How to Replace a Drive Belt on Your Air Handler Blower

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How to replace the belt on a furnace or air handler is fairly simple as long as you have the right tools and take a few precautions.

In other articles I have gone over some fairly technical repairs on heating, ventilation, and air conditioning (HVAC) systems, this article will discuss how to replace the drive belt on the blower of your furnace or air handling unit. Replacing the belt on the blower is easy and can prevent other issues from cropping up such as frozen coils or reduced airflow. Drive belts can last for years, but the more you use you air handler, the faster the belt will deteriorate and need replacement. Replacement belts can be purchased at automotive parts stores, home centers, hardware stores, or online. 



Related Article: Fixing Frozen Air Conditioning Coils

While the belt replacement is simple, there are a few items to make note of as you can damage your blower motor or bearing if you are not careful.

When to Replace your Drive Belt

Most people wait until the drive belt breaks before they run out and buy a new belt and replace it. If you haven’t replaced your belt in 3 years, buy two so you have a spare after you replace the current belt inside your unit.

If you use your air handler for heating and cooling you should replace the belt at least every three years. Check the belt in the spring and fall and look for any abnormal wear on the edges or fraying and cracking. There may be some black powder or dust around the blower and inside the unit. If it appears to be excessive, you should replace the belt. If you take the belt off and twist it gently, it should not be brittle or cracked on the surfaces touching the pulley. It may be shiny which is called glazing.

Types of Belt Failure

Before you Get Started

Make sure that your air handler or furnace is belt-driven and not a direct drive motor. Direct drive units do not use belts, so if you here an unusual noise or the airflow is reduced you may need to replace the motor.

Take a look at the belt to see what size it is. Most residential units will be v-belts, but you may also come across cog belts. Sometimes cog belts are used when v-belts are not on hand, but some contractors like to use them because they last longer and perform better depending on the size of the pulley and motor. If you do not have the owner’s manual on hand for your furnace or air handler, replace the belt with the same one that you find inside.

All belts are marked on the outside with the width and circumference. The old numbering system used the letters A, B, C and D for the width, an X after the first letter means that the belt is a cog belt; and the circumference is a 2-digit number. The newer numbering system uses 3L, 3V, 3VX, 4L, 5L, 5V, 5VX, etc with a 3-digit number which is the circumference in millimeters. For example a belt labeled “AX-36” is a cogged belt with a circumference of 36 inches. If you can’t find the number on the belt, bring it with you to a location that sells drive belts so they can determine the size.

Cog Belts

Tools and Materials:


Adjustable Wrench


Grease or oil

Drive belt


1. Shut the power off to the unit

Locate circuit breaker or disconnect for the furnace of air handler and shut it off. There is usually a switch on the side of the furnace with a red cover that should be shut off as well. Set the fan selector switch on the thermostat should be set to the ON position to ensure that the unit does not start.

2. Access the Blower

Most blowers are located near the bottom of the unit in vertical configurations, or just after the filter rack.

There is usually an access panel on the side or front of the unit. On some furnaces the panel lifts off or is interlocked with another panel that needs to be removed first. Sometimes there are screws that hold an inside panel to cover the blower. You will need a screwdriver to remove this panel if present.

3. Remove the old Belt

One of the biggest mistakes one can make when replacing a belt is to just pull it off. Even though the old belt is worn and stretched out, it can still damage the shaft of the motor or blower if it is stretched around the pulley. With a marker, mark the location of the motor mount so it can be set back to its original position.

Use an adjustable wrench or a set of box wrenches to carefully loosen the mounting bolts of the motor. Just a few turns is enough as the bolts only need to be loose enough to slide the motor toward the blower housing. Once there is enough slack in the belt, pull the belt off around the smaller pulley of the motor and then off the larger pulley of the blower.

Clean off any dirt or debris from the pulleys and blower housing. You may want to vacuum the inside of the blower and squirrel cage also.

Dirty Squirrel Cage

If the motor and blower have oil ports or grease fittings you should lubricate them. Most motors will have an oil port covered by a set screw that can be removed with an Allen wrench. Add a few drops of oil into the hole; be careful not to overfill or get any oil onto the copper motor windings inside.

Adding oil to the blower motor

4. Replacing the Belt

Install the new belt onto the pulleys of the motor and blower, starting with the smaller pulley of the motor first. Slide the motor mount back to the mark you made previously. The belt should be taut, but it should have about 1/2 inch of give when pushed down in the middle of the span with your fingers. Tighten the bolts when the proper tension is achieved. Reattach the access panel and turn the circuit breaker back on.

Check for proper belt tension

Set the thermostat back to the normal settings and the fan selector switch to auto. Check to verify that the fan is running quietly and supplying adequate airflow throughout the house.


Roberta Baxter
Posted on Jul 1, 2011
Abdel-moniem El-Shorbagy
Posted on Jun 29, 2011
Roberta Baxter
Posted on Jun 29, 2011