Why Won't My Furnace Turn On? 5 Things to Check First

The ultimate winter retreat is a toasty home that separates you from the chilly outdoors. This would not be possible without the help of a powerful, efficient furnace managing the indoor temperatures so everyone can stay comfortable. That's why the moment a furnace fails to turn on, that warm and cozy feeling you were once accustomed to quickly vanishes. Fortunately, there are various ways to identify the cause of a furnace that won't turn on. Below are some of the most common reasons why your system isn't running.

When it comes to furnace issues, we always recommend scheduling a furnace repair service visit with an expert HVAC technician who can accurately identify the source of the problem and recommend the appropriate remedy.

Thermostat Settings & Battery

Checking the thermostat is a great first step to identify if there is a disconnect in settings or power.

If the thermostat is set to COOL or is completely turned off, this will cause the furnace to stay off. Turn on the thermostat and/or set it to HEAT with the temperature set a few degrees higher than the current room temperature. The furnace should start running as expected.

Additionally, depending on your thermostat model, you may have dead batteries. Without power to the thermostat, it cannot properly communicate with the furnace to run. Digital thermostats usually have a "low battery" message pop up when the batteries need changing. If you see that message, make sure to change the batteries.

Furnace Circuit Breaker

Furnaces use electricity to run the fan and therefore need a circuit breaker. If the breaker "trips" (cuts flow of electricity), the furnace won't turn on.

The breaker box (or electrical panel) is a gray metallic box or door that's usually found in a utility area. These boxes contain rows of switches usually labeled to identify different areas of the home they control. If the circuit breaker tripped for the furnace, then the switch will have moved to the middle position. If it did trip, you can turn it to the "off" position and then to the "on" position. It's important to note that the switch must be firmly pushed all the way to OFF until it clicks to reset the internal spring mechanism before being pushed to ON.

Breakers trip for a variety of reasons, such as during a sudden electrical surge or system overload. If the furnace breaker continues to trip, DON'T turn it back on. You likely have a more serious electrical issue and need an HVAC technician's help.

Furnace Power Switch

Near your furnace, there is a power switch. It looks just like a light switch that you would find on your wall. Typically, it's located on the side of the furnace cabinet or on a nearby wall/joist. It's possible that someone may have flipped it off by mistake, shutting down your heating system in the process.

Check to make sure the switch is flipped to the ON position.

Condensate Drain Line

A high-efficiency furnace creates condensate that needs to be drained out of your home via a drain line. But if the drain line gets clogged with gunk (mold, slime, etc.), then that condensate backs up into the drain pan or furnace. To avoid water damage, many furnaces have a condensate float switch or overflow safety switch that shuts off the furnace until the condensate drain problem is fixed.

When this issue occurs, it's important to unclog and clean the condensate drain line. Contact a licensed HVAC technician who will be able to assist with this process.

Clogged Air Filter

When was the last time you checked or changed your home's air filters? If it's been more than a few months, then clogged or dirty air filters may prevent your furnace from turning on. Air filters are like lungs; they trap pollutants so clean, conditioned air can enter your home. When dirt and debris consume the filter, it leaves little room for air to pass through. Since airflow is restricted, it overworks your furnace and overheats crucial components in the system. The high limit switch detects this overheating and automatically shuts down the system as a safety protocol.

Broken Down Furnaces Require Expert Repair with George Brazil

For about 70 years, George Brazil has served homeowners across Phoenix, AZ, and the surrounding areas with heating and air conditioning services. Whether your home runs on a furnace or a heat pump, our licensed technicians are highly trained to conduct expert repairs, heat pump repair, replacements, or emergency HVAC services so your home stays warm and cozy all winter long. When your furnace won't turn on, call the pros with decades of experience and schedule your service visit with George Brazil today!