February 2008
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RTU Zoning Systems

Getting more out of your single zone constant volume rooftop units

Steven R. Calabrese

Steven R. Calabrese
Project Manager
Automated Logic Chicago

Contributing Editor

RTZ (RoofTop Zoning) systems are a decent, relatively inexpensive means of providing multiple zoning to a space served by a single piece of packaged heating and air conditioning equipment. They work well when applied properly. This means that the zones must be similar in load. Interior and perimeter zones should not be served by the same RTZ system. It also means that the setpoints to be maintained in the zones are close in value, relative to one another. The number of zones that an RTZ system can have is limited; the more zones a system has, the less of a chance to consistently and continually satisfy the individual zones. While the concept of RTZ is solid, it is not a panacea for all zoning design challenges. An understanding of what to expect from an RTZ system should be thoroughly possessed by the end user, so that there are no disillusions, and no disappointments when all is said and done. With that being the case, then it’s RTZ all the way!

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RTZ systems are microprocessor-based, typically supervised by a single “smart” controller. RTZ system zones are networked together, and back to the system controller. The controller has the ability to “poll” the zones for their comfort needs, and also has the ability to broadcast information to the zones such as time-of-day and rooftop unit operating mode information. The controller is normally a digital controller. It is not programmable but configurable. The basic RTZ algorithms are already burned into the controller, and parameters need to be set by the commissioning technician. This is true with the “store-bought” version of RTZ. DDC contractors, however, will readily implement RTZ with their product line, and are able to provide some “custom” features not offered by manufacturers’ “canned” versions.

The following components minimally make up an RTZ system (plus the rooftop unit, of course!).

  • System Controller

  • Zone Dampers

  • Zone Sensors

  • Supply Air Temperature Sensor

  • Static Pressure Transmitter

  • Bypass Damper

The system controller is the brains behind the operation. This is typically a microprocessor-based controller equipped with all of the capabilities required to supervise and implement the operation of an RTZ system. This includes software/firmware, control algorithms, and possibly even an internal time clock. A computer can typically be connected to the controller, to facilitate human interface.

The RTZ system controller’s main function is to poll all zones on the system, and put the rooftop unit in the appropriate mode of operation. We are dealing with a standard, single zone, constant volume rooftop unit here; instead of a thermostat wired to the terminal strip of the unit, the system controller is wired to it. The controller therefore has full command of the rooftop unit’s heating, cooling, and fan functions. On the other end, zone dampers are networked together and back to the system controller. It is over this network that the system controller communicates with the zones.

Zone dampers are small motorized dampers with controllers. The controllers are typically microprocessor-based, “application specific” controllers, meaning they are specifically designed to perform zone control. The zone damper assembly assumes its place in the duct distribution system, and therefore resides in the ceiling space. Ductwork connects the inlet side of the zone damper to the rooftop unit’s main trunkline. Likewise, ductwork connects the outlet side to diffusers that serve the particular zone of comfort.

Zone sensors reside in the zones served by the zone dampers (one per zone). Each zone sensor is wired up to its respective zone damper controller. The sensor monitors space temperature, and transmits this information to the controller. The sensor in addition may be equipped with an adjustment, allowing the individual user to set his or her own comfort level.

The system supply air sensor is installed in the supply duct, downstream of the rooftop unit, and is wired back to the system controller. The controller uses this temperature information to validate rooftop unit operating modes, and to impose high and low limits, as will be discussed further on.

The system static pressure transmitter is installed in the supply duct as well, normally after the bypass damper and before the first takeoff. It is also wired back to the system controller. The controller uses this information to control the bypass damper, to maintain proper system pressure.

