Furnace Won't Turn On? A Chicago Homeowner's Troubleshooting Checklist

The house is getting cold, the thermostat says heat, and nothing is happening in the basement. No hum, no blower, no warm air. In a Chicago winter that is stressful, but a furnace that will not turn on is often caused by something simple and safe to fix yourself.

This checklist goes from the easiest checks to the ones that need a technician. Work through it in order. If the basic steps do not bring the heat back, you will at least have useful information for whoever comes out for furnace repair.

Safety first: if you smell gas at any point, stop. Do not touch light switches or the thermostat. Leave the building and call Peoples Gas at 866-556-6002 from outside. If a carbon monoxide alarm is sounding, get everyone out and call 911.

Step 1: Check the Thermostat

A surprising number of no-heat calls start at the thermostat. Check that:

• The system mode is set to Heat, not Cool, Off or Auto with a cooling setpoint.
• The setpoint is at least a few degrees higher than the current room temperature.
• The display is on. A blank screen usually means dead batteries or lost power.
• Any schedule or vacation hold is not overriding your setting.

If the thermostat uses batteries, replace them even if the display looks fine. Weak batteries can keep the screen lit but fail to switch the furnace on. Smart thermostats that draw power from the furnace can go dark if the furnace loses power, so a blank smart thermostat may point you to the next step.

Step 2: Check the Furnace Power Switch

Nearly every furnace has a service switch that looks exactly like a light switch. It is usually mounted on a wall, ceiling joist or the furnace cabinet within a few feet of the unit. In basements, especially in older Chicago bungalows and two-flats, it is easy for someone to flip it off thinking it controls a light.

Make sure the switch is on. Some homes have a red switch plate at the top of the basement stairs as well. Check that one too.

Step 3: Check the Breaker

Go to your electrical panel and look for the breaker labeled furnace, heat or HVAC. A tripped breaker sits in the middle position or in the off position. To reset it, push it firmly to off and then back to on.

If the breaker trips again right away or after a short time, do not keep resetting it. Repeated tripping points to an electrical problem such as a shorted motor or failing component, and it needs a technician.

Step 4: Make Sure the Blower Door Is Closed

Most furnaces have a safety switch behind the lower access panel, the blower compartment door. If that panel is off or not seated correctly, the switch stays open and the furnace will not run. This often happens after someone changes the filter or looks inside. Reseat the panel firmly so the switch is pressed.

Step 5: Replace the Air Filter

A clogged filter is one of the most common reasons a furnace shuts itself down. When airflow is restricted, the heat exchanger overheats and the high-limit switch cuts the burners to protect the equipment. Depending on the furnace, it may then refuse to restart until it cools down, or it may lock out after several attempts.

Pull the filter and hold it up to a light. If you cannot see light through it, replace it. Standard 1-inch filters in a Chicago heating season often need changing every one to three months, more often with pets or renovation dust. Our guide to HVAC tune-ups and maintenance covers filter schedules in more detail.

Step 6: Check the Gas Supply

A gas furnace cannot heat without gas. Check:

• The furnace gas valve. There is a manual shutoff valve on the gas line near the furnace. The handle should be parallel to the pipe when open.
• Other gas appliances. If your gas stove or water heater is not working either, the problem may be the gas supply to the house, and you should contact Peoples Gas.

Do not attempt to adjust anything on the gas valve inside the furnace, and never try to light a modern furnace manually. If you smell gas, leave and call from outside.

Step 7: Look for a Blinking Status Light

Most furnaces built in the last few decades have a control board with an LED status light, visible through a small window in the blower door. A steady glow or slow blink often means normal standby. A pattern of quick flashes is an error code.

Look for a sticker on the inside of the furnace door that explains the codes. Common codes point to:

• Pressure switch open, often a venting or condensate drainage issue.
• High-limit switch open, usually overheating from poor airflow.
• Flame not detected, often a dirty flame sensor or ignition failure.
• Ignition lockout after several failed attempts.

Write down the code before you reset the power. Resetting clears it, and the code is one of the most useful pieces of information you can give a technician.

