Diagnosing A Failing Blower Motor
Your furnace's blower is responsible for moving warm air to each room in your house. Without this fan, the heat generated by your furnace would mostly remain near the heat exchanger, or never be transferred to your home's air at all. When a blower begins to fail, you may notice reduced airflow from your vents or no airflow at all. If your home is cold despite a furnace that seems to be running, then your blower motor is a solid culprit. If you suspect that this is the problem, these diagnostic steps will help you to zero-in on the exact cause of the problem.
First: Rule Out Other Issues
When the temperature in your home drops below the desired temperature set on your thermostat, you should be able to hear your furnace fire up. Depending on the placement of your furnace, this may be difficult to hear from elsewhere in the house. Have a helper turn the thermostat up while you stand near the furnace so that you can confirm that the furnace is igniting correctly. When the furnace ignites, you should also be able to hear your main blower motor and exhaust fan, as well. If you don't hear these blowers, then move on to further diagnosis. If your thermostat has a "fan-only" option, turn this on and listen for the blower to engage.
Check Circuit Breakers and Connections
Modern furnaces use electric igniters, so a furnace that ignites is likely to have power. If you have an older furnace, however, you may still have a pilot light instead of an electric igniter. In this case, ensure that your furnace is connected correctly and that it has not tripped its breaker. Always be cautious when working near electricity and contact a professional if you are unsure of where your connection and switch are located. If your blower immediately trips its breaker again, stop your troubleshooting and call in a professional for help since this often indicates a wiring issue.
Thermostat or Blower?
At this point, the two most likely causes for your issue are your thermostat and your blower motor. If your home has multiple heating zones, confirm that you are unable to engage the furnace blower by changing the temperature in a different zone. If all thermostats fail to work, then the problem is most likely the blower itself or with the safety switch that prevents your furnace from running under certain conditions (such as when overheating). While thermostats are relatively easy to replace, properly diagnosing and repairing a blower motor is a more complex job that should be left to a skilled HVAC technician. If you have managed to get this far without finding a solution, then it's time to have a pro evaluate your furnace and handle any heating repair needs that it has.