Updated: Jul 6
In this HVAC training article, we will be discussing how to adjust the fan speeds on an indoor PSC blower motor in furnace and air conditioning units. PSC stands for Permanent Split Capacitor and these motors can be easily identified by their direct connection to a capacitor.
Remember that CFM stands for Cubic Feet Per Minute. Roughly 400 CFM is needed for every 1 ton (12,000 BTU/HR) of heat removal capacity in air conditioning systems. 350 CFM per ton can be used in humid climates to remove moisture from the air. The BTU/HR capacity or tonnage of the air conditioning system should be included within the model number of the outdoor unit.
The blower speed for a gas furnace is selected so that the airflow is comfortable for the building occupant but also so that the Delta T (Temp Rise) does not continue to rise. If the Delta T continues to rise, this is due to low indoor airflow. Usually, gas furnaces have a temp rise of 50 F. Oil Furnaces may have a temp rise of 50-60 F. Remember that a heat pump in heating mode will need to run with roughly the same airflow speed as during AC mode even though it may not be comfortable for the building occupant. This is done so that the maximum amount of heat is gained in the building from the refrigerant.
120v PSC Blower Motor in a Furnace
Always turn off power at the furnace or air handler before changing blower motor speeds on the control board.
120v PSC blower motors may come with multiple power wires that are color coded. The white wire is usually the common and the other colors are for separate speeds. You may ask, how do I determine which wire is the highest speed and the lowest speed on a PSC blower motor? This is done by taking electrical resistance measurements using a multimeter.
In this example, we will be using a 120v PSC motor from a furnace. First, turn the power off to the furnace. Take a pic of where the blower motor wires are mounted. The white will be mounted to the common/neutral bar. Remove the blower motor wires from the control board or other connections. Place the red multimeter probe on the white common wire of the blower motor. Use the black multimeter probe to touch each of the other color-coded wires, one at a time. The electrical resistance from common to each speed is measured.
In this example:
The red and white measures 5.3 ohms.
The yellow and white measures 4.3 ohms.
The blue and white measures 3.3 ohms.
The black and white measures 2.6 ohms.
The resistance value of each pair changes, depending on the second wire selected. The second wire is the one other than the common in the pair. The color wire with the lowest resistance value is the highest speed. The color wire with the highest resistance value is the lowest speed.
In the example above:
Red= Lowest speed
Yellow= Second from Lowest
Blue= Second Highest
Now that we have determined the speed of each of the color wires on the motor, we will look at a 120 volt furnace control board. On the board in this example, the white wire is connected to the neutral/common location. The blue wire is connected on the heat terminal. The black wire is on the cooling terminal. The other wires are connected on M1 & M2. The M1 and M2 can be referred to as spare/park and these terminals do not connect to any circuit on the board. They are simply a place to park the unused speed wire terminal ends so that they do not accidentally short against a ground while the system is running. In this example, in cooling mode the unit is running at its highest fan speed selected. If the heat speed needed to be changed to a slower speed, we would simply turn the power off to the unit and switch the blue with the yellow wire. Then we would turn the system on and check the temp rise in heating mode mode to make sure that it does not continually increase.
I hope this helps you understand how to adjust the airflow speed on a 120v PSC Blower Motor! Keep an eye out for our article on adjusting airflow speed on ECM Blower Motors!
Also, if you are looking for a video to better help understand this topic, check out our "Adjusting HVAC Blower Speed CFM on Furnace & AC Units!" video below!
If you want to learn about refrigerants and how they work in a system, check out our “Refrigerant Charging and Service Procedures for Air Conditioning” book . Test your knowledge with our 1,000 question workbook along with the answer key! We also have quick reference cards for use out in the field! Bundle Packs are a great way to save and get faster shipping! Check out www.acservicetech.com/store
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Published: 05/12/2021 Author: Craig Migliaccio
About the Author: Craig is the owner of AC Service Tech LLC and the Author of the book “Refrigerant Charging and Service Procedures for Air Conditioning”. Craig is a licensed Teacher of HVACR, Sheet Metal, and Building Maintenance in the State of New Jersey of the USA. He is also an HVACR Contracting Business owner of 16 years and holds an NJ HVACR Master License. Craig creates educational HVACR articles and videos which are posted at https://www.acservicetech.com & https://www.youtube.com/acservicetechchannel & https://www.facebook.com/acservicetech/