Attach 08 - HVAC Sequence of Operation.pdf

Air Condensing Unit - Sequence of Operation

Air Condensing Unit shall operate in accordance with a pre-programmed schedule written to the Supervisory Controller Scheduling Program. The system shall index from occupied to unoccupied and back in accordance with this schedule. The resident schedule shall also maintain holiday schedules and Daylight Savings changeover times.

Ventilation Delay Time Mode - Supervisory system shall maintain occupied enable times. The supervisory controller based upon ambient conditions and system historical performance data will calculate a time period. This time period shall represent the Ventilation Delay Mode Time. From the point the system is indexed to occupy, to the expiration of the Ventilation Delay Time, the ventilation mode of the unit's control loop shall remain disabled. Upon expiration of the VDT, the economized dampers shall be enabled as described below.

Fan Operation Occupied - When the system is indexed to the occupied mode, the Supply Fan's Variable Frequency Drives are enabled. Based on the Duct Static Pressure Setting, the fan will modulate in order to maintain the systems static pressure setting. Upon indexing the system to the unoccupied mode, the supply fan shall turn off and cycle based upon the unoccupied temperature set point.

Fan Operation Unoccupied - When the system is indexed to the unoccupied mode, the supply fan shall shut down.

Economizer – When the Outside Air Temperature (OAT) falls below 58 DegF (adjustable), economizer mode is enabled and mechanic cooling is locked out. Outside air dampers and return air dampers shall modulate to maintain mixed air temp on 55DegF (adjustable). When OAT is above 58 DegF, outside air dampers and return dampers shall go to minimum position and mechanical cooling is enabled.

Discharge Temperature Control Occupied - Discharge Temperature Sensor, DAS serving digital controller, shall operate the temperature control loop in the following sequence. Upon the start of supply fan, the controller, shall modulate 3 way hot water value in order to maintain a constant 58° F (adjustable) discharge air temperature. Upon a continued rise in Discharge Air Temperature, Controller shall modulate the outside dampers open and the return air damper closed. Upon a fall in Discharge Air Temperature, the reverse shall occur. Discharge air set point will be automatically reset by controller according to return air temp.

Hot water circulator pump and Boiler control: Hot water circulator and boiler will be enabled at OAT below 65 degF (Adjustable)

Three way hot water mixing wave will reset hydronic radiator loop water temperature to maintain temperature according to the following reset schedule: At 0 DegF OAT, water temperature is 180 DegF.

At 65 DegF OAT, water temperature is 100 DegF.

Safeties - A freeze stat, located in the hot water coil discharge, shall act to shut down the system in the event its set point is exceeded. A system shutdown shall cause the following actions to occur. Supply fan and return fan shall shut down and lock out. Power to the outside, return and relief air dampers shall be shutdown causing those dampers to return to their normal positions via a mechanical spring return action integral to the damper actuators. Power to the heating hot water valve actuator shall be shutdown causing the valve to return to its normal position (full open to the coil) via a mechanical spring return action integral to the valve actuator. This freezestat shall require manual reset. Actuation of the freezestat shall also initiate a network alarm at the Supervisory Controller.

Smoke Detectors - Smoke detectors (by others) located in the supply and return of the system shall operate to shut down the system in the event that products of combustion are sensed in the airstream.

Upon shutdown the supply fan and return fan shall stop and all dampers shall go to their normal positions. These detectors shall require manual reset. This sequence contains no allowances for the interface of duct smoke detectors to the building fire alarm system.

Sequence of Operation

Variable Air Volume Controls

The control damper of the VAV box shall modulate in response to the signal from a flow sensing element at the discharge or inlet of the VAV box to a microprocessor based VAV box velocity controller. The velocity controller shall modulate the box damper from the minimum position the full flow position based on a signal from a space mounted temperature sensor in a sequence designed to maintain an adjustable space set point. This sequence shall be as follows. Upon a rise in space temperature the VAV controller shall modulate the VAV damper open from minimum flow to maximum flow. Upon a fall in space temperature the reverse shall occur. On a rise in space temperature the reverse shall occur.