November 2011

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BAS Interface Methods to Packaged Equipment

Can We Talk?

Steven R Calabrese

Steven R. Calabrese
Control Engineering Corp.

Contributing Editor

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This month we examine the BAS interface methods to the various types of packaged equipment used in commercial HVAC systems. More specifically, how we monitor and control this equipment, nowadays and in the past. Of course these days most of the larger packaged equipment (rooftop units, chillers) will have the option of a communication interface, whether it be any one of the current standards (BACnet, MODBUS, etc.). In fact, more and more these “options” are becoming a standard offering, seeing that, more often than not, the requirement for BAS integration is written into the specifications. Which begs the question, “What did we do before we had this technology at our disposal?”

Here we take a look at four major pieces of packaged equipment, how we managed to monitor and control prior to the “era of integration”, and how we typically do things in the present day.

Rooftop Units

We make a distinction or two here between single zone, constant volume rooftop units, and VAV rooftop units designed to serve multiple zones. For one, the single zone units tend to be small in size, almost “unitary”. The point here is that, as equipment grows in size and complexity, there is more merit in equipping the unit with a communications option. Certainly for a small-tonnage packaged RTU serving a single conference room, there’s not much point of hooking up a comm line to it, even if the unit manufacturer offered this on such a small and uncomplicated piece of equipment. On the other hand, the larger ton units designed to serve larger zones such as open offices and warehouse spaces, may very well be able to be offered with a communications option.

The second distinction between the single zone units and the VAV units is their method of control. For the single zone unit, zone control is typically performed via a thermostat located in the zone and wired back to the unit. The thermostat can be programmable, and can even have a communication option built into it. Or you can stick a digital controller in the unit’s controls compartment, do a terminal strip interface to it, and digitally communicate with the controller. For the VAV unit, one that is designed to serve a system of VAV boxes and/or fan-powered boxes, unit control is typically implemented to maintain two important setpoints: Discharge Air Temperature and Duct Static Pressure. The fact that these units serve not one but many zones almost ensures that their physical size and operational complexity merits a communications option.

So we’ve established that, for rooftop units, the small-ton single zone units will typically not have the comm option, however as these single zone units increase in size, there will be a point at where it makes sense, from an installation and operations standpoint, to have the option. As for the VAV units, these will always merit a comm option, whether it’s purchased and used or not. The control points communicated through the connection include the two mentioned above: Discharge Air Temperature setpoint and Duct Static Pressure setpoint. Of course include start/stop as well, typically based on time-of-day scheduling. Monitoring points abound, and can include a multitude of internal temperatures, electrical attributes (voltages and current values), and heat/cool (furnace/compressor) status.


Starting again with physical size, we see that, as with the single zone rooftop units, size does matter, at least to a point. The thing about chillers is that you’re rarely purchasing a chiller for a single zone application. So chillers, like VAV rooftop units, are manufactured to be applied in multi-zone applications, where the chilled water is pumped out to multitudes of equipment, whether it be the chilled water coils of several large air handling units scattered about the facility in question, or out to dozens and dozens of fan coil units serving single-zone spaces throughout the building.

The point is, chillers will tend to be large and complex enough to merit a good quantity of monitoring and control points that are “built into the package”, so to speak, and therefore are good candidates for performing a communications interface to, in order to gain access to all those internal points. The main control point to be communicated is Leaving Water Temperature setpoint. To be able to send the chiller this setpoint via the Building Automation System has some real advantages. Whereas in the past you would need to establish the setpoint via the chiller’s on-board controller, and maybe have the ability to reset this setpoint by feeding the controller a variable signal, now with direct communications with the chiller’s controller, you can send over the setpoint value, change it at any time based on any conditions, and resend as often as required.

Monitoring points acquired from the chiller controller via the communication connection include internal (refrigerant) pressures, various temperatures, and electrical attributes of the compressors.


More and more these days you see that your standard gas-fired hot water boilers are being offered with a communication connection. So what does this buy you? Well, again you get all kinds of monitoring points that you wouldn’t typically have access to, in addition to some control points that can “make life easier” in terms of operating the boiler via the BAS, especially when there is more than one boiler. For instance, with several boilers and communication to each of them via the BAS, the BAS effectively becomes the “boiler sequencer”, and can perform all functions of boiler sequencing via the communication connection, such as, boiler staging, firing rate control, common hot water temperature setpoint control, and hot water reset based on outside air temperature. Oftentimes boiler manufacturers will offer their own pre-engineered boiler sequencer, which may even have a comm option as well. But when you have a Building Automation System that can speak directly with the boilers, you can effectively “eliminate the middleman” and perform all facets of monitoring and control via direct communication between the boilers and the BAS.

Variable Frequency Drives

VFDs have long been offered with a communications option. For good reason…there’s a ton of info that can be transferred between the VFD and the BAS. Historically the interface between the BAS and a VFD has been one of hardwired points between a digital controller tied into the BAS network, and the VFD’s terminal strip. These hardwired points include start/stop command, speed reference signal, status, and alarm or fault.

There is ongoing debate on whether these traditionally hardwired points can be communicated through the network cable, or should be left as hardwired points, even in addition to having communication with the VFD. In particular, a concern lingers with respect to communicating the speed reference signal via the network. Since this is not a setpoint that is being modified, but an actual control signal, the concern is that the network communication is not fast enough to handle a true PID control loop. So what you typically see is that while the other points that are typically hardwired (start/stop, status, alarm…) may be allowed to be communicated, the speed reference signal will still be a hardwired point, direct from a digital controller’s analog output. While there may likely come a day when we become totally reliant on the network cable for all facets of control, the reality is that we’re not quite there yet, and the consulting engineers will always favor on the side of caution when it comes to guaranteeing an efficient and workable system.

Tip of the Month: Look for more types of packaged equipment to come with factory-embedded digital controllers and communications options. Some that have already hit: packaged fan-coil units and water source heat pumps. Careful though…you need to be sure that these units communicate at the same speed as other controllers that you hang on that same network, as traffic can only go as fast as the slowest device on the network (sounds like the freeway in the morning!).


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