July 2007
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RTU Operation Via Conventional & Digital Controls

Control a packaged rooftop unit with a thermostat, or digitally via a building automation system

Steven R. CalabreseSteven R. Calabrese
Automated Logic Chicago

Steve Calabrese is a Project Engineer with a large controls contractor serving the Chicagoland area, and author of the book Practical Controls: A Guide To Mechanical Systems. You can visit his website at www.pcs-engineering.com.

For those of you that are familiar with the operation and control of single zone, constant volume packaged rooftop units, please consider this as simply a refresher on the topic. For those not so familiar with it, then this one’s for you!

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First, a brief description of the equipment itself. A rooftop unit is a packaged air handler. The three main operational functions of a rooftop unit are its supply fan, its method of cooling, and its method of heating. The supply fan’s function is to draw air through the rooftop unit’s heating and cooling sections and deliver it to the space served. Economizer operation aside (which we’ll hit hard in next month’s column), cooling is mainly accomplished by refrigeration (mechanical or direct expansion “DX” cooling), and the entire refrigeration cycle is within the unit. The heating section of the rooftop unit is typically a gas-fired furnace, however rooftop units with electric heating sections are available, for applications in which natural gas is either not available or not the most feasible option.

The smaller rooftop units, those of which are in the range of a few tons or so (one ton equals 12,000 BTUH), are normally single stage units, meaning they have only one stage of heating and one compressor. The larger units most always have two (or more) stages of heating and cooling. Two-stage heating is accomplished by a two-stage gas valve, which assumes a partially open position upon a call for one stage, and opens fully if there is a call for both stages. Two-stage DX cooling is accomplished by staging compressors, meaning that a rooftop unit with two stages of DX cooling has two compressors.

A rooftop unit employed to serve a single zone is traditionally controlled by a multistage heating-cooling thermostat mounted in the zone and wired back to the unit. These days it is becoming more and more common to control rooftop units via DDC, even in smaller buildings where traditional control methods would have been the norm a generation or so ago. This article will cover traditional control of a rooftop unit, as a prerequisite to describe the more “technologically advanced” methods of today.

Common among most constant volume rooftop units that are designed to perform single zone heating and cooling is the “low-voltage terminal strip”. For a two-cool, two-heat unit, the terminals are as follows: R, Y1, Y2, W1, W2, and G. R is 24 volts “hot” as derived from the rooftop unit’s control transformer. W1 and W2 correspond to stages of heat. Y1 and Y2 correspond to stages of cooling. And G corresponds to the supply fan.

A conventional thermostat basically has the same terminals, and is wired “color for color” to the rooftop unit’s terminal strip. The R terminal on the thermostat accepts the 24 volts from the rooftop unit, and can route it to any of the other terminals as required (hang on, we’ll get there!). These other terminals, when given 24 volts, then “return” the voltage back to the rooftop unit, to its respective terminal.

So far so good? Okay, now if any of the terminals (besides R) at the rooftop unit receives 24 volts as returned from the thermostat, its associated function is engaged. For instance, if the G terminal at the rooftop unit receives 24 volts, then the supply fan runs. Likewise, if the W1 terminal at the unit receives 24 volts, then first stage heating is initiated. And so on.

The thermostat itself determines what happens up at the rooftop unit (whether the fan should run, how many heating stages to engage, etc.). Fan control is typically determined by an on-board user switch labeled “ON-AUTO” The switch allows for selection between continuous and intermittent fan operation. Intermittent operation means that the fan turns on upon calls for heating and cooling, and is off otherwise. Heating and cooling control is based on two parameters: setpoint and space temperature. The setpoint is established by the occupant. Space temperature is measured in a variety of ways. For an electronic thermostat, space temperature is measured by an integral thermistor, which is an electronic temperature sensor. The temperature sensed by the thermistor is compared to the setpoint, and the appropriate actions are taken to engage either heating or cooling. For instance, if the temperature sensed by the thermistor is slightly above the setpoint, then the thermostat will return 24 volts to the Y1 terminal of the rooftop unit, and first-stage cooling will engage. Likewise, if the temperature is substantially above the setpoint, then the thermostat will return 24 volts to both the Y1 and the Y2 terminals of the rooftop unit, to engage both first and second stage cooling. There’s a bit more to all of this, but at least you get the basic gist of it.

