August 2008
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Anatomy of a Typical BAS

Analysis of a hypothetical commercial office building DDC system

Part 3 of 3

Steven R. Calabrese

Steven R. Calabrese
Automated Logic Chicago
Contributing Editor

Unitary Controllers Primer  Typical applications utilizing unit (zone) level control  Part 2 of 3 
Typical Controllers in DDC
Understand the differences in controller types and applications  Part 1 of 3

In this, part three of a three-part series on digital controllers, we investigate the roles played by the various types of systems and equipment that are typically operated by digital controllers, and also how all of these individual systems and equipment come together as one, under the central control of what we universally refer to as a Building Automation System (BAS). Recall from the previous parts of this series that there are four types of DDC controllers: unit level controllers, equipment level controllers, plant level controllers, and building level controllers. Also recall from previous discussions that this is based on what we’ve been referring to as the “traditional model”, remembering that controller types can vary from manufacturer to manufacturer, yet when all is said and done, they still in the end offer the same functionality and perform in similar fashion.

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For our purposes, we consider a hypothetical commercial office building that stands 6 stories tall, with a boiler/chiller plant in the basement, a VAV air handling unit in the penthouse, and terminal units (VAV and fan-powered boxes) serving all of the office spaces on all floors. We’ll start at the plant level, and drill our way down to the zone or unit level. With each segment we’ll present a concise view on the equipment, and on how the chosen controller suits the particular equipment served. Lastly, we’ll discuss networking and front end issues, keeping it brief, for that, my friends, is a topic in itself, and quite possibly the subject of a future column!

Central Plant

The central plant for this hypothetical system consists of a pair of hot water boilers, a pair of hot water system pumps, a pair of water cooled chillers, a cooling tower, a pair of condenser water pumps, and a pair of chilled water system pumps. Its role in the overall building HVAC system is to produce chilled water for the VAV air handling unit, and produce hot water for the reheat coils at the zone level equipment (perimeter fan-powered boxes). Either system (hot or chilled water) can be in operation at any given time, and often will be enabled to operate simultaneously. Control points for these systems are numerous, and include enable and status points for all of the equipment mentioned herein, as well as sensor and transmitter points throughout the piping systems.

A plant level controller is selected to provide total system control of the hot/chilled water plant. As such, this controller will have a large point capacity, and may be a single controller, or multiple modules making up the same virtual controller. The controller will take its place inside its own suitably sized and appropriately located control panel enclosure, and all input and output devices will be wired back to this panel and terminated at the proper I/O. The outside air temperature (and humidity) sensor will typically terminate at this controller, and this information will be used to determine the proper modes of operation for the central plant, and will also be broadcast to other controllers on the network, as global data to be used in determining the operational modes for other systems and equipment. Programming for the central plant will be in accordance with the specified Sequence of Operation if given, and will integrate all subsystems so as to seamlessly function as a coordinated overall system.

VAV Air Handling Unit

The VAV air handler is the building’s main source of cooling. The air handler consists (minimally) of a supply fan, a return fan, a chilled water coil, and modulating economizer (outside/return/exhaust air) dampers. The supply fan will be on a variable frequency drive (VFD), and will be controlled to maintain a suitable supply duct static pressure setpoint. The return fan will also be on a VFD, and will operate to maintain a suitable space static pressure setpoint. The cooling coil control valve and economizer dampers will be controlled to maintain a suitable discharge air (cooling) setpoint. The VAV air handling unit’s prime function is to blow air at a variable rate and at a constant temperature.

An equipment level controller with a suitable point count is chosen for control of the VAV air handler. This controller will provide for all facets of unit control, and will accommodate all input devices (temperature sensors, pressure transmitters, status switches, etc.). This controller, like that for the central plant, will reside in its own control panel enclosure, and will be installed in close proximity to the air handler, as space permits. All air handler I/O will be wired back to this location and terminated at the controller, within the enclosure. Of particular note with this system is the requirement for the supply duct and space static pressure transmitters. These devices will likely be located some distance from the control panel, and will need to be terminated at this panel, unless the consultant will allow for these points to be terminated at a controller that is physically closer to their installation location, and have the information broadcast over the network to the air handler controller (not typically recommended or allowed).