The bypass damper is a motorized damper that is installed between the rooftop unit’s supply and return drops. The bypass damper’s role is to maintain proper pressure in the supply duct, by bypassing supply air straight to the return as zone dampers close off and pressure begins to build.

contemporary Zone Control

Each zone served by the rooftop unit has a zone sensor that transmits zone temperature (and setpoint) information back to its respective zone damper. The damper has the ability to modulate open and closed, to provide varying amounts of rooftop unit air into the zone served by the damper. Consider a single zone. If the temperature of the air that the rooftop unit is providing to the damper is conducive to the needs of the zone, then the proper amount of this air is allowed into the zone, via the damper. The further from setpoint that the zone is, the more air is allowed. The sensor measures the temperature of the zone, and relays this information to the zone damper’s controller. The controller, in turn, modulates the zone damper to introduce the appropriate amount of conditioned air into the zone, in an effort to bring the zone back to setpoint.

Now, what if the temperature of the air delivered to the zone damper is not conducive to the comfort needs of the zone? The damper must close in this event, so as not to provide “opposite mode” air into the zone. For example, if the rooftop unit is in a cooling mode, and the temperature of a particular zone is below its setpoint, the damper serving this zone must close down, so as not to provide cool air into the zone. Typically, the zone damper will not be allowed to close completely, but only to some minimum position, so as to ensure ventilation of the zone at all times.

This in essence is the standard operation of an RTZ system zone damper. If the air received from the rooftop unit is conducive to the needs of the zone, allow it to pass (at the proper amount). If not, then don’t allow it to pass. This is how a single piece of heating and air conditioning equipment can serve multiple zones of temperature control. It’s not perfect, but it’s better than a single thermostat controlling the equipment.

Tip of the Month:
The concept of RTZ is proven and well-established. Its operating principles and methods of control have evolved and are widely accepted. However, their complexity does merit a certain degree of specialization, in terms of design, installation, and commissioning. Moreover, the RTZ system is not the end-all solution to all zoning dilemmas, and care must be taken so as not to oversell the concept. Yet, for what its worth, RTZ is an economical alternative to other methods of zoning, and as long as the end user knows what to expect from his investment up front, and knows of the alternatives and of their associated costs, he will likely be pleased with the performance of his system, and of the flexibility offered with it. 

Rooftop Unit Mode Control

The system controller “polls” all zones, to find out what is needed the most. As such, each zone gets to cast a “vote” for its required mode of operation. Zones whose temperatures are below their setpoints would vote for heating, and zones whose temperatures are above their setpoints would vote for cooling. Zones whose temperatures are pretty much at setpoint do not cast a vote. The zones that are the “majority vote” get their wish, and the system controller puts the rooftop unit into the appropriate mode. The dampers for those zones in the minority will close down, and will have to “sit and wait”, until they become the majority, at which point the rooftop unit will switch to the opposite mode.

This is the basic principle of RTZ. Think of it as a democracy, where all members of the space served by the rooftop unit can “voice their opinion” and have a say-so in how the rooftop unit should operate. It’s not a perfect system, for a rooftop unit can only appease some of the zones some of the time. And possibly some zones never!

In the unlikely event that all zones are satisfied at the same time, and no votes are cast for either heating or cooling, the rooftop unit simply operates in a ventilation mode. You can bet that that won’t last for long though, and soon enough one of those zones out there will call for something, be it heating or cooling, thus affecting all of the other zones, at least in a minimal fashion.

System Controller – Additional Functions

The system controller is the “supreme and benevolent ruler” in an RTZ system. The controller polls its members, finds out the needs of the majority, and operates the rooftop unit in the required manner. The controller also has some other responsibilities, of which we now talk about.

The system controller continuously gathers information from a couple of sensors out on the system, one being the static pressure transmitter. This sensor measures the static pressure in the supply air duct, and transmits this information to the system controller. The system controller processes this information, compares it with the desired setpoint, and positions the bypass damper accordingly, to maintain setpoint.

The system controller also continuously monitors the temperature information furnished by the supply air temperature sensor, and imposes operational “high and low limits”. If an extreme temperature condition occurs, the system controller simply terminates the cause of the condition, whether it be heating or cooling, for as long as need be. The supply fan continues to run during the condition, so that the system continues to receive its required ventilation.


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