Furnace Won't Turn On? Safe Checks for Phoenix Homeowners

When a furnace won't turn on at all, the cause can be as simple as a thermostat setting or as involved as a failed part inside the furnace. Several things are safe for you to check yourself: the thermostat, the power to the furnace, the air filter, the access panel and the gas valve. If those all look right and the furnace still won't start, the next step is a call to a qualified technician.

If you smell gas, skip the checklist and go straight to the gas safety section below. For a broader look at the different kinds of home heating, see our complete residential heating guide for Phoenix Valley homeowners.

Start With the Thermostat

The thermostat tells the furnace when to run, so it's the easiest place to begin. Make sure it's set to heat, with the temperature set higher than the current room temperature. After months of running the air conditioning, a thermostat left in cool mode is an easy thing to overlook.

Next, think about what powers the thermostat itself. If your thermostat runs on batteries, replace the batteries before you move on to the furnace. Fresh batteries rule out one of the simplest causes.

One more clue can help narrow things down. If the blower fan runs but no heat comes out, the issue might be specific to the burner or ignition. That points toward the parts inside the furnace, which we cover further down.

Is the Furnace Getting Power?

A gas furnace burns gas to make heat, but it still depends on electricity. Even gas furnaces need electricity to operate, and a tripped breaker in your home's electrical panel will cut power to the unit completely.

To check, go to your electrical panel and find the breaker labeled for the furnace or heat. It should be in the on position. If it has tripped to the middle, flip it to off and then back to on.

Next, find the furnace power switch. It's usually on the side of the unit or on a nearby wall, and it often looks like a light switch. Make sure it's in the on position too.

Leave the wiring and the electrical parts inside the furnace alone. Internal electrical components should be inspected by a qualified technician rather than replaced as a DIY repair.

Key Takeaway: A gas furnace still needs electricity, so a tripped breaker or a furnace power switch in the off position can keep it from starting.

Check the Filter and the Access Panel

A clogged filter restricts airflow through the furnace, and that can cause the system to overheat and shut down automatically. A fresh filter keeps air moving the way the furnace expects.

Filter care is one of the maintenance jobs you can handle yourself. The ENERGY STAR maintenance checklist suggests that you inspect, clean or change the air filter in your furnace once a month. The checklist also notes that your contractor can show you how.

The furnace's access panel matters too. If you took off the access panel to check the filter or look inside the furnace, put it back in place before you try the heat again. Carrier's gas furnace repair tips explain why: "As a built-in safety precaution, the furnace will not run otherwise."

Key Takeaway: A clogged filter can make a furnace overheat and shut itself off. An access panel that isn't back in place will keep the furnace from running.

Could It Be the Gas Supply or the Condensate Drain?

Do these gas checks only if you don't smell gas. If you have a gas furnace, make sure the gas valve leading to the furnace is open. A closed gas valve or a utility interruption will keep the furnace from producing heat. It also helps to check whether your other gas appliances are working. If they aren't working either, the problem may be the gas supply rather than the furnace. Never attempt to repair a gas valve yourself.

High-efficiency furnaces have one more thing to keep in mind: water. These furnaces produce condensate as a byproduct, and it leaves the furnace through a drain line. If that drain line becomes blocked, a float switch can prevent the furnace from operating to help prevent water damage.

Key Takeaway: A closed gas valve or a gas supply outage can stop a furnace from making heat. On a high-efficiency furnace, a blocked condensate drain can keep it from running.

What to Do If You Smell Gas

If you smell gas, see smoke or flames, or suspect an electrical hazard, stop troubleshooting. None of the checks in this post should come before your safety.

For a gas smell, the winter safety tips from the U.S. Consumer Product Safety Commission (CPSC) say: "If you smell gas or hear gas leaking, leave your home immediately and contact local gas authorities from outside the home. Do not operate any electronics, such as lights or phones, before leaving."

A carbon monoxide (CO) alarm is a separate warning with its own steps. For a CO alarm, the CPSC says: "Never ignore an alarm when it sounds. Get outside immediately and call 911."

