Furnace Won't Turn On? What to Check First

When a furnace won't turn on, the furnace itself usually isn't the first thing to check. Thermostats get left on Cool. Service switches get bumped. Breakers trip.

Any one of those will keep a perfectly healthy furnace from running, and you can rule them all out in about five minutes.

If they all check out, go stand next to the furnace while someone turns up the heat. Is it dead quiet? Or do you hear a hum, a click, maybe a small fan that starts and then stops?

The answer tells you which part of this article you need.

One quick note before we start. This is about a furnace that won't start at all.

A furnace that runs but puts out cool air is a different problem (see our guide to a furnace blowing cold air), and so is one that shuts off every few minutes, which we cover under furnace short cycling.

Start With the Thermostat

Nothing happens until the thermostat calls for heat. That sounds obvious, but after a whole summer in cooling mode, a forgotten setting is one of the most common reasons a furnace seems dead on the first cold night.

It's also why we like a trial run on a mild afternoon, and our guide to the first furnace startup explains what to look for.

Make sure it's set to Heat, with the temperature a few degrees above what the room is now.

If you have a smart or programmable thermostat, look for an away mode or schedule that might be holding the heat back, and switch it to a manual hold while you test.

Dim screen or a battery icon? Swap the batteries.

A screen that's completely blank deserves a second look. Lots of thermostats get their power from the furnace, so a dark screen can actually mean the furnace has lost power.

If fresh batteries don't bring it back, skip ahead to the power checks.

If the Furnace Is Completely Silent

Dead silence, with no light showing in the little window on the furnace door, nearly always means the furnace has no power.

Gas furnaces still need electricity to run their controls and fans, so this is where to look next.

The Service Switch

Somewhere on or near the furnace, there's a switch that looks just like a regular light switch. That's the trouble with it.

It's easy to flip off by accident, especially if you think it controls a basement light. Some homes have a second one at the top of the basement stairs, so check for that too.

The Breaker

Open your electrical panel and find the breaker marked furnace, heat, or HVAC. A tripped breaker sits a little off center. Push it all the way off, then back on.

One reset is fine. If it trips again, leave it off and call us. A breaker that keeps tripping is protecting you from an electrical fault, and flipping it back on won't fix that.

The Door Panel

Most furnaces have a safety switch behind the lower door, and the furnace won't run unless that door is pressing on it.

If someone changed the filter recently, the panel may not have gone back on all the way. Press it in firmly until it sits flat.

If It Has Power but Won't Run

If you can see a light through the window in the furnace door, power isn't the issue. Watch what the light is doing.

A steady glow or a slow, even blink usually means the furnace is fine and simply waiting to be told to run. In that case, go back to the thermostat.

A repeating pattern of flashes is a fault code. Count them and write the pattern down before you turn anything off, because on many furnaces, cutting the power wipes the code.

The chart that explains it is usually stuck to the inside of the furnace door.

A lot of the time, the code means the furnace tried to light a few times, failed, and locked itself out. We'll get to resets in a minute.

No light at all means the control board isn't getting power, even if something else seems to be on. Go back and recheck the switch and the breaker.

If It Hums or Clicks but Never Lights

Now the furnace has power and is trying. Before the burners light, it runs a few quick safety checks, and when one fails, it stops right there.

A couple of those checks depend on things you can look at without opening anything up.

The Vent Pipes Outside

High-efficiency furnaces vent through white plastic pipes that poke out of a side wall. If snow or a bird's nest plugs one, a pressure switch won't close, and the furnace won't light.

What you'll usually hear is a small fan humming away inside the furnace with nothing happening after it.

Around Elyria and Lorain County, it's worth checking those pipes after any big snow, especially if they sit low on the wall where drifts pile up. Clear them by hand. Don't stick anything into the pipes.

The Drain

Those same furnaces make water, which drains away through a small tube or into a little pump. If the drain backs up, a switch can stop the furnace from starting.

See water around the base, or a pump that's full and silent? Give us a call and we'll clear it.

The Gas

Try a burner on your stove. No gas anywhere in the house means it's a supply problem, and that's a call to your gas company.

If the stove works fine, find the valve on the gas line beside the furnace. The handle should line up with the pipe. If it's turned crosswise and you don't know why, call before you touch it.

Parts Inside the Furnace

If the vents, drain, and gas all look fine, a part has probably failed. A worn igniter or a dirty flame sensor is the most likely culprit, though it can also be a pressure switch or the control board.

They all sit right next to the gas valve and burners, so at this point it's a job for our repair team.

The one exception is an older furnace with a standing pilot light. If the pilot's out, the relighting instructions are printed on a label on the furnace, and you can follow them as written.

Don't try it if you smell gas, though. And if the pilot keeps going out, a part needs replacing.

Is There a Reset Button on a Furnace?

Not usually, or at least not one big button. When a furnace fails to light several times in a row, it locks itself out, and many will try again on their own after an hour or so.

You can reset it yourself by turning it off at the service switch, waiting a minute, and turning it back on. Try that once, especially if the power flickered or you just cleared a vent pipe.

If the filter hasn't been changed since spring, put a new one in first. A clogged filter makes the furnace overheat and shut itself down, and that can end in a lockout too.

If it locks out again, don't keep resetting it. The furnace is shutting down for a specific reason, and that reason is still there.

The same goes for the little reset buttons inside the cabinet. Each one is tied to a safety switch, and if one has tripped, we need to find out why.

When to Leave It Alone and Get Help

• If you smell gas, get out of the house without flipping any switches, then call your gas company and 911 from outside. Call us after that.
• If the carbon monoxide alarm goes off, get everyone outside and call 911. Our carbon monoxide guide explains where it comes from.
• If you smell burning plastic or notice scorch marks near the furnace, shut it off and call.
• If the breaker trips again after you've reset it once, leave it off.
• If water is pooling around the base of the furnace, call before running it again.

Keeping Warm Until It's Fixed

Electric space heaters make a safe stopgap. Plug them straight into the wall and keep them well away from curtains and furniture.

Please don't try to heat the house with the oven or a grill, since either one can fill a home with carbon monoxide.

If it's bitter out, open the cabinet doors under sinks on outside walls so a little warm air reaches the pipes.

When to Call for Furnace Repair

Call us if the furnace still won't start, keeps locking out, or is flashing a code.

When you do, tell us what you heard when the thermostat called for heat and what the status light was doing. It gives our technician a head start.

We handle furnace repair all over Elyria, Lorain County, and Cleveland's west side, seven days a week.

Same-day repairs and 24/7 emergency service are available, and you'll get an itemized quote before we start. Call (440) 221-7978.

Plenty of no-start problems come from parts that quietly wore out over the summer. An early-fall furnace tune-up is the easiest way to catch them, and every visit includes a heat exchanger and carbon monoxide check.