Furnace Won't Turn On? What to Check Before You Call

If your furnace won't turn on, start with the simple things: the thermostat setting and batteries, the furnace power switch and breaker, the filter, and the furnace door panel. Those four checks solve a surprising share of "no heat" calls, and none of them require tools or opening anything up. If the furnace still won't run, or you smell gas or burning, stop there and request a repair.

If you do need us, our $49 visit is free if we fix it, and repair requests received by 10 PM are eligible for same-day arrival.

Why do so many furnaces refuse to start on the first cold night?

Picture a typical North Texas fall. Your AC has carried the house since spring, and the furnace has been sitting in the attic, garage or hall closet doing nothing. Then a cold front drops the temperature overnight, and homeowners across Lewisville, Flower Mound and Denton switch their thermostats to HEAT on the same evening.

That first start is the furnace's real test. Dust has settled on the burners and the flame sensor. The filter has been loading up all summer, because the same blower moves air for cooling. Thermostat batteries have quietly run down. Any one of those can keep the heat from coming on, and it's why service schedules fill up fast during the first cold snap.

The good news: several of the most common causes are things you can check yourself, safely, in a few minutes.

Is the thermostat actually calling for heat?

Start at the wall, because it's the easiest fix and the most common one.

• Mode: make sure the thermostat is set to HEAT, not COOL, OFF or AUTO left in a cooling setting.
• Temperature: set it a few degrees above the current room temperature so it has a reason to call for heat.
• Screen: if the display is blank or dim, replace the batteries (many thermostats use AA or AAA batteries even when they're also wired).
• Schedule: a smart or programmable thermostat may be following a schedule or "away" setting that's holding the temperature low. Temporarily override it.
• Fan setting: if the fan is set to ON, air will blow constantly even when the furnace isn't heating, which can feel like cold air from the vents. We explain the difference in our post on thermostat fan Auto vs. On.

Give the system a few minutes after a change. Many furnaces have a built-in delay before the burners light and another before the blower starts.

Does the furnace have power?

A furnace needs electricity even if it burns gas. Two places to look:

1. The furnace switch. Most furnaces have a switch on or near the unit that looks just like a regular light switch. It's easy to bump off during storage or attic work. Make sure it's on.
2. The breaker. Check your electrical panel for a tripped breaker labeled furnace, heater or air handler. If it's tripped, reset it once.

If the breaker trips again, leave it off and request a repair. A breaker that keeps tripping is telling you something is wrong, and resetting it over and over isn't safe.

Could a dirty filter keep the heat from working?

Yes. A badly clogged filter starves the furnace of airflow. The furnace can overheat, and a safety switch shuts the burners down to protect the equipment. You may notice the furnace starting, running briefly and then stopping, or the house never warming up.

Pull the filter and hold it up to a light. If you can't see light through it, replace it with the same size (it's printed on the frame). If you already replaced it this fall, move on to the next check.

Is the furnace door panel closed tightly?

Most furnaces have a safety switch behind the front panel that keeps the system from running when the door is off or loose. If someone recently changed the filter or looked inside, the panel may not be seated all the way. Make sure it's pushed in and latched firmly.

Don't remove panels beyond the filter access, and don't try to bypass that switch. It's there to protect you.

What does the blinking light on the furnace mean?

Many furnaces have a small status light, often visible through a little window in the lower door. It blinks in a pattern, like three short flashes and a pause, that points to a specific problem. The key to reading it is usually printed on a label inside the door or on the side of the unit.

You don't need to diagnose it yourself. Just write down the pattern, or take a short video of it on your phone, before you cut power. That one detail can help a technician get to the cause faster.

What if the furnace clicks, starts, then shuts off?

A furnace starts in a set order: the draft motor comes on, the igniter heats up, the burners light, and then the blower sends warm air through the house. If yours makes it partway and quits, that's a sign something in that sequence isn't checking out, and the furnace shut itself down on purpose.

A common culprit after a long idle summer is a dirty flame sensor, which can make the furnace think the burners didn't light. Other causes include ignition parts, pressure switches or venting problems. These are jobs for a trained technician, not a DIY project, because they involve the gas and safety controls.

High-efficiency furnaces have one more thing to know about: they make water. If the condensate drain line or pump is clogged or full, a safety switch can keep the furnace from running. If you see water around the furnace, or a small pump with a full reservoir, mention it when you request service.

What if you have a heat pump instead of a furnace?

Plenty of North Texas homes heat with a heat pump, sometimes with electric heat strips or a furnace as backup. A few things that look like problems are normal:

• Defrost: on cold, damp days the outdoor unit may stop, make a whooshing sound and give off steam for a few minutes. That's the defrost cycle, and it's normal.
• Cooler-feeling air: heat pump air often feels less hot than furnace air even when it's working properly. We break that down in why heat pump air feels cooler than furnace air.

What's not normal: an outdoor unit that won't run at all, one that's buried in ice for hours, or a thermostat that keeps showing auxiliary or emergency heat. Those are good reasons to request a visit.

When should you stop troubleshooting and call?

Stop and get help if any of these happen:

• You smell gas. Leave the house first, then call your gas utility or 911 from outside. Don't flip switches or try to relight anything.
• A carbon monoxide alarm sounds. Get everyone outside, then call 911.
• You smell burning plastic or electrical burning, or see scorch marks.
• The breaker trips again after one reset.
• The furnace starts and stops repeatedly, or the status light shows an error after you've done the basic checks.

A light dusty or "burning dust" smell for a few minutes the first time the heat runs each season is common, and it should fade quickly. If it doesn't fade, or it smells electrical, turn the system off and request a repair.

Getting heat back on in Lewisville and the rest of DFW

Prime AC and Heat is based in Lewisville, and our technicians handle no-heat calls across the area, including Flower Mound, Highland Village, Carrollton, Coppell, Plano, Frisco and Denton. Learn more about our heating repair service in Lewisville or our heating repair options.

When you request service, it helps to tell us what you've already checked, whether the furnace makes any sound, and what the status light is doing. Our $49 visit is free if we fix it, and repair requests received by 10 PM are eligible for same-day arrival.

If your furnace is running fine but hasn't been looked at in a while, a $59 weekday tune-up is the easier way to catch problems before the first cold night. Here's whether a furnace tune-up is worth it in North Texas.

Request Service Online and we'll confirm your arrival window.

Frequently asked questions

Why won't my furnace turn on when the thermostat is set to heat?

The most common causes are dead thermostat batteries, a thermostat schedule holding the temperature low, a furnace switch or breaker that's off, a clogged filter, or a loose furnace door panel. If you've checked all of those and the furnace still won't start, request a repair.

Is it safe to reset my furnace?

Resetting a tripped breaker once is fine. If it trips again, leave it off and call for service. Don't repeatedly cycle power, remove internal panels, or try to relight a gas furnace yourself.

Why does my furnace click but not light?

A furnace that clicks or starts its draft motor but never lights, or lights and then shuts off, has usually stopped itself because a safety check failed. A dirty flame sensor is a common cause after a long idle summer, but ignition parts, pressure switches and venting can also be involved, so it's best left to a technician.

Can a dirty filter stop a furnace from working?

Yes. A clogged filter restricts airflow, which can cause the furnace to overheat and shut off on a safety switch. Replacing the filter with the correct size is one of the first things to check.

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

Leave the house right away, then call your gas utility or 911 from outside. Don't use switches, phones or anything that could spark inside the home, and don't try to fix it yourself.

How much does it cost to have Prime look at a furnace that won't turn on?

Prime's visit is $49, and it's free if we fix it. Repair requests received by 10 PM are eligible for same-day arrival.