furnace repair

What to do When Your Furnace Wont Start

It might feel stressful to troubleshoot your furnace when your heat won’t work. But it doesn’t have to be that way.

You may be able to avoid a furnace repair call with our DIY troubleshooting guide. You don’t need any technical skills. And most of these fixes are brief and affordable (or even free).

This guide will walk you through how to fix your furnace when it won’t turn on, won’t stay on or won’t light.

When you need a pro in Northglenn, Northglenn Heating & Air Conditioning, Inc. can be there.

We repair and maintain most makes and models of furnaces. If you need a new heating system, we also offer furnace replacement and furnace installation.

Furnace breakdowns are usually caused by neglected routine maintenance. These evaluations often disclose a costly problem before it starts—and causes your HVAC system to fail.

During this service, our NATE-certified professionals will closely inspect your furnace, make sure it’s working properly and lubricate moving parts. A well-managed furnace often lasts longer and operates more efficiently, saving you more on your heating bill.

Ready to begin troubleshooting your furnace? Follow our step-by-step guide below.

Steps for Troubleshooting Your Furnace

Check Your Thermostat

Start by checking your thermostat. Is it telling your furnace to turn on?

If you have a digital thermostat:

  • Change the batteries if the screen is off. If the digital screen is scrambled, you may need a different thermostat.
  • Confirm that the switch is set to “heat” instead of “off” or “cool.”
  • Look to see if the program is displaying the correct day and time and is set to “run.” If you can’t override the program, change the temperature by pushing the up/down arrows and press the “hold” button. This will require the furnace to switch on if thermostat programming is causing complications.
  • Set the thermostat to 5 degrees warmer than the room’s temperature.
Digital Thermostat

Your furnace should turn on within a few minutes. If it doesn’t, double check that it has power by pushing the fan switch from “auto” to “on.” If the fan doesn’t start instantly, your furnace may not have power.

If you’re using a Wi-Fi thermostat—like one made by Nest, Ecobee, Lux, Honeywell or Bosch—check the manufacturer’s website for advice. If you can’t get your smart thermostat to function properly, call us at 303-452-4146 for assistance.

Smart Thermostat

Check Breakers and Switches

After that, you will want to make sure your breakers and furnace switch are on.

  • Head to your house’s main electrical panel. It’s the gray metal box on the wall in your basement, garage or closet.
  • Dry off your hands and feet before working with the panel or breakers.
  • Find the breaker labeled “furnace” or “heat” and make sure that it’s switched in the “on” position. If the breaker has tripped, it will be in the center or “off” position.
  • With one hand, firmly move the breaker to the “on” position. If the breaker trips and pops back to “off” after you do this, leave it alone. Contact a technician from Northglenn Heating & Air Conditioning, Inc. at 303-452-4146 immediately.

Your furnace has at least one wall switch situated on or near it—no matter its age or brand.

  • This switch should be flipped up in the “on” position. It can take your furnace up to five minutes to get working if the switch was off. (Not sure where to find your furnace? Look in your basement, garage or utility closet. It could also be located in a crawl space or attic.)

Replace Your Air Filter

Dirty, closed off air filters often cause complications that are easily avoidable.

  • Your furnace can overheat and shut down too soon, due to dust in the filter hampering airflow.
  • Your energy bills could climb, because your furnace is turning on more often.
  • Your furnace may fail permantly faster, because it has to work harder.
  • Your furnace could lose power, because an extra dirty filter can cause the breaker to trip.

You can find your air filter inside your furnace’s blower component, attached filter case or wall-mounted return air grille. Its position depends upon what model of furnace you have.

Replace furnace filter

When replacing your filter:

  • Turn off your furnace completely.
  • Pull out the filter, hold it up to the light and look through it. Replace it if you can’t see light through it.
  • Replace the new filter with the arrow pointing toward the furnace to avoid damage.

To make the process simpler next time, use a permanent marker on your furnace housing or ductwork to indicate the airflow direction and filter size.

We suggest replacing flat filters each month. Pleated filters usually last about three months. You can also get a washable filter that will work for about 10 years.

If you have children or pets, you may need to replace your filter more frequently.

Look at Your Condensate Pan

Condensate pans, or drain pans, hold water your furnace pulls from the air.

Follow these steps if your furnace is seeping water or there’s standing water in the pan.

  • If your pan has a PVC pipe/drain: Check that it’s open. If it’s not, you can use a special pan-cleaning tablet from a home improvement or hardware store.
  • If your pan has a pump: Find the float switch. If the switch is “up” and there’s water in the pan, call us at 303-452-4146. You will most likely need a new pump.

Check Inside Your Furnace

You can check the condition of your furnace’s blower motor by peeking inside the plastic window. Depending on the kind, this light could be placed on the outside of your furnace.

Contact us at 303-452-4146 if you see anything other than a steady, colored light or blinking green light. Your furnace could be giving an error code that needs professional help.

Clean Your Flame Sensor

Is your furnace attempting to start but shutting off without producing heat? A dirty flame sensor could be to blame. When this happens, your furnace will try to switch on three times. Then, a safety feature will shut it down for about an hour.

You can clean the flame sensor yourself if you feel confident opening up your furnace. We can also do it for you.

Hoping to try cleaning the sensor yourself? You’ll need the following:

  • A 1/4” hex screwdriver or wrench
  • Piece of light-grit sandpaper, steel wool or emery cloth
  • A dry, clean paper towel

Next:

  • Use your furnace’s wall switch or breaker to switch off the power. Shut off the gas also if your gas valve is not electric.
  • Take off your furnace’s front panel and trace the wire to the flame sensor, which looks like a thin, bent rod.
  • Unscrew the rod and use your sandpaper, steel wool or emery cloth to gently clean the metal rod.
  • Use a paper towel to wipe off the rod.
  • Replace the sensor.
  • Put your furnace’s doors back on.
  • Turn the furnace’s power back on. Your furnace may run through a series of checks before it starts as usual. If it doesn’t start, the sensor might need to be replaced. Or something else could be wrong. Call us at 303-452-4146 for guidance if this happens.

Relight the Pilot Light

If your furnace is an older design, its pilot light could be blown out. Relight it following the instructions on the label. You can find the label on your furnace’s doors.

Or you can follow these steps:

  • Locate the switch on the bottom of your furnace labeled “pilot,” “on” and “off.”
  • Rotate the switch to the “off” position.
  • Wait at least five minutes. This avoids the possibility of starting a fire.
  • Move the knob to “pilot.”
  • Hold down the “reset” button as you deliver the flame of a long lighter to the pilot light opening.
  • Let go of the “reset” button once the pilot light is lit.

Call us at 303-452-4146 if you’ve followed the instructions twice and the pilot won’t light or stay lit.

Check Your Fuel Source

Are other gas appliances working? If they’re not, your natural gas service could be off. Or you could be out of propane.

We Can Diagnose Furnace Problems

Made it through our troubleshooting guide but your furnace still won’t turn on?

Call us today at 303-452-4146 or use our online scheduler. We’ll come out to your home and find out what’s wrong.

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