Problem:
Supply fan not controlling to duct static pressure setpoint
Description:
There are times when the supply fan will ramp up or down regardless of the active duct/supply static pressure reading and does not follow the active duct/supply air pressure setpoint. More than likely if the unit has BACNet controls, AO-39 (Supply Fan Speed Command BAS) is In-Service. This Analog Output commands the unit's supply fan speed through a BAS therefore supply air pressure control no longer has priority. The most convenient way to confirm this is by looking at the unit's Human Interface in the General System Status Submenu.
If the Member Role shows SLAVE then AO-39 is In-Service. They can also verify this with TU or if they are not at the unit through their BAS.
Note: Although we do not get many calls about it, if AO-40 (Exhaust Fan Speed command) is In-Service, it will also show as SLAVE in the Member Role as well.
Resolution:
Place AO-39 Out of Service and now the unit should control to the Active Supply Air Pressure Setpoint.
Note: There are times when it is desired to have BO-39 or 40 In Service. The most common application is multiple units on a common duct.
Note: Same issue could be present on MZVAV Symbio units. AV-10136 (Supply Fan Speed Setpoint) and AV-10137 (Exhaust Fan Speed Setpoint) are points available to control supply and exhaust fan speeds. These points should ONLY allow yo to control fan speeds if the Supply and Exhaust Fan Speed Setpoint Enable is set to ENABLED through BV-10122 and BV-10123 respectively.
WARNING:
Information in this article is intended for use by individuals possessing adequate backgrounds of electrical and mechanical experience and who comply with all federal, state, and local laws, rules, orders, or regulations related to the installation, service, or repair of a heating or central air conditioning product. Any attempt to install, service, or repair a heating or central air conditioning product may result in personal injury and/or property damage. The manufacturer or seller cannot be responsible for the interpretation of the information contained herein, nor can it assume any liability in connection with its use.