Programmable thermostats are electronic thermostats that have an on-board time clock function. This allows the occupant to set up a time schedule whereby in the occupied modes, the unit will operate as desired to maintain occupied comfort levels, and in the unoccupied modes, the unit will “hibernate”, so to speak, only to come on if it gets too-too-hot or too-too-cold in the space (please forgive the gratuitous use of overly technical terms!).

The overwhelming trend these days, especially when it comes to new construction, is to provide Direct Digital Control (DDC) of all of the facility’s HVAC equipment, and install a Building Automation System (BAS) with a web-based operator workstation and internet connectivity (hey…why have it any other way???). The thermostats of yesteryear are done away with, and replaced by networked digital controllers and electronic space temperature sensors. So how do we physically control a packaged rooftop unit, one that has thermostatic control as part of it’s traditional design origin?

Start with a digital controller, one that has enough outputs to accomplish basic control of the rooftop unit (fan control, cooling & heating stages), and enough inputs to accommodate all sensors and monitoring points to be picked up. Output counts for this type of application generally won’t exceed five, however you might throw one more in there (of the analog variety) for taking direct control of the economizer actuator (again, more on that next month).

[an error occurred while processing this directive] As far as inputs go, you need at least one, for a space temperature sensor. The sensor occupies the same physical space as its predecessor, the conventional thermostat, would once occupy. The sensor itself may have setpoint adjustability built into it, and if so the digital controller would require an input for that function. A discharge air sensor comes in handy, if not for control purposes then at least as a useful monitoring point. Other monitoring functions can be added as well, in the form of temperature sensors (return air, mixed air), current sensing switches (supply fan status), and pressure switches (filter status). At present count, the quantity of inputs is at seven (and rising!). Anything else you can think of, feel free to add, as long as you can accommodate it with your digital controller.

Now that the unit is monitored and under significant control, time to take advantage of the network and bring some global information into the mix. Central scheduling performed by the maintenance crew can be extended to the controller, in order to establish reasonable occupied and unoccupied modes of operation for the equipment. And outside air temperature and humidity information can be used to limit the modes of operation of the unit, and to optimize start-ups and shut-downs and maximize run-time efficiencies.

Rooftop unit control has come a long way from the simple thermostat, and the benefits brought forth by networked DDC are substantial. This article has only touched upon these benefits, but has hopefully given some insight, as to where we’ve come from, and where we’re heading!

Tip of the Month: When doing an RTU terminal strip interface with a digital controller, oftentimes the controller manufacturer recommends that you provide “interposing relays” for each output. These additional relays are to isolate the digital controller’s delicate “pc board level” output devices, protecting them from the larger currents that can sometimes be generated at the rooftop unit’s terminal strip. However, if the unit’s terminal strip is nothing more than a board level connector, then it’s a good indication that the unit’s terminals are low level electronic signals that are processed via the unit’s on-board (microprocessor-based) control system. If that’s the case, the requirement for interposing relays may be nil, and therefore eliminated from the design. Check with the manufacturer, and follow their advice as to whether or not these extra devices are really necessary.

About the Author

Steve Calabrese earned his BSEE degree in 1990 from the University of Illinois at Chicago (UIC). He has since spent much of his professional career working for a mechanical contracting company, in various roles including mechanical systems design, control systems design, project management, and department management. Currently employed by a large Chicagoland controls company, Steve couples his broad mechanical knowledge and experience with a strong background in the area of electricity and electronics. His control systems expertise includes electrical and electronic stand-alone controls, as well as microprocessor-based direct digital controls (DDC) and networked Building Automation Systems (BAS).  You can visit his website at www.pcs-engineering.com.

In 2003 Steve’s book, Practical Controls: A Guide To Mechanical Systems, was published. Geared toward the HVAC professional, the book details practical methods of controls and defines the role of HVAC controls in an easy-to-understand format. Steve brings his mechanical and controls contracting experience to this writing, and offers practical approaches to control systems issues.


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