When the Problem Is Inside the Furnace

If the basic checks all look right and the furnace still won't start, call for service. A technician can then look at the parts that light the burner and control the furnace. A few of the parts involved are:

• Igniter: Whether your system uses a pilot light or a modern electronic igniter, if it can't light the burner, the furnace will not cycle. Electronic igniters can crack or fail, and then the gas will not ignite.
• Flame sensor: This part detects whether a flame is present. When it's coated in carbon or dust, it shuts the gas valve as a precaution.
• Other components: A furnace may have a failed component, such as the motor or the circuit board.

Problems with the control board, heat exchanger or blower motor are best left to a certified technician, and so are recurring failures. We're a family-owned team in Chandler, and our heating repair service covers gas furnaces across Phoenix and the East Valley.

Regular upkeep also plays a part. Maintaining your equipment helps prevent future problems, and ENERGY STAR recommends having a contractor do annual pre-season check-ups. According to its checklist, a typical check-up should include checking the gas connections, gas pressure, burner combustion and heat exchanger. Our post on heating system maintenance for Chandler winters walks through the steps of winter upkeep.

Key Takeaway: Problems with the control board, heat exchanger or blower motor are best left to a certified technician. An annual pre-season check-up by a contractor helps prevent future problems.

Frequently Asked Questions

Why won't my furnace turn on at all?

A furnace that won't start can have a simple cause. It could be a thermostat that isn't set to heat, dead thermostat batteries, a tripped breaker or a furnace power switch that's off. It could also be a clogged filter, an access panel that isn't back in place, or a closed gas valve. On a high-efficiency furnace, a blocked condensate drain can also keep it from running. A failed part, such as the igniter, can be the cause too. If the simple checks don't solve it, call for service.

Does a gas furnace need electricity?

Yes. Even gas furnaces need electricity to operate, and a tripped breaker in your home's electrical panel will cut power to the unit completely. Find the breaker labeled for the furnace or heat and make sure it's on. If it has tripped to the middle, flip it to off and then back to on. The furnace's own power switch, which often looks like a light switch, also needs to be in the on position.

Can a dirty filter stop a furnace from working?

It can. A clogged filter restricts airflow through the furnace, and that can cause the system to overheat and shut down automatically. Filter care is a job you can do yourself. ENERGY STAR suggests that you inspect, clean or change the air filter in your furnace once a month. Your contractor can show you how.

Why won't my furnace run after I changed the filter?

Check the access panel first. If you removed the access panel to check the filter or look inside the furnace, it needs to go back in place. The furnace won't run without it. That's a built-in safety precaution. If the panel is back on and the furnace still won't start, work through the other checks in this post, and check the gas valve only if you don't smell gas.

What should I do if I smell gas near my furnace?

Stop troubleshooting. The CPSC says: "If you smell gas or hear gas leaking, leave your home immediately and contact local gas authorities from outside the home. Do not operate any electronics, such as lights or phones, before leaving." A carbon monoxide alarm is a separate warning. For a CO alarm, the CPSC says: "Never ignore an alarm when it sounds. Get outside immediately and call 911."

Can I repair furnace parts myself?

It's best to leave repairs to a professional. Never attempt to repair a gas valve yourself. Internal electrical components should be inspected by a qualified technician rather than replaced as a DIY repair. Problems with the control board, heat exchanger or blower motor, and recurring failures, are best left to a certified technician.

How can I help prevent furnace problems before winter?

Have a contractor do a check-up before the heating season. ENERGY STAR recommends annual pre-season check-ups by a contractor, and it says maintaining your equipment helps prevent future problems. According to its checklist, a typical check-up should include checking the gas connections, gas pressure, burner combustion and heat exchanger.

When the Checks Don't Solve It

When your own checks all look right and the furnace still won't start, it's time to call for service. If you smell gas at any point, stop and follow the CPSC's steps above before anything else.

Still no heat after going through the list? Call us at (480) 689-5167 or send a note through our contact page, and we'll help you figure out what's going on with your furnace.