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“TAB of SMALL TONNAGE ROOFTOP UNITS LESS THAN 25 TONS”
Ronald Ballard – AABC Certified TAB Technician
The Department of Energy (DOE)mandated that commercial air conditioners become more energy efficient by the year 2018. These new standards have made balancing of these units much more burdensome. We must first understand how to ensure the units are operating in high speed before proceeding with TAB. Manufacturers are deploying ECM type fan motors or VFD type to achieve these standards. ECM motors are deployed on direct drive type fans, while VFD’s are utilized on both direct drive and belt drive configurations. The question becomes how we ensure the unit is operating at high speed, the days of simply turning the fan switch on are all but gone. Manufactures are adjusting fan speeds to achieve these efficiencies. Typically, the fan is operating at 50% design cfm with just a call for fan only. The manufacturers are adjusting fan speed based on unit demand a call for fan only, staged cooling, dehumidification, and heat etc….
We need to investigate what type of fans the manufacturer is utilizing and how they are controlling them, and what types of controls are being utilized (thermostat, BAS) to operate unit. Manufacturers typically operate fans at 100% speed on a call for heat. This can be problematic with space comfort during testing. They also typically operate the unit at 100% fan speed on a 100% demand for cooling, determining if unit is at 100% cooling can also be problematic, time delays on units and thermostats can delay 100% cooling. Some manufacturers also factor in supply and return temperatures to control cooling demand. which will affect fan speed. If unit has dehumidification capability this also affects fan speed, if the unit has both a call for cooling and dehumidification, the speed will be set by the dehumidification demand (typically 80% of high-speed setting). There are strategies that need to be used based on a particular manufacturer’s configuration, and it’s time to explore some of those.
Many of the energy efficient units have digital displays installed and we can configure thru these displays to operate unit at high speed. Some manufacturers also allow VFD to be indexed to the hand mode, and high speed can be achieved. When VFD is in the hand position the outside air is still being controlled by unit controls (thermostat, BAS). It should be noted on belt drive configuration utilizing VFD’s, we must adjust or replace sheaves to achieve design cfm and NOT adjust maximum hertz on VFD. The reason is the manufacturer uses maximum hertz as a reference to determine fan speed when demand is less than 100%, adjusting maximum speed on drive can cause alarms and failures of unit at less than 100% demand.
Carrier RTU utilize 2 types of speed controls, the smaller units utilize an axial fan with an ECM motor, adjustments are made with a potentiometer on control board based off of external static pressure, a chart is attached to unit with information. The larger belt drive units utilize a VFD which can be operated in the hand position, some units have a jumper (yellow & white wire that is labeled) can be connected to operate drive in high speed, eliminating the need to operate the VFD in the hand position. The VFD is located in the blower section which also results in some challenges. A cable to extend drive display is available to remote drive display outside of blower section. It should be noted that the drive display is a factory option on these units. In order to ascertain amp draws from VFD a blue tooth type volt/ amp meter must be utilized, allowing amps to read remotely. All VFD data (amps, volts) must be obtained from leads that are on the outlet of the drive connecting to motor. Carrier units also deploy low and high speed settings for economizer minimum position.
Lennox RTU utilize a controller that has a balance function which can be utilized to place unit in high speed mode. The balance feature operates unit for (2) hours, then returns to normal operation. Units utilizing direct drive fans can be programmed through controller to set permanent high speed setting through cfm settings, in fan menu. Units utilizing VFD and belt driven configuration can be set to high speed through balance mode, fan menu, select damper then select high speed minimum oa position. CFM adjustments are made with sheave package. Lennox also utilizes low and high speed minimum outside air position.
Daikin RTU are set to high speed through unit controller, different controllers are utilized, so contacting Daikin tech support is probably the best option, unless unit literature is available. Daikin tech support phone number is 315-282-6434.
York/Johnson RTU are set to high speed through unit controller. Please see attachments. Johnson also utilizes low and high-speed minimum outside air minimum positions. The VFD is in the blower section which also results in some challenges. A cable to extend drive display is available to remote drive display outside of blower section. In order to ascertain amp draws from VFD a blue tooth type volt/ amp meter must be utilized, allowing amps to read remotely. All VFD data (amps, volts) must be obtained from leads that are on the outlet of the drive connecting to motor.
Trane RTU are set to high speed through unit controller, Trane utilizes both direct drive and belt drive units, Trane units come with VFD’s that do not allow hand operation. Trane also has low and high speed “OA” settings. Please see attachment for changing speeds.
Finally, when all else fails, read the O&M Manual that comes with the unit. Many manuals detail adjusting fan speed. If manual is not present, an internet search with model # for O&M manuals usually reveals digital versions.