ignite the burners. If these parts fail, there won’t be any light, and your furnace will stay cold.

Solution:

Check If the Ignitor Is Glowing

A hot surface ignitor should begin to glow red when the furnace starts a heating cycle. To inspect it, first turn off the switch on the side of the unit. Remove the access panel and locate the thin, flat, rectangular metal ignitor near the burners. If you turn the thermostat up to a higher temperature and the ignitor doesn’t glow red, then there’s an issue with it.

Listen for Clicking Sounds 

If you have an intermittent pilot or direct spark ignition system, you should listen for a clicking noise when the furnace turns on. These systems produce sparks to ignite the burners. If you hear continuous clicking but the burners won’t light, the ignitor may be defective and needs replacement.

Check the Gas Flow and Valve 

Your furnace relies on gas to function. If the gas pipe is blocked or the valve doesn’t open, the burners won’t ignite. When the inducer motor starts, you should hear a hiss as gas flows in. If there’s no hiss, there might be a problem with the valve or gas supply.

Ensure Proper Ignition 

If the ignition system isn’t working properly, the burners won’t light up. A spark ignition system generates sparks to ignite the gas. If you don’t see a light or sparks, it’s time to check the ignitor and its connections.

7. The Condensate Pan is Full

If your furnace isn’t turning on, there could be a variety of reasons why it’s not working. One common issue is that the condensate pan is full or blocked. This may seem like a minor problem, but it can stop your furnace from functioning properly.

Solution:

If your furnace isn’t kicking on, it might be due to a full condensate pan. This happens when the system collects moisture from the air during heating. Over time, the pan can fill with water, causing a blockage or overflow. Here’s how to resolve the issue:

1. Turn off the power: First, ensure that the power to the furnace is turned off. You can do this by switching off the furnace switch or flipping the circuit breaker to cut off the electricity.
2. Inspect the condensate line: The PVC pipe that exits the furnace and leads to the floor drain is crucial. Over time, debris can build up in the line or drain, leading to a clog. It’s important to inspect the end of the pipe for any blockages.
3. Remove any debris: If you find any debris in the pipe, use a brush or another tool to remove it. This will ensure the line is clear and the water can flow freely.
4. Check the cap: The cap at the top of the line can also cause issues if it’s clogged or obstructed. Be sure to remove the cap and check for any blockages.
5. Flush the line: Once you’ve removed the clog, flush the tube with a mixture of white vinegar and hot water. This solution helps clear out any remaining gunk or buildup. Pour the mixture through the pipe and allow it to drain.
6. Repeat the process: If necessary, you can repeat this process several times to make sure the line is completely cleared and the water can flow without obstruction.
7. Use a wet/dry vacuum: If the pan is particularly full, you may need to use a wet/dry vacuum to drain the excess water. This will prevent any further buildup and ensure the system works efficiently.

Preventing Furnace Issues in the Future

If you’re finding your furnace not kicking on, it’s likely that one of several furnace issues is at play. Rather than waiting for an inconvenient and costly repair, there are steps you can take to prevent these issues from happening in the future.

Schedule regular preventative maintenance 

It’s essential to get your furnace checked out each fall before winter. Scheduling annual inspections and tune-ups will ensure that everything is in top condition and ready to go for the colder months. This will help keep your furnace running smoothly, avoiding unnecessary breakdowns when you need it most.

Check the air filter 

One of the most common causes of furnace issues is a clogged air filter. If it gets too dirty, it can block airflow and cause your furnace to stop working. Check your filter every month, and make sure to replace it when it gets dirty — typically every 1-3 months. A clean air filter is crucial for efficient operation and energy savings.

Inspect vents and ductwork

Clear and clean vents and ductwork are vital for proper airflow. Any blockages or obstructions like dirt, dust, or debris can reduce airflow, causing your furnace to overheat or fail. It’s important to keep these areas clear to avoid wear and tear on the system.

Look for leaks and check the pilot light 

Take a moment to inspect your furnace for any possible leaks. Leaks can cause inefficiencies and lead to higher energy bills. Also, ensure the pilot light is on and functioning properly. If it goes out, your furnace won’t turn on.

Listen for unusual noises and smells 

If you hear strange noises or detect unusual smells coming from your furnace, it could signal an underlying issue. These issues can often be fixed early before they require professional attention, saving you from a much larger repair bill later.

When to Call a Professional

While some furnace problems can be resolved with basic troubleshooting, others require professional service. If you’ve checked the thermostat, power supply, and filter but the furnace still isn’t kicking on, it’s time to call a furnace repair technician. A professional can also clean dirty burners, inspect the combustion chamber, and check for carbon monoxide leaks, ensuring your system operates safely and efficiently.

Conclusion

If your furnace isn’t kicking on, start by checking the thermostat settings, inspecting the power supply, and examining the furnace filter. By following these easy steps, many common furnace problems can be fixed, but always remember that safety should come first. If you encounter signs of more serious issues, such as gas smells or strange noises, contact a furnace repair company immediately. Attempting to fix complex problems yourself can lead to further damage or safety risks.

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