Best practices for mixed-use buildings: HVAC

Taking on a mixed-use structure—such as one that includes retail and residential portions—can be an engineering challenge. With all the different engineered systems involved, it can be like working on and integrating several different projects at once. HVAC systems and indoor air quality (IAQ) challenge engineers on many levels.

By Consulting-Specifying Engineer September 29, 2014


  • Anil Ahuja, PE, RCDD, LEED BD+C, CxA, President, CCJM Engineers Ltd. Chicago
  • Jason R. Gerke, PE, CxA, LEED AP BD+C, Mechanical and Plumbing Team Leader, GRAEF, Milwaukee
  • Keith Lane, PE, RCDD, NTS, RTPM, LC, LEED AP BD+C, President/CEO, Lane Coburn & Associates, Bothell, Wash. 
  • Brian A. McLaughlin, PE, Associate, Arup, Los Angeles

CSE: What unique HVAC requirements do mixed-use structures have that you wouldn’t encounter on other structures?

Gerke: One unique challenge is the changing code requirements as the systems move floor-to-floor in the building. There may be firewall-type separations that create a change of occupancy from one space to another. Those types of divisions are not typically found in single-occupancy/use buildings.

CSE: What changes in fans, variable frequency drives (VFDs), and other related equipment have you experienced?
Ahuja: The biggest change in VFDs has been the overall acceptability throughout the industry. Ten years ago, engineers typically did not specify a VFD in a building due to the noise and frequency distortion issues that the drive created. Because the drives have been changed and modified over the years—and are less expensive—most building staff want to upgrade to the drives because of the benefits they provide. 
Gerke: VFDs are used all the time. We rarely design systems without them, unless they are very small single-zone arrangements that do not have VFDs on the fans. Any and all pumps should have VFDs unless they are direct-drive with built-in speed controllers. A fan manufacturer that we base many of our designs on currently offers built-in VFDs on direct drive motors. We have received positive feedback from the field from both test-and-balance and the controls contractors on how packaging this equipment helps them save time setting up the systems.
CSE: What indoor air quality (IAQ) challenges have you recently overcome? Describe the project, and how you solved the problem.
Ahuja: A public school had an issue where there was little to no airflow in classrooms on the second and third floors, and in classrooms farthest away from the fans. The system was an old tunnel system where the supply fan discharged air into a tunnel, and the air was then treated by reheat boxes within the tunnel. After some field investigation, it was found that the air tunnel had many cracks, holes, and leaks that reduced the overall pressure so much, air was not being properly supplied to the building. The tunnel was completely sealed to be airtight, new doors were provided, and all penetrations were properly sealed. No complaints have been recorded since the work was completed.
CSE: In your experience, have alternative HVAC systems become more relevant? 
Gerke: We are now considering chilled beams and variable refrigerant flow (VRF) systems on all office or residential projects. These are the typical system options reviewed with the owner/developer for efficiency, first-cost savings, and long-term maintenance. This variety of systems is reviewed along with the standard options of airside systems historically used in the project locations.
Ahuja: I don’t know that they have become any more or less relevant in the industry. There is always a niche that these systems will fill, and there will always be projects where it is simply a better solution to provide a variable air volume (VAV) air handling unit (AHU) with skin heating. Our job is to understand the client’s needs, the occupancy, and help guide them to provide the best solution at a cost that they find acceptable. 
CSE: What unique dedicated outdoor air system (DOAS) projects have you encountered in these buildings? 
Gerke: The use of DOAS allows for many unit configurations, system types, and control strategies to be considered. We typically take the opportunity to “throw everything on the table” and sort through what will work best for this particular project. Options might include how to connect the heat gain of one part of the building or system with the heat loss of another system, or how do we simply not dump all that cooled air out in the summer and keep the warmth of the air inside in the winter. DOAS units can be configured with energy recovery wheels, hydronic or refrigerant coils, gas burners, or many different fan options as standard options from many manufacturers. This large number of standard options allows the project mechanical engineer to be creative designing the most energy-efficient system.