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RTU Zoning Systems
Getting more out of your single zone constant volume rooftop units
Steven R. Calabrese
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!
The following components minimally make up an RTZ system (plus the rooftop unit, of course!).
Supply Air Temperature Sensor
Static Pressure Transmitter
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.
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:
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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