The thermometer reached 75 this past weekend in Chicago, which means we’re officially entering cooling season. Cooling season is the industry term for when we use our air conditioning units – compared to heating season for when we use our furnaces.
Most people cross their fingers when turning on their air conditioners for the first of the year and they hope everything will function properly. That’s not really the best approach. This guide can be used as a high level overview of the residential HVAC system components used during cooling season and how to troubleshoot some problems that might arise.
The two main components of your residential HVAC cooling system that we'll focus on in this guide are the indoor air handler and the outdoor condenser unit. Both need to be functioning properly in order for your house to stay cooled and comfortable.
Let’s start with your indoor air handler.
Mounted inside most residential air handlers is a direct drive blower motor. This is the motor that circulates cooled air in the summer, and warmed air in the winter, throughout your house. To move air, a blower wheel – sometimes called a squirrel cage – is secured directly onto the shaft of the blower motor using a set screw.
Some OEM blower motors have 3 or 4 legs welded to the side of the motor that are used to mount the motor to the blower housing. This mounting style is called the torsion flex mount. There are torsion flex mount replacement blower motors available, but they’re usually much more expensive than traditional blower motors.
It’s good practice to replace your run capacitor when replacing your motor and it’s important to note that the correct run capacitor is chosen by the requirements of each specific motor. You might be replacing a ½ HP blower motor that requires a 7.5 microfarad capacitor with a ½ HP motor that requires a 10 microfarad capacitor. Avoid the headache by replacing the run capacitor when replacing the motor.
Now, let’s take a look at your outdoor condenser unit.
Your outdoor condenser unit is the heart of your central air conditioning system. Without getting too far lost in the weeds of thermodynamic principles, refrigerant is circulating through the system’s coils where it transitions from hot to cold, gas to liquid depending on where in the line it is at. To help cool the very hot gases after they pass through the compressor, a condenser fan motor is used to pull air from outside of the unit over the unit’s coils and out the top of the condenser unit.
Most residential air conditioning units use a thermostat as a control to turn the system on and off as the room’s temperature fluctuates in and out of a defined temperature range. When it’s time to supply power to the condenser unit, the thermostat sends a signal to a switch called a contactor – which closes the circuit and supplies electricity to the compressor and the condenser fan motor.
Residential condenser fan motors are most commonly mounted to the top screen of the condenser unit with the motor's shaft pointing down. Other condenser units require the condenser fan motor to be mounted with the shaft up or even horizontally. While many condenser fan motors are totally enclosed and rated for all angle mount, others are open ventilated and manufactured for a specific mounting direction. It's important to select a motor that is rated for the corrected orientation of your condenser unit.
The condenser fan blade is mounted to the condenser fan motor shaft using an interchangeable hub. The proper hub is selected by considering the fan motor's shaft diameter. Most residential condenser fan motors are 48 frame motors with a 1/2" diameter shaft and many commercial condenser fan motors are 56 frame motors with 5/8" diameter shaft.
When replacing a condenser fan blade, it's important to select one that is properly rated for your condenser unit and your condenser fan motor. We use a formula that considers the diameter of your existing fan blade, the HP rating of your new condenser fan motor and the motor's RPM rating. Selecting an improper condenser fan blade can cause your system to not function properly, so it's important to double check your work and make sure that you're using the proper replacement condenser fan blade.
Solution: Most direct drive blower wheels have sleeve bearings which need to be maintained. For sleeve bearing motors, it is recommended to lubricate the bearings with a few drops of twenty weight non-detergent at the start of each heating and cooling season.
Problem: The blower motor shaft is completely locked up.
Solution: If your blower motor’s bearings have completely failed and the shaft is locked up, the motor will need to be replaced. Bearings often fail because the fan blade, or blower wheel, that the motor is driving isn’t balanced properly – which causes the shaft to rotate in an oval pattern and not a circular pattern. This puts added stress on the poles of the bearings which leads to their failure. If your blower motor’s bearings are looked up, it is highly recommended to replace the blower wheel when replacing the blower motor.
Problem: Your motor is having a hard time starting or simply won’t start.
Solution: If your motor is having a hard time starting or seems sluggish, you’ll want to check the motor’s run capacitor. Use a multimeter in capacitance mode to get a microfarad reading from the run capacitor. If the microfarad reading is in line with capacitor’s rating, then the capacitor is good. If the microfarad reading is significantly lower, then the capacitor is bad and will need to be replaced.
Problem: The condenser fan motor doesn’t run or the air conditioning system isn’t cooling properly.
Solution: As with the direct drive blower motor, if the condenser fan motor doesn’t start or the system doesn’t seem to cool properly, the first item that you’re going to want to check is the unit’s dual run capacitor. A dual run capacitor is rated for both your condenser fan motor and also your compressor, so you’ll have to pay close attention to make sure any replacement is rated properly for both items.
Tip: Set screws often require the use of an Allen or Torx wrench – so it’s wise to have one available.