Step 8: Check the Condensate Drain on a High-Efficiency Furnace

If your furnace vents through white PVC pipes rather than a metal flue, it is a high-efficiency condensing furnace. These furnaces produce water as part of normal operation, and that water has to drain away through a trap and tubing, sometimes with a small condensate pump.

If the trap clogs or the pump fails, water backs up and a safety switch stops the furnace. Look for:

• Water around the base of the furnace.
• A condensate pump with a full reservoir that is not running.
• Kinked or blocked drain tubing.

In Chicago, condensate lines that run near exterior walls or into unheated spaces can also freeze during a cold snap. Our guide on what to do when your heat goes out in a polar vortex covers cold-snap problems like this.

Step 9: Check the Outdoor Vent Pipes

High-efficiency furnaces draw combustion air and exhaust through PVC pipes that usually exit the side of the house. Heavy snow, drifting or ice buildup can block them. When that happens, the pressure switch prevents the furnace from firing.

If it is safe to reach them, check the vent terminations outside and gently clear snow or ice by hand. Do not use tools that could crack the pipe. After a big Chicago snowfall, this is one of the most common reasons a high-efficiency furnace stops working.

Step 10: Try One Reset

Once you have checked the items above, try a single reset:

• Set the thermostat to off.
• Turn the furnace power switch off and wait about 30 seconds.
• Turn the power back on.
• Set the thermostat to heat a few degrees above room temperature.

Listen for the start-up sequence: the inducer motor, a click, the ignitor and then the burners. If the furnace starts and runs a full cycle, keep an eye on it. If it starts, runs briefly and shuts off, or tries several times and stops, it is locking out for a reason. Do not keep resetting it. Repeated resets can mask a real safety issue.

What It Means If the Furnace Tries but Will Not Stay On

If you hear the inducer start and maybe see the ignitor glow, but the burners light only briefly or not at all, the problem is usually in the ignition and flame-proving system:

• A dirty flame sensor cannot confirm the flame, so the furnace shuts the gas off after a few seconds.
• A cracked hot surface ignitor cannot light the gas.
• A pressure switch problem prevents the burners from lighting at all.

These parts are common repairs. Our guide to furnace ignitor and pilot light problems explains each one, and our post on furnace short cycling covers furnaces that start and stop repeatedly.

What It Means If the Blower Runs but the Air Is Cold

If the fan runs but you only get cold air, the furnace is turning on but the burners are not heating. That points to ignition, gas supply or a safety switch. It is a different problem from a furnace that will not start at all. We cover it in detail in Furnace Blowing Cold Air? 7 Causes to Check in Chicago.

Things You Should Not Try

Some fixes you will find online are not safe for homeowners. Avoid:

• Bypassing or jumping safety switches, such as the pressure switch, rollout switch or limit switch.
• Removing or adjusting the gas valve or burners.
• Working inside the control panel with the power on.
• Repeatedly resetting a furnace that keeps locking out.
• Using a gas oven or outdoor heaters to heat the house.

Safety switches exist to prevent fires and carbon monoxide exposure. If one keeps tripping, something is wrong, and the switch is doing its job.

Keeping Warm While You Wait for Service

If the furnace will not start and it is bitterly cold, close curtains, close doors to unused rooms, and keep the family together in the warmest part of the home. If you use a space heater, use a modern unit with tip-over and overheat protection, plug it directly into a wall outlet, and keep it at least three feet from anything that can burn. Never run a space heater unattended or while everyone is asleep. Protect plumbing on exterior walls by opening cabinet doors under sinks.

When to Call for Furnace Repair in Chicago

Call for service if:

• The basic checks do not restore heat.
• The breaker trips again after a reset.
• The furnace shows an error code you cannot resolve.
• The furnace starts and then locks out.
• You see water around a high-efficiency furnace and cannot find a simple cause.
• You smell gas or a carbon monoxide alarm sounds. Leave first, then call.

When you call, have the error code, the furnace make and model from the rating plate, and a description of what happens when the thermostat calls for heat. That helps a technician arrive with the right parts. If it is late at night or temperatures are dangerous, see how emergency HVAC service works, or call (312) 638-0812.

If your furnace is old and failing often, it may be time to weigh repair against replacement. Our guides on how long furnaces last in Chicago and furnace replacement can help you decide.