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Unitary Controllers Primer
Part 2 of 3
Steven R. Calabrese
We start this month’s column by reiterating some of the points laid out in the first part of this three-part series, that segment which pertained to unit level, or unitary controllers. As we recall from that column, there are basically four types of controllers that make up the “traditional model” of a manufacturer’s DDC product line, listed again here as Unit Level, Equipment Level, Plant Level, and Building Level controllers. Please feel free to reference the previous column featured back in April of this year. Part three of this series will venture to depict a Building Automation System based on the traditional model of controllers, and showing each style of controller as applied to the particular equipment its best suited for.
So with that aside, let’s recap, in point form, the basic features and functions of the unit level controller:
• Alternatively referred to as a unitary controller, zone controller, application-specific controller, etc.
• Designed to control unitary equipment, i.e., equipment manufactured to serve a single zone, or area of temperature control.
• Small footprint, able to reside in the controls compartment of the unitary piece of equipment that it’s controlling.
• Limited point count; non-expandable.
• Digital outputs may or may not be true dry contact closures.
• Analog outputs may or may not be true voltage varying signals.
• No battery-backed realtime clock onboard (updateable software clock).
• Programming limitations (canned programs, customizable personalities to choose from).
• Designed and manufactured to keep costs down.
With regard to point counts, we can generally say that the unit level controller has a fixed, non-expandable point count. Which is not to say that any given controller manufacturer won’t have a variety of unit level controllers to choose from, with differing point counts, manufactured to fit various unit level applications. It is not unusual to see a unit level controller with only two inputs and two outputs, for there may very well be applications that only require this number of points. On the flipside, you may find a unit level controller with upwards of twenty points on board! Again, certain unit level applications could call for this quantity of points, and certain manufacturers recognize this by producing such a beast. As a rule of thumb, below are some typical point counts for these unit level controllers:
• Digital & Analog Inputs – you will likely find these in the form of “universal inputs”, meaning that any given input can function as either a digital or analog input, depending upon how it’s configured, i.e., how some jumper or DIP switch on the circuit board is set. You could see up to eight of these, however five is a more typical input point count for the majority of unit level applications.
• Digital Outputs – As with the universal inputs described above, you could see up to eight of these, however most higher-scale unit level apps could be handled with only five or six DOs.
• Analog Outputs – The most of these you’ll ever see on a unit level controller is probably three. We’ll consider an application using all three analog outputs.
Common to all of the upcoming applications is the requirement for a space temperature sensor. All zone level temperature control applications mandate the existence of a temperature sensor in the zone, that sensor perhaps being the most telltale sign of the existence of a zone. Anyway, the space sensor won’t be much more than a thermistor, or resistance-based temperature sensing device, housed in a decorative enclosure. Most often, the sensor enclosure will also contain a setpoint adjustment and an unoccupied mode override pushbutton. The temperature sensor consumes an analog input at the controller, as does the setpoint adjustment, while the override button usually is set up to “short out” the sensor input to the controller when pushed and held for a moment. Conversely, the controller is pre-programmed to acknowledge this action as a request for an override of the unoccupied mode of operation. Finally, the sensor enclosure will have a port on it, for local access to controller parameters either via a laptop computer or via a proprietary handheld service tool. Now that we’ve covered the requirement and the functionality of the requisite space temperature sensor, it’s time to turn our discussion to typical applications of unit level controllers.
Single zone rooftop units
In its simplest form, a single zone constant volume rooftop unit is controlled by a thermostat. The thermostat controls all on/off facets of rooftop unit operation: fan (G terminal), heating (W1 and W2 terminals) and cooling (Y1 and Y2 terminals). Now take away the thermostat and provide a digital, unit level controller. Place the controller in the rooftop unit controls compartment (providing that the controller’s ambient operating temperature range makes it suitable for outdoor use), and locate the temperature sensor in the space, where the thermostat would have been located. Wire the temperature sensor up to the controller, and you’ve just gobbled up two input points. Take control of the unit’s fan, heating, and cooling functions, and you’ve used up five digital outputs. No analog outputs required here, unless you want to take control of the rooftop unit’s economizer cycle (see August ‘07 column). Lastly, perform some monitoring (discharge air temperature, supply fan status, filter status, etc.) using a few more available inputs. Point counts for this specific application tally up as follows:
• Universal Inputs: 5
• Digital Outputs: 5
• Analog Outputs: 0
Fan coil units
Fan coils consist minimally of a fan and (you guessed it!) a coil. A four-pipe fan coil unit will have a hot water coil and a chilled water coil (a pair of pipes per coil, one supply and one return, hence the four-pipe designation). Throw in an outside air connection to it, in addition to the return air connection, and you have the opportunity for an economizer cycle. Each of these three components (heating coil, cooling coil, and economizer) will require an analog output. The coils will be fitted with proportional control valves, and the economizer dampers (outside and return air damper) will be fitted with a single proportional damper actuator. As with the rooftop unit application discussed previously, fan control will require a digital output. Likewise with the rooftop unit app, throw in some additional monitoring points, and you end up with a point count looking something like this:
• Universal Inputs: 5
• Digital Outputs: 1
• Analog Outputs: 3
The phrase “terminal unit” has been designated to encompass VAV and fan-powered boxes, from the cooling-only VAV box, to the series fan-powered box with variable speed fan and hot water heating coil. These days it is typical for a unit level controller, one specifically designed for terminal unit control, to have the requisite flow sensor and damper actuator built right in to the controller. As such, these two points don’t count against the controller’s available point capacity. So for a cooling-only VAV box, the only wired points would be those from the space sensor. Contrast that with a fan-powered box, which will require an output for the fan, at least one output for the heating coil, and an input for a discharge air temperature sensor. If the heating coil is hot water, the coil will be fitted with a proportional control valve, and require an analog output. If the coil is electric, it will require a digital output for each stage of control (up to three stages typical). A unit level controller specifically designed for terminal unit control will have a maximum (field-wired) point count as follows:
• Universal Inputs: 3
• Digital Outputs: 4
• Analog Outputs: 1
Tip of the Month: Utilize a spare input on a unit level controller, to pick up a nearby point associated with a completely separate system…? Often overlooked or not thought about, this practice is in theory quite feasible, provided that the input is capable of being polled for its value via the communication network. A word of caution though: if the point is a critical point utilized in the operation and control of some other system, this might not be such a good idea. One of the true beauties of networked DDC is that you can do something like this. On the other hand, if you lose the network, you lose access to that point. Oftentimes consulting engineers will require that critical points belonging to a particular system be wired to that particular system’s controller, and not to some other “more conveniently located” controller. Yet for non-critical points, this practice is definitely something to consider, and granted approval by the engineer, makes life just a little bit easier in those types of circumstances.
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