Furnace Not Heating? A Step-by-Step Checklist Before You Call

When the house is cold and the furnace is silent, it helps to check the simple things in the right order. This checklist starts with safety, moves through the checks a homeowner can do without tools, and tells you where to stop and bring in a professional. LocalRepairLine is a referral service, not a contractor, and this guide is general information, not a substitute for a technician.

Step 1: check the thermostat

• Set it to Heat and raise the target at least 5 degrees above the room temperature.
• Make sure the fan is set to Auto, not Off.
• Replace the batteries if the display is blank or dim.
• A programmable or smart thermostat can lose its schedule after a power flicker, so confirm the time and the active schedule.

Step 2: check power to the furnace

• Look for the furnace switch on or near the unit. It looks like a regular light switch and is sometimes switched off by accident during storage or cleaning.
• Check the breaker in the panel. If it has tripped, reset it once. If it trips again, stop and call a professional, because that points to an electrical fault.
• Make sure the blower compartment door is fully closed. Many furnaces have a safety switch that cuts power when the panel is loose.

Step 3: check the air filter

A clogged filter restricts airflow and can make the furnace overheat and shut itself off. Slide the filter out and hold it up to a light. If you can't see light through it, replace it with the same size printed on the filter frame. Most homes do well with a change every one to three months, and sooner with pets or dusty rooms.

Step 4: check vents, registers and the outdoor vent

• Open the supply registers and move furniture, rugs or curtains away from them.
• For a high-efficiency furnace with white plastic pipes, look outside at the intake and exhaust terminations. Snow, ice, leaves or a nest can block them and make the furnace shut down.
• Clear only what you can reach safely from the ground, and keep your hands away from anything hot.

Step 5: check the gas supply valve

Find the gas line to the furnace. The shutoff handle should sit in line with the pipe when the gas is on. If it sits crosswise, the gas is off. Do not force a stuck handle. If other gas appliances such as the stove or water heater are also not working, the problem may be the supply, so call your gas utility.

Step 6: look at the control board light

Many furnaces have a small window on the control board showing a blinking light. The number or pattern of blinks is a code. A label inside the furnace door usually explains what the code means, for example a pressure switch, a limit switch or a failed ignition. Write the code down, because it helps a technician understand the fault before they arrive.

Step 7: check the condensate drain (high-efficiency furnaces)

High-efficiency furnaces produce water that drains through a small line. If the line is clogged or the trap is full, a safety switch can stop the furnace. If you see water around the unit or a full condensate trap, stop there and call a professional rather than disassembling parts.

Common symptoms and what they often mean

• Furnace clicks but never lights: often a worn igniter, a gas supply issue or a dirty flame sensor.
• Starts, then shuts off after a few minutes: commonly a dirty flame sensor, a clogged filter or overheating.
• Blower runs but the air is cold: the burners may not be lighting, or the gas is off.
• Loud banging, rattling or squealing: a loose panel, a failing blower motor or a delayed ignition that needs prompt attention.
• Uneven heat between rooms: usually airflow, closed registers or duct issues.

When to stop and call a professional

• You smell gas, hear a loud boom when the burners start, or see yellow, flickering flames or soot.
• The breaker trips again after one reset.
• A part needs to be opened, replaced or wired. Gas and electrical repairs aren't do-it-yourself work.
• The furnace is old and keeps failing, which is a good time to ask a technician whether repair or replacement makes more sense.

What to tell the technician

• The make and approximate age of the furnace, if you can see the label.
• The control board light pattern.
• What you already checked: thermostat, filter, breaker, gas valve.
• When the problem started and whether it happens every time.

Furnace and heating repair by city

Wilmington, DE · Dover, DE · Philadelphia, PA · Pittsburgh, PA · Columbus, OH · Cleveland, OH · Cincinnati, OH · Denver, CO · Toledo, OH · Fort Wayne, IN

More guides: Furnace making noise Furnace blowing cold air Frozen pipes: what to do Water damage and home insurance