Why Won’t My Furnace Turn On?

Without electricity, your furnace won’t kick on. Even a gas furnace needs power to run the control board and blower motor. The first thing you should check is your circuit breaker.

It is possible that a power surge or overload caused the breaker to trip, cutting the power supply to the unit.

Troubleshooting Tip: Check your Circuit Breaker

1. Locate your home’s electrical panel.
2. Look for the switches labeled “Furnace” or “HVAC.”
3. If the switch is in the “OFF” or neutral middle position, it has tripped.
4. Firmly flip it to “OFF” and then back to “ON” to reset it.
5. Ensure the furnace switch (often a light switch located on the side of the unit) is also flipped UP to the “ON” position.

If you have a fuse box instead of breakers, check for a blown fuse and replace it if necessary. If the breaker trips again immediately, do not force it—this indicates a dangerous electrical short that requires a professional.

It can be frustrating to come home to a cold house. You may be wondering, “Why won’t my heater turn on?” A clogged or dirty air filter is one of the most common furnace problems that homeowners overlook.

When airflow is cut off, the heat exchanger can overheat. Modern furnaces have safety limit switches that detect this overheating and shut the system down to prevent fire or damage.

Troubleshooting Tip: Replace Air Filter

1. Turn your system OFF at the thermostat.
2. Locate the filter rack (usually between the return duct and the furnace).
3. Remove the filter and hold it up to a light source.
4. If light cannot pass through the fibers, the filter is clogged and needs to be replaced.
5. Install a new filter, ensuring the arrow on the frame points towards the blower motor.

If you are unsure which size you need, check the owner’s manual. A clean filter helps your system operate efficiently and protects your heat exchanger.

Energy.gov reports that replacing a dirty filter with a clean one can lower your energy consumption by up to 15% and prevent costly heat exchanger damage.

Sometimes when your furnace won’t start, the problem isn’t the furnace at all—it’s the brain controlling it. Your thermostat is the mainline of communication for your HVAC system. If the thermostat settings are incorrect, or if the unit has lost power, it cannot tell the furnace to fire.

Troubleshooting Tip: Reset/Double Check Thermostat

1. Check Settings: Ensure the switch is set to “HEAT” and the fan is set to “AUTO” (not “ON”).
2. Check Temperature: Make sure the set point is at least 5 degrees higher than the current temperature inside the room.
3. Replace Batteries: If the display is blank or fading, replace the AA or AAA batteries. A faulty thermostat often just needs fresh power.
4. Inspect Wires: Pull the thermostat cover off and check for loose wires. Tighten them with a small screwdriver if needed.

If your thermostat is programmable, check that it isn’t following an old schedule that keeps the heat off during the day.

If your furnace won’t turn on but you hear a hum, or if the burners light but then quickly shut off, you may have a dirty or failing furnace fan (blower motor).

The blower motor’s job is to pull air through the return vents, push it over the hot heat exchanger, and distribute it through your home. If the motor is clogged with dust, it can seize up.

Troubleshooting Tip: Clean out your Blower Motor

1. Turn off power to the furnace at the breaker.
2. Remove the front access panel.
3. Use a shop-vac with a brush attachment to gently clean dust off the blower fan blades and the motor housing.
4. Spin the fan wheel by hand. It should spin freely. If it feels stiff or grinds, the motor bearings may be shot, requiring a replacement.

If your furnace won’t ignite but the fan is running, your ignition system may be to blame. In older furnaces, this is often a pilot light that has blown out. In modern furnaces, it is usually a dirty flame sensor or a cracked igniter. If the system does not detect a flame within a few seconds of opening the gas valve, it shuts down immediately to prevent a dangerous gas buildup.

Troubleshooting Tip: Check the Pilot Light

• For Pilot Lights: Look through the sight glass. If the flame is out, follow the instructions on the furnace label to relight it. You may need to hold the reset button for 60 seconds.
• For Electronic Ignition: Listen for a “click” followed by a “whoosh.” If it clicks but doesn’t light, your igniter may be cracked. If it lights but shuts off after 3 seconds, your flame sensor is likely dirty.
• Cleaning the Sensor: Turn off power, remove the sensor with a screwdriver, and gently clean the metal rod with a dollar bill or fine steel wool (do not use sandpaper).

Note: “According to the EPA, ensuring your heating system is vented and inspected annually is the single most effective way to prevent Carbon Monoxide (CO) poisoning in your home.” If you aren’t comfortable checking these components, call a pro.

Now that you have completed these troubleshooting steps, if your furnace still won’t start, it is time to put it in the hands of a professional HVAC technician.

We employ NATE-certified technicians to ensure your repair is done right the first time, maintaining your manufacturer’s warranty.

At Alpha Mechanical, we have been serving the Sacramento area for over 20 years. Whether you need a simple heating repair or a full system diagnosis, our team is here to help you get warm again.

• Need a Pro? Contact us today for fast service.
• Service Area: Check if we cover your neighborhood on our Service Area page.
• Save Money: Don’t forget to check our current specials and ask about SMUD rebates for heat pumps.

If you are considering an upgrade, SMUD currently offers rebates for qualifying heat pump heating and cooling systems in the Sacramento area.