Furnace Not Starting? Here Are Some Simple Reasons Why!
They’re Cheap to Fix Too!
By Tim Wojnar
Three different types of pilot/ignition systems for furnaces
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.
Intermediate Pilot – The main reason the standing pilot is now outdated for furnace units is that it is not efficient to have a pilot flame lit at all times.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.
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
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.
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.
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.
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.