Overriding Existing Schedules

Your facility most likely has some HVAC schedules already in place for your day-to-day operation of your HVAC equipment.  To use Events2HVAC, you will eventually either need to deactivate those schedules or take steps to override them so that Events2HVAC can send commands to equipment automatically. Ideally, you want to leave your existing schedules in place in case the server is shutdown and automatic scheduling through Events2HVAC is disabled. It is always a good practice to have a backup plan.

Most schedule objects in HVAC systems can be setup to work on a fixed daily schedule, a manual event schedule, or a holiday schedule.  The schedule object commands one or more individual points that turn the equipment on or off.  The command sent to these items are sent at a specify priority level. When setting up Events2HVAC, you will need to determine the individual points to command and the priority level at which the schedule object is sending the commands. When you define the actions for equipment, you will need to send the commands to those same points using a priority that is higher than your existing schedule object.

There are several schedule override strategies that can be employed when integrating Events2HVAC to your existing systems.  Some require little, if any, preparation of the existing HVAC system.  Others may require your HVAC technician to make some programming changes and/or add some additional objects. This document describes four different methods for overriding schedules in the case of an outage.

1.  Overriding at a Higher Priority

Note: This method has minimal impact to your existing HVAC system, but requires more manual steps to revert back to backup scheduling.

 

Figure 1- Overriding Priorities

This example shows a BACnet schedule object that has a weekly schedule assigned to turn on AHU-1,2,3 from 8am to 8pm.  This object sends start commands at a priority of 15.  In Events2HVAC, we created 3 equipment items AHU-1, 2, and 3.  For each item, we created an action that sends a command on each events start/stop time.  This action will send the command at priority 14 (higher) so that it will override any command that is sent from the schedule object.

In this scenario, individual rooms can be assigned to each AHU in Events2HVAC, allowing individual room schedules to turn on/off each AHU.

The proper method to reverting back to default schedules in this case is to release the priority 14 commands on each of the commanded zone objects.  The objects will then revert to the commands coming from the schedule objects at priority 15.

2.  Override at Higher Priority and Release

Note: This method has minimal impact to your existing HVAC system, but requires more manual steps to revert back to backup scheduling.  This method well suited for the need to have existing base schedules for equipment or a method create exception schedules on the HVAC side.  EventsHVAC will turn the system on when needed at an overriding priority and when done the equipment is release back to the base scheduler.

Figure 2 - Overriding and Releasing Priority

Use this strategy if you want your default base schedules to operate during normal business hours.  For after-hours events, E2H will override the individual HVAC equipment for the start of the event.  When the event is finished, the override priority (14) is released so that the last commanded priority 15 state will take over.

This method also allows users a way to enter exception schedules using there HVAC scheduler whenever they have an event that may not be entered in the reservation system.

The proper method to reverting back to default schedules in this case would be to release the priority 14 commands on each of the commanded zone objects.  The objects will then revert to the commands coming from the schedule objects at priority 15.

3.  Commanding the Schedule Object Directly

Note: This method requires additional scheduling objects to be created in your HVAC system.

Figure 3 - Commanding Schedule Object Directly

In this scenario, according to the BACnet specification, if you set the schedule object's OUT OF SERVICE flag to TRUE, then the present value property of the schedule object will no longer be calculated internal and can be commanded directly by an external source.  To revert to the default schedule, set the schedule object's OUT OF SERVICE flag to FALSE.

Since most scheduling objects are created for fixed building schedules, these schedules will typically start/stop the entire system including the individual room VAV boxes.   Choosing this method requires the creation of room level scheduling objects since the ultimate goal is to perform dynamic room level scheduling.

You could expand the user-friendliness of the switchover by adding a global switch binary object and have it set the OUT OF SERVICE flag directly to all of the schedule objects as an interlock so that the operator only has to switch one point to enable or disable the automatic scheduling.

4.  Isolating and Adding Logic for Events2HVAC commands

Note: This method is probably the most eloquent and user-friendly switching method, but will require additional programming and object creating on the HVAC system.

 

Figure 4 - Switchover Logic

This option adds logic to the controllers and binary points to regulate the commands coming from Events2HVAC.  Additional binary points are created in the HVAC system for each item that Events2HVAC is commanding; and a global binary object is created to indicate how you want to control your rooms.  

If the global switch point (SCHED-SW) indicates that you want default schedules, all of the commands originating from default schedules are sent to the room points.  If the global switch point indicates that you want Events2HVAC to control rooms, default schedules are blocked and the binary room points that you added for each control point will command the direct room points.

This kind of control takes more time because you have to add all of the binary points and the necessary programming logic. However, it will allow the Events2HVAC binary points to be tracked using point history and provide a more orderly switch over when it is needed.

For automatic switch-over strategies see: Schedule Fail-over Strategies

Summary

Whatever method you choose, make sure you discuss it with you HVAC representative and/or your local HVAC operators.  They may have some insight into a better way of accomplishing the end result. Also, remember to document the procedure so that the person on-call knows what to do in case of an outage.

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