A single controller may be able to accommodate all required points for this type of system, however most manufacturers will allow for expansion by offering the capability to install add-on modules in the case that a single controller doesn’t fit the bill. And be mindful also that oftentimes these systems are specified to have spare points available, in the event that additional I/O needs to be accommodated for somewhere down the line. In the end, it’s just good practice to have more I/O available than what is currently required, as in practice it always seems that another point or two is added to the scope of the system, and it’s nice to have this available rather than trying to re-engineer the system in the field to make room for one more point.

A final note: as with any controllers these days that make up a distributed DDC system, the equipment level controller operating the VAV air handling unit will operate completely “stand-alone”, meaning that, if network communication goes down for any reason, the controller will continue to operate the equipment in a stand alone mode using its local I/O, and using “last known good values” for global shared data such as outside air temperature and time-of-day schedule. A good reason to have those supply duct and space pressure points terminated back at the air handler controller!

[an error occurred while processing this directive] Zone Level Terminal Units

At the zone level, we have the terminal units. For the interior spaces we have cooling-only VAV boxes. And for the perimeter zones we have parallel fan-powered boxes with hot water reheat coils. These terminal units are employed to provide zone level control. They all receive cool air from the VAV air handling unit. The interior units are little more than zone dampers, and operate to provide varying amounts of cool air into the spaces served, in order to achieve and maintain zone setpoints. The perimeter units are capable of providing heat into the zones as well. For a parallel style fan-powered box, the fan is the first stage of heat, and cycles on to draw air from the plenum space when the zone temperature drops below setpoint. On further drop from setpoint, the hot water valve is modulated open to provide additional heat.

Each and every terminal unit will be equipped with a zone or unit level controller, suited to fit the application. As for the cooling-only boxes the point count would typically be limited to a couple of AIs from the space sensor. That in addition to the points already provided (and factory-wired to the controller) by the on-board flow sensor and damper actuator. For the perimeter units, additional points required would be as follows: fan control (DO), hot water valve control (AO), and discharge air temperature sensor (AI).

Again, as mentioned above and typical with all digital controllers, these unit level controllers will operate in a stand-alone mode. If network communication is lost, rest assured that the individual zone level equipment will continue to operate and provide comfort control to the space served, right up until that network problem is diagnosed and communication is restored.

Network and Front End

All of the aforementioned controllers are connected together, in daisy-chain fashion, back to a network level controller. As I keep reiterating throughout this column, all of these digital controllers making up the Building Automation System (BAS) are completely capable of operating in stand-alone fashion, with no other information or support from any other controllers. However it is only when you connect all of these controllers together and back to the network controller, that you can call this system a true BAS. The network controller coordinates the operation of all of the various interdependent subsystems, by distributing data among the controllers, sharing system-wide data such as outside air temperature/humidity and true time-of-day, implementing central time schedules, and collecting trend data and logging alarms. The front end is simply the operator workstation computer that is used to access the BAS, and is connected to the network level controller, via a hardwire connection, or more commonly nowadays, virtually through some web-based means of connectivity. The technology afforded in this day and age allows access to a modern-day building’s BAS from any computer worldwide, requiring nothing more than an internet connection and a password! Need to check your building’s lobby temperature from your vacation niche in the Bahamas? No problem at all!

Tip of the Month: A very popular practice among controls contractors, with regard to terminal unit controllers, is to have the controllers shipped to the VAV box factory, for factory mounting and wiring. This requires focused coordination between the contractor and the manufacturer’s rep, for even one small wiring error gets replicated many times, depending upon just how many terminal units you are required to control! For fan-powered boxes with electric heaters, this course of action does have its merit, if articulated properly. But what about the cooling-only VAV box? Or the fan-powered box with a hot water coil (for which the requisite control valve is installed and wired in the field)? Does it make any sense to employ factory installation of controllers in these cases? For the cooling-only VAV box, probably not, seeing as the only thing the factory will be doing is mounting the unit level controller (with on-board damper actuator and flow sensor) and making the flow sensor poly tubing connections to the flow probe assembly. For the fan-powered box with hot water heat, maybe, as additionally the factory would be charged with wiring the fan relay and possibly the discharge air sensor. The tip here is to think about this before blindly sending a hundred controllers to the factory for third-party installation.


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