By Erik Dlugajczyk
A proper project design review with an emphasis on TAB can vary from one company to another, but the standardization of such reviews can assist NEBB Certified Professionals (CPs) in achieving job completion with fewer headaches and delays. The reality is that many problems can be accurately predicted just from a proper design review long before Certified Technicians (CTs) are dispatched to a job. Since there are a variety of types of projects, I will focus on one type: Air Handling Units with VAVs, and the potential pitfalls of each. In this article, the Variable Air Volume (VAV) system design will be the primary focus.
Variable Air Volume Zones
When reviewed independently and outside of the HVAC system in which they are installed, VAVs are capable of certain minimum and maximum air flow parameters. However, when reviewed within the system in which they are installed, their limitations may be drastically different. In this example, we will utilize Titus DESV zones, with the individual zone parameters highlighted below.
An important note is mentioned on the bottom of the Titus submittal page: the controls parameters may be quite different. In fact, most controls transducers are rated for a minimum of 0.01” w.c., which in standard air conditions translates to 400 fpm. Most of these zones listed in the submittal above, on the minimum side, calculate to an average of 255 fpm, which is far below the minimum standard of most controls transducers. When faced with this situation, a request for controls submittals is necessary, followed by an RFI to the design team to address the discrepancies. Let’s take a look at a sample job:
A Tale of Two Zones
Let’s explore two individual zones located on opposite sides of the building.
The Design:
Zone 3-01 is a 10” round inlet DESV exterior zone, requiring 1,320 CFM at maximum flow and 400 CFM at minimum flow. Calculated velocity at the inlet of the zone, at maximum, is 2,422 fpm. This would require somewhere between 2.0 – 2.5” w.c. of static pressure at the main duct just upstream of the zone inlet to achieve this design flow.
The zone serves the northwest corner office and is expected to be at its highest peak in the afternoon when the sun heats up the western side of the building. This zone is located 125 feet away from the source of the air (measured duct length).
The calculated friction loss is 0.372” at the main duct. Additionally, upstream of the zone inlet are three 90-degree rectangular elbows with turning vanes which are estimated at a pressure drop of 0.22” per elbow. The total system loss, upstream of the inlet of the zone, calculates to an estimated ≥1” w.g. ((0.22” per elbow x 3) + 0.372”). The mechanical schedule lists the air handling unit external static pressure (ESP) as 2.0” w.c.
Using those numbers, the discharge static pressure necessary at the air handling unit would need to be over 3” w.c., far exceeding the design external static pressure of 2” w.c. This zone should be included in the RFI as it will be the “key zone” in an attempt to set the operating static pressure, as this is the zone with the estimated highest amount of resistance. The design ESP of the AHU may not be sufficient to satisfy the maximum zone airflow requirement; design team input would be valuable prior to the start of the balance process.
Located on the eastern side of the building, Zone 3-09 is a 6” round inlet DESV interior zone requiring 100 CFM at maximum flow and 30 CFM at minimum flow. While this zone does not have some of the friction loss pressure challenges of zone 3-01, it is designed for an air flow amount lower, at minimum CFM, than the DESV submittals require. It is likely that this zone will not control at minimum airflow as the airflow through the transducer may be too low to control.
Analysis of Design
Zone 3-01 is located so far away from the source of air, and with so much estimated friction loss, that it is unlikely to achieve airflow even at optimum conditions. Not only is the main duct sized to produce high velocity/high friction loss, the inlet of the zone is sized for a high velocity, creating a high likelihood that the system is not capable of producing that much airflow at that great of a distance. Hopefully, the RFI produces a redesign of a 12” zone inlet as well as some main duct resizing, which would produce a much higher likelihood of success for the maximum CFM requirement, while still satisfying the minimum flow requirement.
Zone 3-09 is oversized for the minimum flow setpoint, and hopefully the RFI produces a redesign of a 4” zone inlet which would satisfy both the maximum and minimum requirements.
With no changes to the design—aside from the specific flow issues already presented above—one additional issue will likely arise during normal operation. On a typical Southern California afternoon with the western sun beating on the side of the building and causing zone 3-01 to modulate to maximum flow, the interior zone 3-09 will likely be at minimum flow (at least part of the time). This would result in the VAV damper for 3-09 being open less than 10% due to the high static pressure in the duct required to satisfy zone 3-01. This may cause a whistling effect through the VAV damper. Since this is a library with Noise Criteria (NC) requirements, such a result would not be favorable or acceptable to the end user.
At this stage of the job, it would be very advantageous for the NEBB CF DCP to have backup emails, showing the issues were already flagged during the design review process and that this is not a balancing issue, but rather a problem for the design team to tackle. The problems presented here have no resolution available to be implemented by the test and balance contractor. Remember, this is the stage of the job when everyone is out of time and money. The owners, contractors, and inspectors just want a report, and all the balancer sees are problems. If some of those problems have already been addressed by the NEBB DCP, (even if those concerns are initially ignored by the design team), they are no longer the balancer’s responsibility.
