Photograph of attic air conditioning air handler, condensate drips on floor HVAC Blower Fan Testing & Diagnosis
Blower Unit or Blower Fan Won't Run or Airflow is Weak

  • BLOWER FAN OPERATION & TESTING - CONTENTS: Heating or Air Conditioner / Heat Pump Blower Fan Testing & Diagnostic Procedures - How to diagnose a blower fan that won't start. Also CONVECTOR UNIT FANS: Air Conditioner or Heater Wall Convector Unit Blower Fan Inspection & Diagnosis
  • POST a QUESTION or READ FAQs about heating or cooling system air handler blower fans
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HVAC blower fan testing & diagnosis guide:

This article discusses how to inspect and test a heating or air conditioning indoor air handler blower fan that is not working.

We also discuss convector unit fans and we suggest diagnostic steps for squirrel cage blower fan squeaks and noises.

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Heating or Air Conditioner / Heat Pump Blower Fan Testing & Diagnostic Procedures

Air handler blower assembly(C) Daniel Friedman

How do I fix my indoor air conditioning or furnace blower fan: the air conditioning (or heat pump or furnace) blower fan just won't run

Our page top photo shows an air handler unit located in a building's attic - we removed the cover to show the blower fan assembly just to the left of the red tag) in this image of a Lennox™ horizontal HVAC system.

[Click to enlarge any image]

The blower fan is located inside a horizontal air conditioning unit in many home air conditioning systems, especially when the air handler is located in an attic or crawl area.

The location of a blower fan in vertical "up flow" or "down flow" heating and cooling systems is illustrated in additional sketches and photographs below.

Article Contents

Initial, simple diagnostic checks of the air handler system are also described at DUCT SYSTEM & DUCT DEFECTS: Basic checks of the indoor air handler (blower), air ducts, and filter systems.

My issue is about an air conditioner fan that just won't start. The fan motor is not seized, and we had a recent blower motor starting capacitor change. The blower fan relay appears to pull in okay, the electrical connections reseated and tight.

But the cooling fan will still not start. This is an early 1990's York home air conditioning unit. - R.S.

Our photo (left), shows a modern blower assembly inside of an air handler. In this case the blower is a direct-drive unit - the electric motor that drives the air conditioner blower fan is mounted inside of and at the center of the blower assembly itself.

Other HVAC blower units may mount the motor separately from the squirrel cage fan, connecting the motor to the fan using a set of pulleys and a fan belt [image].


Heating & Cooling Blower Assembly (C) Carson Dunlop Associates

For completeness we have listed some blower fan diagnostic steps that you have already tried, as well as additional things to check.

The blower assembly is the green component in this illustration from Carson Dunlop Associates The Illustrated Home.

  1. A/C won't start or lost cooling capacity: If your air conditioning or heat pump system has lost its cooling capacity or won't start see REPAIR GUIDE for AIR CONDITIONERS. Make sure that all electrical power switches for the system are "on" and that the thermostat is properly set.
  2. Heat won't start or lost capacity: if your heating system or heat pump (warm air heat) won't start, see the links above, or see HEATING LOSS DIAGNOSIS-FURNACES.
  3. If it appears that it is specifically the blower fan for your HVAC system is not working, first be sure that the thermostat is calling for heating or cooling.
    • For heating systems the set-temperature on the thermostat must be set higher than room temperature.
    • For cooling or air conditioners the temperature must be set lower than room temperature. See THERMOSTATS.
  4. Try overriding the thermostat by forcing the blower fan "on" using the thermostat. A simple test is to disconnect the thermostat wires completely at the control and jumper the thermostat terminals there, or more conveniently, remove the wall thermostat and simply (temporarily) connect the controlling thermostat wires there.

    The latter approach does not rule out a broken thermostat wire however. (See FAN ON AUTO MAN Thermostat Switch)

    or to test the blower unit by using manual fan-on switch see FAN LIMIT SWITCH.

    This switch is also illustrated at FAN LIMIT SWITCH TROUBLESHOOTING.
  5. If the blower fan motor won't turn ON or is noisy,

    First check for a cover ajar on the air handler. See BLOWER DOOR SAFETY SWITCH


    Don't forget also to look for other switches that could prevent the air conditioner from even starting, such as a condensate overflow pan sensor switch (see CONDENSATE DRIP TRAY DEFECTS) that can shut down the air conditioning system to avoid damage from overflowing condensate pans.

