How to Adjust AC Fan Speed and Why It May Be Necessary

September 19, 2022
AC Fan Speed in Seattle, WA

The blower fan inside your home is one of the most important parts of your air conditioning and heating system. When your HVAC system is on, the fan is what circulates the heated or cooled air throughout the building.

If the fan isn’t running at the optimal speed, it can reduce the effectiveness of your heating or cooling system or create other issues such as high indoor humidity. This means that having an HVAC technician adjust the speed of your blower fan could hold the key to improving your home’s comfort and overcoming any AC or heating issues. Before we focus on how a technician adjusts the fan speed, let’s look more closely at how fan speed affects your HVAC system and why the speed may sometimes need to be adjusted.

Single-Speed vs. Variable-Speed Fan Motor

It is first important to note the differences between single-speed and variable-speed blower fans. The majority of residential HVAC systems use single-speed fans, which means that the fan will always run at the same speed. If you have a high-efficiency AC or furnace, you may have a variable-speed fan. In this case, there is no need to ever worry about the fan speed since the system will adjust this automatically as needed.

While single-speed fans only run at one speed, this can still be adjusted to either low, medium or high. As long as your AC system was installed or calibrated correctly, you really shouldn’t ever need to change the fan speed. Nonetheless, there are times when this can help to improve the efficiency of your AC or heating system.

How Fan Speed Affects Airflow Rate

The speed at which the blower fan operates determines the speed at which your home is cooled or heated. The faster the fan runs, the more air it will circulate every minute. Usually, as a result, your home will be heated or cooled more quickly to the desired temperature. Fan speed is more important for your AC than it is for heating as it will also affect the rate at which the evaporator coil absorbs heat from the air inside your home.

For your AC system to cool efficiently, the fan needs to be able to move a certain amount of air every minute. For every AC ton, you need between 350 to 400 cubic feet of air per minute (CFM), i.e. a 2.5-ton AC needs 875 to 1,000 CFM, a 5-ton AC needs 1,750 to 2,000 CFM, etc. The specific airflow rate is determined by the size of the fan motor and what speed it runs at. The ability to change the fan speed is important as the same size fan can be used with different sizes of AC units as long as it is set to the correct speed setting.

Reasons AC Fan Speed May Need to Be Adjusted

If the fan speed is too low, there won’t be sufficient air volume to effectively cool your home. This will obviously impact your home comfort. It also results in more wear and tear on your AC system and higher energy costs since your AC will need to run for much longer to sufficiently cool your home. This problem is quite rare since the technician that installed your AC should have tested to make sure the fan was set to the correct speed.

The bigger issue in humid climates like Seattle is if the fan speed is set too high. In areas that experience higher humidity, it is always recommended to have your fan run slightly slower than you would in a dry climate. In this situation, running your fan at 350 CFM instead of 400 can provide major benefits in terms of managing your home’s humidity level.

If the fan runs faster, your AC system will cool your home much more quickly. The issue with this is that the system won’t be able to absorb that much moisture from inside the home when it runs for a shorter time. Reducing the fan speed also reduces the rate of cooling. This means that your AC will run for longer each time, which allows it to remove much more moisture from the home.

While having your AC run for longer may seem like a bad thing in terms of energy costs, the difference is usually minimal since your AC uses a huge amount of electricity each time it turns on and off. In addition, reducing the humidity also reduces the strain on your AC as it is more difficult to cool humid air than it is dry air. Once the system has effectively reduced the indoor humidity level, it will actually end up needing to run less often and for shorter times since it will then cool more effectively each time it turns on.

How an AC Technician Changes Fan Speed Settings

Changing the speed of your blower fan is a relatively simple task. However, it is not something that you can do on your own as it requires specialized knowledge of fan motors and how they are wired. In addition, attempting to change the fan speed on your own or performing any other work on your HVAC system will void its warranty. You are also likely to cause serious damage if the job isn’t done exactly right. For these reasons, it is essential that you give the team at Brennan Heating & Air Conditioning a call if you think your fan speed may need adjusting.

The blower fan has three color-coded wires. One wire is for high speed, one is for medium and one is for low speed. Changing the fan speed involves disconnecting the speed wire and then connecting one of the other wires instead.

Before making any changes, the technician will first need to determine which speed the fan is currently set to. This can be difficult if you don’t know what you are doing as each manufacturer has its own color-coding system. The yellow wire on one blower fan may be for high speed, but it could be medium or low speed on a different fan.

If the low-speed wire is connected, your fan is already running as slow as it can. As a result, you would need to find some other option for controlling your home’s humidity, such as a whole-home dehumidifier. If either the high- or medium-speed wire is connected, the technician can switch out the wires so the fan runs at a lower speed.

They will then need to turn the system back on and measure the airflow rate to ensure that there is enough volume for the system to work effectively. This is important as, again, the system won’t work properly if the speed is too low. In most cases, this problem will cause your evaporator coil to quickly freeze up every time your AC runs. This puts a huge amount of stress on the compressor motor and can cause it to burn out. A frozen evaporator coil will also cause the system to blow hot air, and it won’t work correctly again until it has fully thawed out, which could take up to a day.

If you think you may need to have your fan speed adjusted or any other AC repairs or services, the team at Brennan Heating & Air Conditioning is ready to help. Our certified technicians work on all brands of AC equipment, and we will inspect your entire AC system to quickly determine what is causing your specific issues. We also install, repair and maintain heat pumps and furnaces in Seattle and the surrounding areas. To schedule a time to have your AC inspected, give us a call today.

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