In order to service and troubleshoot a large Air Conditioning System with a supply and return fan a service technician must understand the unit’s sequence of operation. This is the order of events the system undergoes to cycle itself on and off. Knowing how the unit operates properly aids in determining where to start troubleshooting when the system doesn’t operate properly. Where the system varies from its normal sequence is a major clue to the solution of a problem So let’s take a look at a typical supply fan systems sequence of operation with Direct Digital Controls.
The sequence of operation:
The DDC controller uses a temperature sensor mounted in the supply air duct to modulate control valves or mixing dampers to maintain a supply air temperature setpoint. In most systems that employ a heating and cooling coil, the hot water valve and the chilled water valve should be modulated in sequence.
When the supply air temperature falls below setpoint, the hot water valve begins to modulate open and consequently, the cooling valve begins to modulate closed. If the supply air temperature continues to fall below the setpoint, the heating valve will open fully and the cooling valve will close completely.
When the supply air temperature rises above setpoint, the hot water valve begins to modulate closed and consequently, the cooling valve begins to modulate open. If the supply air temperature continues to rise above the setpoint, the heating valve will fully close and the cooling valve will open completely.
A temperature sensor located in the mixed air stream (between the unit filters and the coils) is used to provide mixed air control. When the temperature sensed by this element falls below the setpoint, the outside air damper fully closes, the return air damper fully opens, the exhaust air damper closes to a minimum position, and the valves on all coils will fully open. This sequence should always be used on systems with wetted coils. When the unit fan is turned off, the outside air damper fully closes, the return air damper fully opens, the exhaust air damper fully closes, and all control valves return to their “normal” positions.
By Roger J. Desrosiers
About the Author:
Roger has over 40 years experience in Air Conditioning and Refrigeration. He is also a member of R.S.E.S., CM, The Association of Energy Engineers, Certified Energy Manager, ASHRAE, Certified Pipe Fitter, United Association and is 608 Universal Certified. Roger is a contributing faculty member of HVACReducation.net.
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