    Regardless of whether your air handler is for both heating and cooling, or for just one of these, see FAN WONT START to see a list of common reasons that the blower fan won't run or won't deliver air.

    Next, there may be an ELECTRIC MOTOR OVERLOAD RESET SWITCH that has tripped off due to thermal overload or an electric motor or voltage problem.

    If the problem is suspected to be the blower motor itself, that is if all of the controls are calling for the blower to run and the motor has electrical power, then see ELECTRIC MOTOR DIAGNOSTIC GUIDE to check for a motor that won't start.

    If the blower motor hums or is slow or is having trouble starting, also see CAPACITORS for HARD STARTING MOTORS.

    Keep in mind that if the blower motor or fan bearings are shot the wobbling fan can make a horrible noise and can eventually ruin the motor shaft bearings. And if your blower motor misbehaves after checking bearings and the start/run capacitor, consider that the motor itself may be bad. An expert can measure current draw as part of diagnosing a failing electric motor.

    Blower Fan No Start / No Stop - weird blower behavior can also be diagnosed and fixed - this discussion, found in our article titled AIR CONDITIONER WON'T START provides a detailed list of things to check.
  6. If the blower fan motor runs but little or no air is coming out of your heating or cooling supply registers, see LOST COOLING CAPACITY or HEATING LOSS DIAGNOSIS-FURNACES.

    The problem may be as simple as a dirty air filter (AIR FILTER CLOGGED), or the problem may be a DIRT CLOGGED SQUIRREL CAGE FAN itself,

    closed air supply registers (AIR SUPPLY DUCTS),

    disconnected air ducts (AIR DUCT LEAKS),

    a loose fan belt (LOOSE FAN BELT PHOTO) connecting the blower motor to the blower fan (if your fan is not a direct drive unit)


    Check the blower assembly drive belt: if the blower assembly uses a separate motor to drive the blower fan assembly (as opposed to direct-drive blower fan units) an electric motor drive shaft spins a pulley that is connected to a separate pulley wheel on the blower fan assembly by a drive belt.

    If the belt is broken the blower wont' spin. If the belt is too lowe the blower may not turn.

    How tight should the furnace or air conditioner blower drive belt be? In the absence of a manufacturer's specification we set about 1 1/4" of free play in the blower drive belt.
  7. If the blower fan motor won't run, see the motor test procedures at ELECTRIC MOTOR DIAGNOSTIC GUIDE

    Check to see if voltage is present at the fan motor wires. If voltage is present and if none of these steps will turn the blower fan on, and if you are certain that the motor starting capacitor is good (see CAPACITORS for HARD STARTING MOTORS), then we suspect that the motor may be seized.

    Sometimes if we have to replace a starter capacitor that gets things going again for a while, but ultimately we discover that the motor itself was increasingly hard to start as it was moving towards seizing. Keeping the fireplace flue closed (if you have one) when not in use will improve both heating and cooling efficiency.

    This photo shows a blower fan assembly with a motor starter capacitor installed on the upper right side of the squirrel cage fan.
  8. If the air conditioner or furnace blower fan won't stop when it should, for example if the blower keeps running and blows cold air onto room occupants during the heating cycle, see the diagnostic article found at FAN WONT STOP - LIMIT SWITCH

Thanks to reader Randy Shaffer for suggesting this topic.

Air Conditioner or Heater Wall Convector Unit Blower Fans

Photograph of wall convector heating and cooling system

Wall convectors are often used for both heating and cooling in commercial installations and high-rise apartment buildings.

The unit shown has its own compressor mounted right in the cabinet, visible at lower center in the photo.

Wall-mounted heating and cooling convector installations may be designed with one central heater or cooling system which feeds multiple units with chilled or heated water or possibly refrigerant from a single remote heating and cooling heat pump.

Our photo (left) illustrates dual squirrel cage blower fans typically found in the bottom of a fan/convector heating or cooling unit such as this one found in a New York City apartment.

If the convector fan motors run and the squirrel cage fans spin but not enough air is coming out of your convector unit, turn off power and take a closer look at the fan blades themselves - you may need a flashlight and a mirror to make this check without disassembling the unit further than shown here (we removed the convector unit cover).

A/C indoor convector unit with cover off (C) D Friedman

Watch out: Dirt on the squirrel cage blower fan blades can significantly reduce airflow through the unit. We have seen a 40 to 50% improvement in air flow simply by cleaning this blower fan assembly, yet it's something people rarely check.

