Furnace blower assembly Blower Fan Won't Start
A/C or Heating Air Handler Fan Will Not Run

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AHU blower fan troubleshooting diagnosis & repair:


This article series describes in detail the testing and diagnosis of problems with air conditioning fans and with forced warm air heating blower fans found in the air handler or AHU.

When the blower fan won't start the problem could be something as simple as a blower compartment door that is not fully closed, or a clogged condensate drain.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

How to Diagnose & Fix a Blower Fan that Will Not Start

Furnace blower motorWhy won't my furnace blower fan start on its own? It will run if I give it a "spin"

My gas heater isn't working correctly. My thermostat signals the heater correctly and the burner ignites, but the fan doesn't always turn on. If I open the panel and spin the squirrel cage a little, it immediately begins to blow air and runs the heating cycle. What could this be? - Mike.

Photo: an electric motor using a pulley and belt to drive the fan in an air handler.

[Click to enlarge any image]

Reply: Check or replace the blower fan motor start/run capacitor


Often when an HVAC fan won't start on its own but will start and keep spinning when you give it a push that means that the start-capacitor for the unit has failed.

It's an inexpensive part that can be installed or replaced by your HVAC service tech or by a homeowner who has expertise in electrical wiring, using a VOM, and following simple wiring instructions.

12 Reasons that an Air Handler, A/C or Furnace Blower Van Does Not Run

Air handler blower assembly(C) Daniel FriedmanPhoto: a direct-drive air handler blower motor on a nicely-cleaned AHU.

Before we focus on the motor and controls of the air handler blower fan (used in both air conditioning and heating systems) let's ask if the trouble is not specifically the blower fan but more basically: the air conditioner or heater itself just won't start in response to a call for cooling or heating.

To rule out trouble outside of the fan or air handler itself, check:

Now we can continue by assuming you're convinced the trouble is right in the air handler. So take a look at these most-common causes of failure of the AHU blower fan to run. I've arranged this list roughly in order of probability and also in order putting the easiest and simplest things to check first.

Most homeowners can check the first six of these things by simple observation.

Watch out: turn off the electrical power before opening and inspecting the interior of the air handler or its blower compartment. There are risks of chopped off fingers or even death by electrocution if the equipment is not completely turned off.

Certainly most homeowners (except my mom) can check to see that the thermostat is set to call for heating or cooling, and you can look to see if a blower compartment door is open or the condensate tray is full of water.

But for electrical inspection and testing of components or switches, if you are not trained, comfortable and confident with using basic electrical test tools like a neon tester or VOM or DMM to check for electrical power, do not try it and don't touch anything. Instead call your HVAC service company for help.

  1. Controls are not calling for heating or cooling - a thermostat or sometimes electronic control board fault. See FAN AUTO ON THERMOSTAT SWITCH
  2. No electrical power to the fan or its controls - check for a blown fuse or heating or cooling switch set to off. Check for a loose or disconnected wire or a corroded electrical connector.
  3. Open blower compartment door - a safety switch may be preventing fan operation - see BLOWER DOOR SAFETY SWITCH
  4. Flooded condensate drain tray - for heat pumps and air conditioning systems that use a sensor switch to detect water in the condensate overflow pan, this condition can prevent the air handler from running in air conditioning and heat pump systems - CONDENSATE PAN SWITCH LOCKOUT
  5. Loose or broken fan beltOn belt-driven blower assemblies - the blower fan motor runs but the fan does not turn
  6. Loose pulley on belt driven blower assemblies - the blower fan motor runs but the fan does not turn
  7. Damaged blower assembly shaft or bearings - a seized fan assembly (unusual). The motor will draw abnormally high amps.
  8. Bad fan motor start/run capacitor - See details at FAN MOTOR START CAPACITORS
  9. Bad fan control relay in air conditioners or air handlers that use a control board or relay to operate the fan
  10. Bad fan limit control switch in heating furnaces - see FAN LIMIT SWITCH TROUBLESHOOTING
  11. Overheated electric motor, off on thermal reset - see MOTOR OVERLOAD RESET SWITCH
  12. Bad electric motor for the fan or blower itself - see ELECTRIC MOTOR DIAGNOSTIC GUIDE

Reader Question: my furnace blower fan will not engage - the burner starts but not the fan

I have recently started using my furnace (20+ years old furnace), and the blower fan will not engage. The pilot and gas burners start, but the fan does not. I have ensured that the fan is set to "auto", on the thermostat. Is there any procedure I can do to troubleshoot the issue? I have seen several topics, however none seem to be helpful. - J.B. 12/31/2013


A competent onsite inspection by an expert usually finds additional clues that would permit a more accurate, complete, and authoritative answer than we can give by email alone. But here are some suggestions to start troubleshooting the problem of a furnace blower fan that does not start:

Step 1: I would start by confirming that the blower fan will operate. Your system probably has a manual FAN ON switch on the thermostat, or if not, there may be a FAN ON push pull switch right on the fan limit switch in the air handler.

If you can make the blower run then we know the problem is not with the blower motor or drive belt or assembly and we'd go to step two.

See  FAN AUTO ON THERMOSTAT SWITCH for the switches on the thermostat;

For a manual FAN ON switch found on some limit switches in the blower or air handler assembly itself see CONTINUOUS BLOWER FAN OPERATION

Step 2: I would continue diagnosis by watching the behavior of the fan limit switch as the furnace heats up; presuming nobody has messed with the switch to try to change its settings, it should act as described in the article above beginning at FAN LIMIT SWITCH TROUBLESHOOTING

This article series answers most questions about central heating and air conditioning system troubleshooting, inspection, diagnosis, and repairs. We describe how to inspect, troubleshoot and repair heating and air conditioning systems to inform home owners, buyers, and home inspectors of common heating system defects. Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.


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