What is the Adjacent Zone?

ASHRAE Standards 55 and 62.1 summarizes ASHRAE’s standards for thermal comfort and minimum ventilation and explains how to apply air outlets to comply with each of these standards. Even though the article also references the definition of the ‘adjacent zone’ that exists in close proximity to Thermal Displacement Ventilation (TDV) diffusers, it’s also important to understand how displacement ventilation products can differ with respect to occupant comfort.

To briefly recap, displacement diffusers used in fully-stratified systems deliver low velocity cooling directly to the occupied zone. Since the supply air is cooler than the room air, it cascades down the face of the diffuser and travels across the floor in a thin layer generally no more than 4 inches deep.

Adjacent Zone

The ‘adjacent zone’ is defined as any portion of the room where discharge velocities exceed 40 fpm. This area is not recommended for stationary occupants who would likely feel a chill around their ankles.

Although displacement ventilation diffusers are available from most manufacturers in a myriad of shapes and sizes, there are really two basic types of designs being manufactured:

• Fixed Air Pattern

• Adjustable Air Pattern

Although all displacement diffusers include a perforated face plate and a rear supply plenum, fixed air pattern diffusers are characterized by a perforated central baffle. The purpose of the central baffle is to further slow and spread the supply air evenly over the face plate. Diffusers of this design are less costly to produce but they must be selected very carefully to ensure that the symmetrical adjacent zones they create will not result in thermal discomfort for stationary occupants.

Fixed air pattern diffusers also lack versatility in situations where spaces are being reconfigured or re-purposed. Rather than used a fixed perforated central baffle, adjustable air pattern diffusers include a baffle fitted with air pattern controllers. These sturdy steel pattern controllers are easy to remove and reset and do not raise concerns about plastic materials in the air stream. Although these pattern controllers ship out of the factory in a default arrangement to create a standard symmetrical discharge pattern, they provide adjustability for enhanced versatility and improved occupant comfort.



Left: Fixed air pattern displacement diffusers in standard operation. Right: Adjustable air pattern controllers allow for adjustability of the adjacent zone for maximum occupant comfort.



 Rather than settle for a fixed discharge pattern, Titus displacement ventilation products can be easily adjusted to direct air away from occupants and areas that may result in comfort issues (now or in the future). Titus displacement ventilation products include adjustable pattern controllers so that you can control your ‘adjacent zone’.

Randy Zimmerman – Titus Chief Engineer