Why? Because even a small amount of dirt in the cupped fan blade edges reduces airflow significantly, but it's not visually obvious.

You have to look carefully at the fan assembly. In our wall convector unit above you'd use a good flashlight and a mirror to inspect the blower assembly fan blades.

Also check the cooling or heating coil fins for blockage by dust and debris - a more common source of air flow blockage at heating and cooling convector units like the one shown.

A/C indoor convector unit with cover off (C) D Friedman

Our photo (above left) illustrates a condensate handling problem in the cooling convector unit for the same apartment unit introduced above.

Air conditioning condensate was leaking inside of the convector unit due to a clogged condensate drain line.

The condensate leak exited the bottom of the convector, ran through a raised floor cavity, entered apartment building walls, and ran around the wall interiors in a metal stud-framed wall sill plate where it led to major toxic mold contamination over a wide area, floor damage, and the need for costly cleanup and repair work.

Details about wall convectors used for heating or cooling and about convector blower fans are found at WALL CONVECTORS HVAC and


Also see CONDENSATE HANDLING, HVAC for more about air conditioner or heat pump condensate drainage handling.

Where is the "Blower Fan" or Air Handler blower located?

Which air conditioner, heat pump, or furnace fan is located where?

Air conditioner or heat pump basic schematic (C) D Friedman

In our simple air conditioning (or heat pump) system sketch shown at left, the light blue fan shown at left in the sketch (above the red compressor box) is the outdoor compressor/condenser fan discussed at FAN, COMPRESSOR / CONDENSER UNIT.

The dark blue fan shown at right in the sketch is the indoor air handler or blower compartment or cooling coil fan found inside the building. This fan blows building air across the evaporator coil (or cooling coil) to cool and dehumidify indoor building air.

We discuss this indoor fan at FAN, AIR HANDLER BLOWER UNIT.

In a warm air heating system that does not include air conditioning as well, the indoor fan is still located in the indoor air handler and is discussed at FAN, AIR HANDLER BLOWER UNIT.

At BLOWER FAN ASSEMBLY CLEANING we describe and illustrate a detailed, thorough cleaning of the blower assembly in an air handler that is used for both heating and air conditioning.

Squeaky Squirrel Cage Blower Fan Noises: Diagnosis, Cure

Question: What do I do about a squeaking, squalling squirrel cage? It doesn't do it every time the A/C comes on but more frequently than not. Not alot of room to see in there. WD-40? Any suggestions? - Erma

Reply: Check for Loose Blower Fan Parts, Lubricate with the proper oil or grease

The squirrel cage fan to which Erma refers is found inside the indoor air handler unit. This fan moves building air through the duct system and across heating and/or cooling elements to condition the air as it is then supplied to the building through supply ducts and registers.

In an air conditioning system this is the dark blue fan shown in the right side of our sketch above. For combined heating/cooling systems the same fan assembly can blow heated air into the building and an additional FAN LIMIT SWITCH will be found inside the furnace cabinet.

More about HVAC blower fans including squirrel cage fans is in this article, above beginning at BLOWER FAN OPERATION & TESTING but to start addressing a squeaky blower fan, note that your fan may be repairable by tightening one or more allen screws that secure the fan cage assembly (or the bearings that carry the fan) to its housing through which passes the fan motor drive shaft.

With power OFF for safety, if the fan and all components appear to have no loose screws or mounts, but you can still wiggle the fan from side to side, then the bearings are worn and need replacement.

If your fan is pulley-driven, check also that the pulley is secure on the shaft and that the fan belt is not so loose as to be squeaking.

I would not use WD40 on or near electric motor parts nor on greased bearings as it's not intended for a motor or bearing lubricant, though to be honest, I've sprayed WD40 on almost every problem that has ever arisen at one time or another. Better would be a lubricant intended for motors and motor shafts.

Some HVAC blower fan assemblies use a grease fitting not oil. In that case your service tech may need to re-pack the bearing with grease; often on older units there is a grease cap over the fitting that, if tightened, forces more grease into the bearing, so if you see one of those, try first giving the cap a turn. .

at NOISY AIR CONDITIONER / HEAT PUMP and at NOISE CONTROL for HEATING SYSTEMS we discuss a range of noises can be traced to air conditioning and heating systems.


Continue reading at BLOWER FAN OPERATION & TESTING FAQs or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.






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