What Size Furnace Blower Motor Do I Need: A Complete Guide

Furnaces are considered lifelines for temperature controls. These furnaces are made of many components. Among them, some of the components should be familiar to you. “Blower motor” may be the most famous and confusing of them all.

Nowadays, most people seem to be confused about the size of the blower motor of the furnace. And it’s okay to be not so sure about the size, we’re with you.

So, what size furnace blower motor do i need?

The size of your furnace blower motor depends on its physical and electrical specifications. The length & diameter, shaft sizes, mounting type, etc are very important for choosing the motor size. Additionally, to ensure compatibility, you’ll need to know the HP, RPM, amp & voltage limits.

To find out the correct size, only this portion won’t be enough for you. I’ve compiled everything you need to know before choosing the blower.

Get started then!

What Factors to Consider Before Choosing Furnace Blower Motors?

Choosing blower motors is an easy task. Yet, a bit of calculation is needed when making the choice. 

Let’s see some of the factors you have to consider when choosing the appropriate furnace blower motor.

Length And Diameter

This is the most important factor of all. You have to calculate the length and the diameter of your blower motor.

When you’re getting a new blower motor, matching the previous size is necessary. 

If the sizes don’t match, the new furnace blower might not fit in the furnace! Motor length can be from 3.3” up to 7.5” and above.

Mounting Type

There are 3 types of mounting types and they are-

• Ridgid base, 
• Belly band 
• Stud mount. 

These mounts vary from motor to motor. 

You can look up your furnace manual and find out the mounting type for your motor.

Shaft Size

The shaft size of blower motors is essential because the motor needs to fit. You need to have registers that support the shaft size of the furnace blower motor. 

When the register supports the air shaft, the air can circulate freely around the house.

The greater the shaft size, the more air the blower can push through. The most common shaft sizes range from 0.5 to 1 inch.

The shaft size greatly impacts the heating duration of your furnace. The bigger the shaft size, the less time it will take to supply heat in your whole house or room.

RPM

RPM is the abbreviation for revolutions per minute. This basically indicates how many rotations the blower motor can complete in 1 minute.

The greater the RPM, the more air can circulate through the blower. Most furnace blowers have 1.2-1.5K RPM. Going with the 1.2K RPM option is the safest bet here.

Voltage And Amp

These two mainly indicate how much electricity the blower motor will be using.

Your electric outlet should support the amount of voltage and amp. That’s how you can match the size of the furnace blower motor!

Voltages can range from 135 to 230V, depending on the motor. You can also pair up motors and run them at a combined voltage! 

The amps also decide the number of wire gauges. You can easily find the gauge count from any chart of wire gauges.

Horsepower

If the blower motor is multi-speed, the speeds will be mentioned. The mentioned speed will be the HP for the blower motor.

Matching the blower motor HP with the furnace HP is supercritical. The compatibility should be maintained. Be sure not to buy motors consisting of different HPs! 

For instance, there will be different sizes and specifications on a 12 hp vs 34 hp blower motor. 

Also, different types of fans have different HPs. Furnace blowers have more than 1/8 HP whereas utility fans are less than 1/15 HP.

You can find all this information in the specification sticker or in the manual. 

Remember to check these stats of your old furnace blower motor before switching to a new one.

Now we know what factors are necessary when choosing the blower motor size. Now, let’s see what size furnace blower motor you need.

What Size Furnace Blower Motor for You?

Whether you’re replacing the blower motor or setting up a new HVAC system, you’ll need to know the size required. It’s always better to replace it with the blower motor you previously had like this guy.

So you might be wondering, what size blower motor do I need? 

Well, in most cases, you can find the model number of your furnace to find the size. Match all the physical factors ( length, shaft size, diameter) and electrical factors ( RPM, Voltage, Amp, Horsepower) mentioned above to find the right blower motor!

If you can’t find the exact blower motor for your furnace, don’t stress. There are many universal blower motors, which fit most of furnaces.

