Furnace Not Starting? Here Are Some Simple Reasons Why!

They’re Cheap to Fix Too!

By Tim Wojnar

When it comes to Murphy’s Law, your old furnace not igniting in the dead of winter is no exception.  You check your thermostat and it is set to the typical 68 – 70 F in heating mode, but no hot air is coming out of your supply registers.  You go downstairs to check on the furnace and you don’t see the flames from the burners on, but why?  Over a 50 percent of the time, the issue has to do with your furnaces pilot/ignition system and the good news is that most of the time the repair is fairly simple and cheap (even free at time).  So before you call your local HVAC tech and have them charge you a couple hundred dollars for a service fee to look at your furnace (I had a client charged $350 for a thermocouple replacement that costs $10 and 5 minutes to repair), continue to read this article to see if you can diagnose and fix the problem yourself.
Before we get started – It is important to make sure that the furnace has power and gas is still being supplied to the unit before proceeding because if they are not, that is why you unit is not working.  When you set your thermostat level to start the unit, you should hear the furnace fan turn on in preparation for the furnace to start.  If this happens you know the power is on, if not try locating the emergency shutoff switch nearby the unit (looks like a light switch) and flip its position (sometimes people accidentally flip the switch thinking its for lighting).  If gas does not appear to be flowing, check the gas shutoff valve to ensure it is in the open position.

Three different types of pilot/ignition systems for furnaces

Depending on when your furnace was manufactured and the quality of furnace you purchase, you will have one of three types of ignition systems.  While the ignition concept is fairly similar between each, knowing which type you have is essential for diagnosing any issue.

In this image, you can see what a standard thermocouple looks like.

  1. Standing Pilot – If your furnace is old (20 years plus), you most likely have a standing pilot system.  For this type of system, a small pilot flame is always lit, so when you turn on your furnace the gas that flows out of the burners can be ignited and starts the furnace heating your home.  Close to the standing pilot, is a temperature sensor called a thermocouple.  Its job is to ensure the pilot light is on and confirms this prior to the furnace valve opening.  If the thermocouple does not sense the hot pilot flame temperature, it will not allow the automatic furnace gas valve to open because that would then lead to non-ignited gas escaping the furnace and freely filling your home.
  2. Intermediate Pilot – The main reason the standing pilot is now outdated for furnace units is that it is not efficient to have a pilot flame lit at all times.   An intermediate pilot system solves that problem by lighting the pilot right before the furnace is ready to turn on the burners.  An electric spark is created to ignite a pilot light and then the burner gas is release by the furnace valve and ignited.  The pilot is then extinguished to not burn additional fuel for efficiency sake.  Like the standing pilot, the Intermediate pilot has a temperature sensor called a flame detector.  If the detector senses that the pilot is not ignited, it will stop the furnace valve from opening just like the thermocouple.
  3. Hot Surface Igniter – This last type of ignition system is the simplest.  The hot surface igniter uses electrical current to heat a plate (In the same way an incandescent light bulb works)  to a temperature hot enough to ignite the burners when the furnace cycle is turned on.  The surface gets so hot you will see it glowing prior to ignition.  Just like the intermediate pilot system, this type of an ignition system has a flame detector to ensure that there is a burner flame within 7 seconds of the gas valve opening.

Troubleshooting Your Ignition Problem

Before you start the trouble shooting why your furnace is not firing, it is important you know which type of ignition system your unit has (I would look this up in your owner’s manual if possible).  Another way to determine if you have a standing pilot versus the electronic ignition systems is if your furnace gas valve says “on-off-pilot” versus just “on-off”.  Last note, I would go down the list below in order to troubleshoot from the easiest (and cheapest) issue to the slightly more difficult ones.

If your furnace valve says pilot on it, you have a standing pilot ignition system.  If it says just on and off its an eletric system.

  1. Your pilot light is out – This issue only applies to standing pilot furnaces, so move on if you have an intermittent pilot or hot surface igniter system.  Sometimes your pilot flame gets blown out and needs to be re-lit.  To relight the pilot, I suggest watching this video that give you the step by step.  If the pilot ignites, but turns off once you have pressed down the pilot button for 30 seconds, then go on to the next step.
  2. Thermocouple or Flame detector need cleaning – If you standing pilot light does not stay on the issue is probably with your thermocouple.  For electronic ignition systems, I would start with your flame detector which acts similarly to a thermocouple.  These temperature probes may get dirty over time with carbon deposits and need a quick cleaning to properly function.  Here is a quick video  on how to clean a flame sensor.  A thermocouple may be cleaned in a similar fashion.
  3. Thermocouple or Flame Detector needs to be replaced – If cleaning the temperature sensors doesn’t work, then they may need to be replaced.  Now you might be nervous to touch anything in your furnace, but if you know how to use a wrench, you can easily replace these elements.  Most thermocouples and detectors cost around $10 and can take 5-10 minutes to change out.  Here are some good videos on how to change out a thermocouple and a flame detector.
  4. Electric igniter needs to be replaced –  If the flame detector has been cleaned and subsequently replaced and nothing else is working, the last step is to replace the electric igniter.  If you have a standing pilot this step does not apply to you.  Remember there are two types of electric ignitors, a spark igniter for the intermittent pilot system and a hot surface igniter.  To replace a spark igniter type, you may have to replace the whole pilot assembly which could cost over $100 for the part so you may just wish to call an HVAC tech.  For the hot surface igniter, the part is usually around $10 and can be replaced easily as shown in this video.

If you have worked your way through each troubleshooting step and your furnace is still not igniting (or staying ignited) there is probably more complicated issue causing the problem.  At this point, it is best to call an HVAC tech to properly diagnose the issue.