December 7, 2020

Challenges During a PHASED TURNOVER in a HVAC system with a Common Plenum

The four-story building was supplied by a fourth-floor mechanical room consisting of seven variable air volume (VAV) air handling units (AHU), each supplying air into a common supply plenum and common return plenum. These supplies and return plenums were routed through several chases and served each of the four floors throughout the building. This design approach was built with redundancy in mind so that, if any particular AHU among the seven needed to be brought offline for maintenance, then it could be isolated from the common plenum via dampers at each AHU and the remaining six AHUs could provide HVAC to the building without the need for an outage. After design, the owner of the building requested a phased turnover of each floor within the building. Level one would be turned over first, then levels 3, 4 and 2 respectively. Level 2 was a tenant build-out space which would be turned over significantly later than levels 1, 3 and 4. No upfront design consideration had been given to this phased turnover approach. This phased turnover proved problematic. Under ideal conditions, the TAB contractor would be provided a complete HVAC system to balance, after which the commissioning agent would perform functional testing of the HVAC systems. Then the facility would be turned over to the owner after these activities were completed. But with the phased turnover approach, level 1 would need to be ready before levels 4, 3 and 2 were even balanced or even ready for balancing activities.

Several questions would need to be answered:

  • How can the cleanliness of the air be maintained being provided to level 1 (each AHU was equipped with HEPA filters) given that the doors of the level 4 AHUs were propped open to provide a source of air to the AHUs since the return ducts to the remaining three floors were not opened? This is a common practice to prevent return ductwork from becoming dirty. Return ductwork is left isolated until the floor from which it draws air is architecturally clean.
  • Multiple exhaust fans served multiple floors. At what point will the exhaust fans be able to be balanced?
  • How will each floor’s pressurization be maintained/ stabilized during each phase of the turnover?
  • How can the commissioning agent ensure that the building systems serving each floor can be relied upon for occupancy of that floor if the remainder of that building system is not yet constructed or balanced?

An organized approach started by viewing the mechanical air riser diagrams (The outside air ductwork was removed for simplicity) (See figure 1). The first idea was to determine if it was feasible to close all of the doors of the seven AHUs and utilize the two upstream pretreated outside air units (PTOAs) serving these seven AHUs to provide the equivalent make-up air that would ordinarily have been returned to the AHUs from each floor. The two PTOAs were scheduled with a maximum airflow of 30,000 CFM each. There is a maximum of 60,000 CFM available to serve the seven AHUs should the option to close all AHU doors be considered. Each of the seven AHUs was scheduled for 26,000 CFM, of airflow of which 8,575 CFM of outside air was scheduled to be sourced from the outside air handling units. Ignoring the exhaust of each floor (because we couldn’t open return ductwork or exhaust ductwork due to needing to keep the ductwork clean), this meant that (26,000 CFM supply air – 8,575 CFM outside air) = 17,425 CFM return air per AHU would be ordinarily returned to

 

each of the seven AHUs at maximum airflow conditions (these are VAV AHUs.) But since we could open neither exhaust nor return ductwork, the two PTOAs would need to supply all make-up air to all seven AHUs. 26,000 CFM x 7 AHUs = 182,000 CFM required by the PTOAs. This yields roughly a 122,000 CFM deficit in air that needed to come from somewhere. This idea was immediately abandoned because, with no relief of the supply air being supplied to each floor, the building would be dangerously overpressurized. This yielded the all-too-familiar “you can’t get there from here” scenario. Obviously, another solution would need to be devised. There would need to be some pathway for the supply air to be relieved from each of the four floors until the final return and exhaust ductwork was permitted to be opened to receive airflow. The next idea discussed was to isolate the HVAC at levels 2, 3 and 4 while performing the TAB activities on level 1. This could be accomplished by closing the supply and return ductwork to levels 2, 3 and 4 by temporarily capping the ductwork at these floors. This would enable the supply and return to be balanced on level 1 by isolating it from the other “dirty” levels. However, the general contractor had strong objections to this approach as it would not allow for conditioned air to the other three levels during the completion of this work on level 1. The general contractor advised they would require conditioned air to the remaining levels to complete their architectural finishes of drywall and painting. Without conditioned air, there would be no means of drying/ curing the drywall compound and paint finishes required throughout the space. So this idea was also abandoned. The following proposed idea was to utilize the stairwells within the building that connected each floor as the return air path. This approach would utilize the stairwell as the return air path on levels 2, 3 and 4 while opening the return and supply ductwork only to the clean level 1. The TAB contractor could perform their scope of work on the clean level 1, while still maintaining conditioned air to the other dirty levels. The return air ductwork on levels 2, 3 and 4 would remain capped to protect its integrity while the stairwell doors would remain open on these levels to provide a pathway to the AHUs returns. In order to provide sufficient static pressure in both the supply and return ductwork to serve level 1, one or two of the AHU’s return air doors would be closed and the return dampers at these respective AHUs would be opened to the return ductwork to pull air from level 1 via the return duct plenum. The remaining AHUs would have their return air side doors remain open to draw air from the stairwells (See figure 2). Once the TAB scope of level 1 was completed, and level 3 was architecturally ready for balancing (clean), then the same process would be repeated for level 3. Level 1 would remain connected to the clean return ductwork and the level 3 return ductwork would then be connected to the return riser. The remaining levels 2 and 4 (which were still dirty) would remain isolated from the return ductwork and continue to utilize the stairwells as their return air path to the AHUs. To build sufficient static pressure in the return ductwork to perform TAB scope on level 3, additional AHU return air side doors would be closed and their respective isolation dampers would be opened to the return air plenum to draw air from the plenum and sufficiently serve level 3 (See figure 3). This same logic would then apply to level 4 when it was architecturally ready to balance. The only hiccup in the plan was

that the level 2 space construction was going to lag significantly behind the other three floors by several months. This was problematic because the balancing scheme up to the final floor level 2 utilized the stairwells as the return air path. But the owner would be occupying levels 1, 3 and 4 well in advance of level 2 being clean enough to open it to the return ductwork. And once levels 1, 3 and 4 were occupied, the stairwell doors would need to remain closed for life safety concerns. Another strategy needed to be considered. It was finally decided that the engineer of record would design a special filter rack to be attached to the level 2 return ductwork, thereby allowing it to be connected to the return air plenums (See figure 4). The exhaust fans served multiple floors via risers. However, their airflows were negligible in comparison to the return airflows. As a result, it was decided that the exhaust fans could be balanced as soon as the floors they served were clean. In order to verify the functionality of the AHU sequences of operation, each of the seven AHUs would have its functional test performed by the CxA once its return air section doors were closed. This would be done for all seven AHUs. Upon completion of the final balance of all seven AHUs by the TAB contractor, a final retest of the seven AHUs would be performed by driving all of them to maximum cooling to verify total system capacity and functionality.