The speed at which your HVAC blower fan runs has a direct impact on how quickly the system cools and how much moisture it can remove from your home. Most HVAC systems have a single-speed blower fan, but some more energy-efficient systems have a variable-speed fan instead. This can improve energy efficiency as the fan can automatically change speeds to increase or decrease the rate at which it cools.
While a single-speed fan always runs at the same speed, an HVAC technician can still adjust the fan so it runs faster or slower. This is generally not necessary, but there are times when it can help to improve indoor comfort and energy efficiency.
AC Airflow Rates
The size of your AC unit determines how much air your blower fan needs to circulate to cool your home effectively. In turn, how fast or slow the fan spins determines on how much air it moves per minute. For a 1-ton AC, the blower fan needs to move between 350 and 400 cubic feet of air per minute. This doubles with each additional AC ton, i.e., 1,050 to 1,200 cubic feet per minute (CFM) are needed for a 3-ton unit, and 1,575 to 1,800 CFM are needed for a 4.5-ton AC.
How Airflow Impacts Rate of Cooling and Humidity
When your air-conditioning system was installed, your blower fan was most likely calibrated to provide 400 CFM per AC ton. The reason is that the more air volume the system circulates, the faster it will cool your home down to the desired temperature.
If the fan runs too slowly and doesn’t circulate enough air, it will cause your AC to need to run for much longer than it would if the fan was at the correct speed. As a result, your energy costs will be much higher since your system will run more frequently and for longer each time. This problem can also lead to your AC system freezing up all the time, which results when there isn’t enough hot air flowing over the evaporator coil. Luckily, having the fan run too slowly is quite rare since the HVAC technician should have measured and calibrated the airflow rate when the system was installed.
The bigger issue in Indiana and other Midwestern states is when the fan speed is set too high, as it can drastically limit how much moisture your AC can remove from your home. When your fan speed is set higher, your AC will likely run for only 15 minutes or so each time.
While the system is running, the evaporator coil is constantly absorbing heat and humidity from the air. The difference in temperature between the hot air and the cold refrigerant in the system causes condensation to collect on the evaporator coil, and this is how ACs help to reduce indoor humidity.
If your AC runs for only a short time, it will obviously absorb far less moisture than if it cooled more slowly and ran for longer. This can make it impossible for the system to have much of an effect on indoor humidity. This is why it is always recommended to have your fan speed set lower in humid climates than you would in a drier area. In this situation, adjusting the fan so it circulates 350 CFM per ton will help keep the home far less humid than if your fan ran at 400 CFM per ton.
How Is the Fan Speed Adjusted?
Adjusting the speed of your blower fan is something that an HVAC technician would do as it involves rewiring the fan. The process usually takes only a few minutes, but it can damage your AC system and reduce its effectiveness if not done correctly. Your fan should have three different wires that control speed. How fast or slow the fan runs is determined by which of the three wires is connected.
Once the speed wire is disconnected and replaced, our technicians will then need to test the system to make sure the fan is working correctly. They will also need to measure the airflow rate to ensure that the fan isn’t now running too fast or too slow for the system to cool effectively and efficiently.
At Tyler’s Heating & Cooling, our certified HVAC technicians can measure your airflow rate and adjust your fan speed if needed. If your fan speed is as it should be, we can also check to see if there are any other issues that are affecting how efficiently your AC cools and how well it manages your home’s humidity. We repair and service all brands of central AC and heating equipment. Our team also installs new HVAC units for residential and commercial customers in Mishawaka and other nearby towns in Indiana and Michigan. To schedule an appointment to check your fan speed, call us today.