Furnace Not Turning On: What to Check on the First Cold Night

Natural gas furnaces heat most detached homes around Kennesaw and Marietta, so this rule comes first. A faint dusty smell for the first few minutes of the season is normal. That is dust burning off parts that sat all summer. A smell that lingers or smells electrical means turn the system off and call.

A North Georgia furnace may sit idle from March to November. Then the first real cold front arrives, half of Cobb County turns the heat up the same evening, and parts that weakened over the summer get their first test. Igniters crack, flame sensors are coated, and condensate traps on high-efficiency furnaces clog.

Winter lows here average just above freezing, with occasional hard freezes. In December 2022, an arctic blast pushed metro Atlanta into the single digits. On nights like that a house cools quickly and pipes on outside walls are at risk, so work through the checks below calmly but promptly.

Set the mode to Heat and raise the set point at least three degrees above room temperature. Then wait. Many furnaces take a minute or more before anything visible happens.

• Blank screen: replace the batteries.
• Shows heat on, but nothing happens: move on to the power checks.
• Has an Emergency Heat or Aux setting: you likely have a heat pump. Its cold-weather problems are covered on our heat pump repair page.

In two-system homes, confirm which thermostat controls the unit that is not working. Upstairs systems are often in the attic, with the downstairs furnace in a basement, garage or closet.

Every furnace has a service switch nearby that looks like an ordinary light switch, often on the wall beside the unit or near the attic stairs. It gets flipped off by accident more than you would think. Make sure it is on.

Then check the breaker box for a breaker labeled furnace or air handler sitting in the middle or off position. Push it fully off, then on, one time only. If it trips again, leave it off and call. A furnace that trips its breaker has an electrical fault, often in the blower motor or control board.

A badly clogged filter starves the furnace of air. The heat exchanger overheats, a high-limit switch shuts the burners off, and after repeated trips some furnaces lock out. Replace the filter if you cannot see light through it. One that went all cooling season without a change is a common culprit.

Furnaces also have a door safety switch that cuts power when the blower door is not seated. If your filter slides in behind the lower door, make sure that door is pushed fully into place after a change. Do not remove the panels or tape the switch down.

Most modern furnaces have a control board with an LED that blinks a code, and on many models you can see it through a small window in the lower door. Count the blinks and look for the code chart on a label on the door or cabinet. Photograph the label and the model number on the rating plate.

You do not need to decode it yourself. Passing the code and model number to the technician helps them arrive with the likely part.

Stand near the furnace while the thermostat calls for heat. A gas furnace starts in a set order, and where it stops is a strong clue:

1. The small draft inducer fan starts.
2. A pressure switch clicks to confirm the vent is clear.
3. The igniter glows orange or ticks.
4. The burners light.
5. The main blower starts pushing warm air.

Nothing happens at all: power, thermostat, door switch or control board.

Inducer runs, nothing lights: often a pressure switch problem. On a high-efficiency furnace with white PVC vent pipes, a clogged condensate trap or drain is a common cause. A cracked igniter is the other usual suspect.

Burners light, then shut off within seconds: the classic dirty flame sensor. The furnace retries a few times, then locks out.

Do not keep cycling the power to force more attempts. A technician can clean or replace the part and check the heat exchanger and venting while there. Our furnace repair page explains those repairs.

Where the furnace sits changes what you can safely check. In two-story Kennesaw subdivisions, the upstairs furnace is often in the attic, reached by pull-down stairs. Without a floored path and good light, skip the attic and tell the dispatcher where the unit is.

Older homes in Marietta's historic districts and surrounding 1970s to 1990s neighborhoods often have furnaces in tight crawl spaces or basements, sometimes past 20 years old. A gas furnace typically lasts 15 to 20 years or more. Past that point, a cracked heat exchanger or failed control board is a reason to discuss replacement as well as repair.

The first hard cold snap of the season brings the same handful of no-heat calls across Kennesaw and Marietta. Knowing them helps you describe the problem and judge the fix.

Know which ignition your furnace has

Most furnaces installed since the 1990s light with a hot surface igniter or a spark, and have no pilot flame to relight. Some older furnaces in Marietta's historic homes still use a standing pilot with a thermocouple. If your furnace has a pilot, the relighting steps are printed on the label inside the door; if it does not, there is nothing to relight, and repeated resets only lock the board out.

Confirm the gas is actually on

If other gas appliances, such as the stove or water heater, also fail to light, the problem may be the gas supply, not the furnace. Call the gas utility. Do not try to open or adjust gas valves you are unsure about.

The repairs that come up most

• A dirty flame sensor. The burners light, then shut off within seconds. A routine fix that a fall tune-up usually prevents.
• A cracked igniter. Hot surface igniters are brittle and often fail on the first startup after a long summer off.
• A pressure switch fault on a high-efficiency furnace. A clogged condensate trap, or a PVC drain that freezes during a hard cold snap like December 2022, stops the furnace before ignition.

Read the code, then let it be tested

Most boards blink a fault code, and the decal inside the door translates it. Treat the code as a starting point: the technician still tests the part before replacing it.

If the thermostat, switch, breaker, filter and door all check out and the furnace still will not run, call Hooten Air Pros at (321) 478-0693. We answer 24/7, including nights and holidays, and dispatch the nearest available technician for emergency heating repair. You get an arrival window on the call and a price before work starts. Every HVAC contractor we send is licensed and insured in Georgia.

Until help arrives, close doors to unused rooms and open cabinet doors under sinks on outside walls. Use space heaters only as the manufacturer directs, and never heat the house with a gas oven or grill.