A basic air handler is nothing more than a piece of equipment that circulates air through a building or space. As such, air handlers may be among the simplest pieces of equipment that service technicians deal with. However, nothing in the field is ever as simple as it seems. A malfunctioning air handler can cause huge comfort and energy efficiency problems for building owners and occupants.
When there’s no air circulating through an air handler, the most common cause is a broken belt. When repairing this problem, verify that the motor pulley and fan pulley are in good condition. It's also important to align the pulleys to prevent premature failure of the belt, the motor pulley, or the fan pulley.
If the belt is in place but the fan is still not turning, begin troubleshooting backwards on the electrical circuit. Check the incoming power to make sure that the overload reset isn’t tripped. Once you verify that the motor is indeed receiving power, then you can begin to troubleshoot the motor itself.
There are many reasons for a motor to fail, including:
- Improper voltage to the motor. This can be caused by incoming power being outside of the motor's operating range, as per the manufacturer’s specs.
- Improper wiring. Check the incoming voltage and ensure the motor is wired to the required specifications. A motor can operate at different voltages and it's important to wire it for the correct voltage. As part of the troubleshooting process, verify that there are no failed splices on the electrical wiring from the incoming electrical circuit breaker to the magnetic starter and to the motor. Failed conductivity through an electrical splice tends to cause breaks in the electrical circuit. This, in turn, will cause high current and may damage the electrical windings in the motor.
- Improper fuses. Verify that the disconnect fuses and overload heaters are sized accordingly to the motor's required amp rating. Using incorrect fuses can cause motor damage.
- Motor bearing failure. It's important to maintain the motor bearings and keep them properly lubricated in order to prevent premature motor failure. Bearing failure is a major reason why motors fail in air handlers. Bearing failure can be caused by over-lubrication as well as under-lubrication.
Check Fan Rotation
Let's say you've checked the air handler itself. The belt's in good shape, the motor's running, the fan's spinning, and you've changed the filters — but you still don't have proper airflow. Now's the time to check the correct rotation of the fan.
You must verify the direction of fan rotation to get proper air distribution. When the fan motor's running backwards, you'll get some pressure in the system, but you won’t get the full force of the air handler. This will directly affect comfort, because you’re not moving the proper amount of air. On a three-phase system, you can change two of the motor's electrical legs to change the rotation. On a single-phase system, you must change the wiring in the motor itself, if possible, to change the rotation.
After checking the belt, motor, filter, and fan direction, take one step back from the actual air handler and check the system's manual and fire dampers. Closed dampers will affect the direct distribution for the airflow.
Now, you should verify the operation of the high-static switch, which can trip if there are restrictions in the airflow. Check the main circuit breaker, where the unit's main power supply originates. Verify that it's in working order, verify voltage, and make sure you have the correct number of lines coming in. This is critical; on a three-phase system, you must have three lines of power, operating in balance. Make sure you have the correct voltage. It's unusual to see a motor installed in an air handler and wired for the incorrect voltage, but it does happen at times.
When it Can Get Complicated
If you're still not getting proper airflow, things might get complicated. Many air handlers now function off of a building automation system, which has the power to turn the system's fans on or off, and open and close the dampers. Verify the building control system's time clock, to ensure that it's giving the fan the correct command to operate, and is opening and closing the appropriate dampers.
Building automation systems generally just send an on/off command to air handlers themselves, but they draw the air handler into a more complex world. Many systems have variable frequency drives (VFDs) that adjust the air handler's fan speeds based on static pressure in the ductwork. When a VFD isn’t receiving a signal from the building automation system, it can either default to its minimum speed, or it can go to its high speed. In either case, that can also lead to a problem with your building comfort levels.
VFDs are a big part of air handlers nowadays due to the energy conservation push to get all possible efficiency out of the equipment, but, they unfortunately can wear out the bearings in your air handlers' motors. If you get bearing wear often, you may want to install a grounded collar around the motor shaft to help prevent premature bearing failure.
You must become knowledgeable about how the building's control system operates if you want to relate it to troubleshooting the system's mechanical equipment. It's important to understand the sequence of operation for each piece of equipment, and for the system as a whole.
Martin Naranjo is HVAC service manager at A.O. Reed, San Diego, CA. The company is Contracting Business.com’s 2007 Commercial Contractor of the Year. Naranjo can be reached at 858/565-4131, ext. 335, or by e-mail, at [email protected] aoreed.com