Why Won’t My Furnace Light?

How a modern furnace lights

Most gas furnaces around Munster, and pretty much every one installed in the last 25 years or so, don’t have a standing pilot light. They light themselves every time the thermostat calls for heat. Knowing the order of events helps you figure out where it’s getting stuck.

• The thermostat calls for heat and the control board wakes up.
• The small inducer fan starts, pulling air through the burners and pushing exhaust out the vent.
• A pressure switch confirms the vent is drawing. No proof, no ignition.
• The hot surface igniter glows bright orange, or on some models, a spark igniter clicks.
• The gas valve opens and the burners light.
• The flame sensor confirms there’s a flame within a few seconds. If it can’t, the board shuts the gas off.

Most furnaces will try this a few times. If they keep failing, the board goes into lockout and stops trying for a while. That’s a safety feature, not a malfunction.

The usual suspects

The igniter

Hot surface igniters are brittle and wear out over time. A cracked one may not glow at all, or may glow only faintly. If you look through the sight glass and see nothing glowing after the inducer starts, the igniter is a strong suspect. It’s one of the most common winter repairs we do.

The flame sensor

This one fools people. The furnace lights, runs for a few seconds, then shuts off and tries again. The flame is fine; the thin metal rod that senses it is coated with residue and can’t prove the flame is there. Cleaning or replacing it is quick, but it lives inside the burner compartment, so we’d rather do it for you.

The pressure switch and venting

If the inducer runs but nothing ever glows, the pressure switch may not be closing. On high-efficiency furnaces, the common causes around here are a clogged condensate drain, a cracked hose to the switch, or the intake and exhaust pipes outside packed with snow or frost after a lake-effect storm. Clearing snow from the pipes is a safe check. Anything past that is ours.

The gas supply

If the gas valve at the furnace was turned off during other work, or if the gas meter or service has a problem, the burners have nothing to light. A quick test: does the stove or water heater work? If nothing gas-fired works, contact NIPSCO. If only the furnace is out, the problem is likely in the furnace.

What you can check safely

Start with the simple stuff. A surprising number of our no-heat calls end at the thermostat.

• Set the thermostat to HEAT, raise it three to five degrees and replace the batteries if it has them.
• Find the furnace power switch, usually on the wall or ceiling near the unit, and make sure it’s on. Check the breaker.
• Make sure the lower blower door is fully closed. A loose door leaves a safety switch open and the furnace won’t run.
• Look at the filter. A badly clogged one can make the furnace overheat and shut down.
• If you have white PVC pipes on an outside wall, clear any snow or ice around them.

You can turn the furnace off at the switch, wait about a minute and turn it back on once. That clears a lockout. If it fails again, stop there. Resetting it over and over doesn’t fix anything and can hide the real problem.

Safety first, every time

If you smell gas, which many people describe as rotten eggs, don’t flip switches, don’t try to relight anything and don’t use your phone inside. Get everyone out and call NIPSCO or 911 from outside. If a carbon monoxide alarm sounds, leave the house and call 911. Those calls come before any call to us.

If you have an older furnace with a standing pilot, follow the lighting instructions on the label inside the furnace door exactly. If the pilot won’t stay lit, the thermocouple is a likely cause and it’s a repair, not a reason to keep trying.

What the furnace is telling you

Stand near the furnace after you raise the thermostat and pay attention to the order of sounds. It narrows things down fast, and it’s useful information when you call.

• Nothing at all: no fan, no click. Think power, thermostat, blower door or a control problem.
• Inducer hums, nothing glows: often the pressure switch, a blocked vent or a clogged condensate drain.
• Igniter glows, no flame: the gas isn’t getting to the burners, or the gas valve isn’t opening.
• Flame lights, then goes out in a few seconds: classic flame sensor, though grounding and gas pressure can do it too.
• A blinking light on the control board: many furnaces flash a code. Count the blinks and check the chart on the inside of the door, or just tell us the pattern.

When to call a pro

Call us when the simple checks don’t bring it back, when it lights and drops out, or when it locks out again after one reset. A real person answers 24/7. We’ll ask what the furnace is doing, and often we can tell from the description which part is likely involved.

When we get there, we find the failed part and write the repair price down before a wrench turns. Our techs aren’t on commission. A furnace that won’t light almost always needs one part, and we’ll tell you that plainly.

We’re based in Munster and cover about 20 miles around. See furnace repair in Munster, furnace repair in Highland, or the full service area. For the bigger picture, start at our furnace repair page. If the furnace runs but the air coming out is cool, read why a furnace blows cold air. If it lights and shuts off again and again, see furnace short cycling.

For general guidance on furnace safety and efficiency, the U.S. Department of Energy’s Energy Saver site is a good, plain-language resource. Heat out now? Call (219) 555-0142.