For example, you have a 12 HP blower motor and can’t locate one with the same model number. You may buy a universal blower motor that runs at 14, 1/3, or 12 HP depending on the wiring used.

Here are some top Universal Furnace Blower Motors that you can choose from. All of these motors are recommended by previous users so feel free to choose any.

The right-hand rule is to get 440 CFM from your blower motor. If the size and electricity constraints match, then you’re good to go.

It’s not mandatory to find the exact size. So, you might ask, can i put a bigger blower motor in my furnace? 

The answer to this question is- yes! Yes, you can. However, you need to find a motor that satisfies the HP, RPM, voltage, etc factors. If the motor isn’t within limits, accidents might happen. 

For instance, fitting a higher voltage motor than recommended, might cause the ventless gas heater smell.

Now, these problems may get you worried. But you know what? If you take care of your blower motor, it’ll stay problem-free! 

Read the next segment to know how to maintain furnace blower motors.

Types of Furnace Blower Motors

There are many types of furnace blower motors. Let’s see some of them.

One-speed Vs. Multi-speed

One-speed blower motors can blow the air at one constant speed. You cannot alter the speed, it will stay the same.

On the other hand, multi-speed blower motors can be tweaked according to your preference. You can adjust the speed from low to high and vice versa. Also, it helps to create better airflow, a quieter system, and improved efficiency.

Source: Fire&Ice

Direct Drive Vs. Belt Drive

The installation of the direct-drive motor is done directly to the blower wheel. If you’re constantly trying to upgrade your tech, go with direct drive motors.

On the contrary, the belt drive blower motors are a bit rare; and cheap! The motor is connected to the blower wheel by a pulley in this system. 

If you’re looking to save some cost and settle with old techs, go with belt drive motors.

How to Maintain Furnace Blower Motor?

When considering any kind of equipment, taking care of it increases its longevity and performance. The same goes with furnace blower motors. Here are 2 tips regarding blower motor maintenance-

Cleaning the Blower Motor

Cleaning the blower motor will get rid of the dust and grease inside. The grease and dust buildup can cause a lot of problems; which you will be avoiding!

Source: Total Mechanical System

Additionally, the motor will heat less when it’s clean. Less heating will boost the efficiency of the blower motor. Cleaning the motor every 2-3 years is a mandatory task.

Here’s a tutorial guide on how to clean the blower motor.

Oiling the Motor

This is mainly for older furnaces. Old furnaces have blower motors that require oil. Now you might be wondering what oil to use on furnace blower motor.

Well, non-detergent motor oils can do the work. If you’re planning to buy the oil, check here-

If you use these oils, your blower motor can function a lot better!

So, if you clean and oil the blower motor, you can keep it healthy and efficient! Remember to turn off the power of your furnace before attempting any of the methods.

FAQs

Are duct booster fans worth it?

Duct boosters are worth it when you need aid for the fans of your furnace. These fans by no means can replace any component of the ductwork. However, the booster fans do a superb job at pulling air out of the furnace. You can consider booster fans as an inexpensive toolkit to avoid expensive repairs!

Does duct cleaning improve airflow?

No, duct cleaning will not improve the duct airflow. If there are small particles interfering with the airflow, then the cleaning can help. If you want temperature changes from the airflow, the duct cleaning won’t help. Additionally, even if the airflow improves, it’s not a permanent solution.

What speed should the furnace fan run?

Furnace fans should run at 1200 rpm. So, every 60 seconds, the fan should rotate 1200 times. Although, you have to check the furnace to know the speed limits of the fans. Most residential furnaces have a maximum 1500 rpm limit. If that’s the limit for your furnace, tone it down to 1200 rpm.

The Final Words

This is the end of this article. Hopefully, I have answered the question, what size furnace blower motor do i need?

If you’re in confusion, you can seek professional help! Contact your nearest HVAC professional to sort things out.

All the best!

Tim David

HVAC tech with over 30 years of experience. Retired and doing repair work on the side around Madison County, AL.