This is a very common call description that I receive. While there are endless reasons why this can happen, I will go over a few of the most common reasons.
- Blocked air filter/ tripped high limit switch
A blocked air filter (shown in photo below) is by far, the most common reason for a furnace to not run correctly or run at all. A blocked air filter can starve the furnace of air moving over the heat exchanger.
This will cause the unit to overheat and trip the high-limit switch. When the limit switch trips, it triggers the blower to run to cool down the heat exchanger, once it cools down, the limit resets and allows the unit to start again. If it does not restart, then the limit switch has failed from repeated tripping and will need replaced.
- Dirty flame sensor
The next common issue would be a dirty flame sensor. (photos below shows a dirty flame sensor and after cleaned)
When a flame sensor is dirty, you will notice the furnace starting, turning the burners on, then the flames go out after a couple seconds. Simply remove the sensor, clean it with some scotch-brite or fine sand paper and reinstall it.
- Failed igniter
Most modern furnaces incorporate a hot surface igniter. These igniters are made from either silicon nitrade or silicon carbide and are very fragile. Hot surface igniters generally last around 5 years. You will know these are operating properly by their bright orange glow when the furnace is starting its ignition sequence. When they fail, the ignition sequence will start but you will see no orange glow and hear the gas valve turn on and get a smell of gas from the furnace. The photo below shows a failed hot surface igniter.
The last one that we will include in the most common issues to cause the furnace to come on is the thermostat. I was not going to include this because it is so obvious, but over my many years in the service industry, I see this time and time again. The thermostat is the control for when the furnace turns on or not. If your thermostat is blank (shown in the photo below) or unresponsive, your thermostat could be your issue.
If your thermostat is blank as in the photo above, the first thing you should do is replace your batteries. If your thermostat is “hard-wired”, meaning it has a wire hooked up to the “C” (common) terminal, then you need to be looking at your furnace power as it is likely turned off or the breaker is off or tripped. Change the batteries and restore the power to the furnace. If the breaker trips again, you have bigger issues than a thermostat and will need to call for a diagnostic from a reputable hvac company or if you are a capable, do-it-yourselfer, you can use our virtual diagnostic tool and have one of our senior technicians walk you through the trouble-shooting process to help you figure out what is causing the issue. Simply click on the Virtual Diagnostic button below to get started!