Critical Zone Reset – Dealing with Rogue Zones
For most modern variable-air-volume systems (VAV), codes and best practice now mandate critical zone reset, such as static pressure reset (fan pressure optimization), primary supply air temperature (SAT) reset, or both. Critical zone reset strategies can save significant fan energy and reduce the need for reheat.
For instance, with fan pressure optimization, the building automation system (BAS) monitors the position of each VAV box and determines the maximum terminal position in the system. Then, the supply duct static pressure is continually adjusted such that at least one box is always wide open (usually 90-95% open in practice). For most part-load conditions, this will ensure that each box has enough airflow to meet the load, while keeping fan operating cost at a minimum at all flow conditions.
One challenge, though, is dealing with so-called “rogue zone” boxes that are always wide open. In these zones, the thermostat setpoint is never satisfied for some reason, perhaps due to poor design or a change in use that changes the load in the space. With a pressure-dependent VAV system, the BAS determines that the rogue zone is the critical zone, and keeps resetting the duct static setpoint higher and higher. The result is wasted energy, and usually a noisy, over-pressurized system.
As an example of a typical rogue zone, while recently commissioning a K-12 VAV system, trends indicated a lower-than-expected, and constant, SAT setpoint of 55 F even during cool weather (SAT was set to modulate between 55 F and 65 F based on outside air temperature and critical zone reset). The space temperature of one zone in particular was consistently several degrees higher than setpoint. When we investigated, we found that a large copier had been installed directly under the temperature sensor, making it appear that space temperature was 4 to 6 degrees warmer than actual conditions.
After removing the rogue zone from the critical zone calculation, SAT reset behaved as expected, and gradually increased, reducing reheat in the remaining zones.
In summary, all critical zone reset sequences should have an easy way for operators to identify and remove bad zones from the calculation list. In many canned OEM programs, this may be as simple as unchecking that particular box from the list. Floor plan graphics can also highlight visually which zone(s) are driving resets.
Most importantly, facility operators should be on the lookout for these “tail wagging the dog” situations and be trained to remove zones as needed until the root cause can be addressed.
