Staged Air Volume (SAV) is a mandatory code as of January 1, 2018, and is used on Two-Stage Cooling Commercial Rooftop Units, some Single Stage Rooftops and Commercial Fan Coils. The purpose of the SAV is to match the blower speed to refrigerant stage.
Bryant uses a Variable-Frequency Drive system, which consists of a two-speed relay board and VFD control with a motor. A simple sequence of operation would be with a two-stage thermostat calling for G, Y1, or Y2. When the stat calls for one of these modes, the stat sends 24 AC volts to the main board, known also as the CTB. Once the 24 AC volts is applied to the CTB, it is routed through the CTB and it finds its way to the two-speed relay. It then comes out of the two-speed relay as 24 DC volts to the VFD, which then starts the motor. The motor will start off very slowly and gradually get to running speed.
The motor runs at two speeds, low or high. Low speed is 40 Hertz, 66% of its design speed, and high speed is 60 Hertz, a full 100% of its design speed.
When a rooftop unit is shipped from the factory, the VFD is programmed and ready to go at the correct speeds, with no further programming needed. If the original VFD fails then the replacement VFD needs to be programmed. The replacement VFD will come with instructions and a keypad; the appendix included in the instructions will provide the correct parameters. First, check what VFD controller you have. The original VFD controller has the part number on it, which will start with HK30WA. The second item you need to know is the Blower capacity, either Standard, Medium, or High. Match up the two items in the appendix and you will have your parameters.
Some VFD have been unnecessarily replaced, due to repeated lockouts. Just because a VFD repeatedly locks out it doesn't necessarily need to be replaced. The most common reason for lockouts is due to voltage, which you can check your Bryant remote keypad.
We have experienced some low voltage issues, but have mostly seen high voltage. To correct the voltage you must contact the utility, or install an aftermarket phase monitor. Some phase monitors will reset when all voltage faults are back to normal.
Important: If the keypad asks if you want to use the Startup assistant mode, please select no.
For further information on VFD operation, refer to a document called VFD Installation, Startup and Service in catalog #IIVFD-06. For more information on checking for proper voltage, refer to the installation instructions. The installation instructions will tell you how to determine if the voltage is balanced by checking leg to leg. Units with a VFD you should also check each leg to ground.
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