TAB is a Prerequisite for Functional HVAC System Performance
No amount of commissioning can make an unbalanced system work.
I was reminded recently of the value of testing, adjusting, and balancing (TAB) coordination for enhancing the efficiency and success of HVAC commissioning. Balancing of all air and water flows serving HVAC equipment is critical for realizing the performance required of that equipment. No amount of commissioning can make an unbalanced system work.
Therefore, successful TAB completion and documentation is a prerequisite for starting HVAC system functional performance testing. In many projects, even though TAB is far from trivial, TAB contractors are typically not well integrated into the project team. Regardless of who holds the TAB contract (e.g., mechanical contractor, general contractor, construction manager, or owner), the TAB contractor is often the last person brought onto a job site. Unfortunately, that is sometimes at or after the substantial completion date.
This is not a recipe for success. The following sequence of events would be considered an excellent scenario in today’s project delivery world.
- The TAB contractor is awarded the subcontract at the beginning of construction using a price the TAB contractor submitted based on the bid documents (drawings and specifications) available at the time.
- The TAB contractor is scheduled to balance the systems at the end of construction.
- In preparation for the field work, the TAB contractor:
a. Refreshes his or her memory about the bid documents and requests the latest set of drawings;
b. Gathers submittals associated with the systems being commissioned in order to understand the technical specifics of the installed equipment; and
c. Submits questions/RFIs regarding technical information needed for successful TAB execution.
- The TAB contractor goes to the project site and does the best he or she can with the systems as installed, submitting a report and, if necessary, documenting issues that prevent completing the TAB scope as understood by the TAB contractor.
- The TAB contractor returns to the project after the deficiencies have been corrected, completes the balancing work, and submits a final report.
In practice, this is far from ideal and often further from reality. First of all, the time gap between Step 1 and Step 2 could be six months to three years (or longer), depending on the size of the project. That creates a major disconnect between the TAB contractor and the project’s team and technical requirements. A lot can happen during the construction phase to impact the TAB requirements that is not shared with the TAB contractor.
That brings us to Step 3, during which the TAB contractor is scrambling to come up to speed on the project and what he or she needs to do on-site.
Obtaining the latest construction documents and approved equipment submittals and clarifying expectations depends on the responsiveness and thoroughness of other project team members. Having time to properly review and process that information, once received, is a function of the amount of advanced warning the TAB contractor receives before balancing is scheduled to begin.
At the end of construction, schedule is king, and the TAB contractor will be expected to show up to perform the balancing (Step 4) regardless of his or her level of understanding regarding system performance expectations and documentation requirements (Step 3). It is also common for some of the HVAC systems to not be ready for balancing when the balancing is scheduled. This is the result of balancing being planned for a time when the construction schedule dictates the system’s need to be complete, which is not necessarily the same as when the systems will actually be complete.
With a conscientious TAB contractor, all of the above can result in a Step 4 TAB report documenting many mechanical or electrical deficiencies that render TAB impossible, missing critical system performance information (e.g., control system set points), and not accounting for some systems altogether.
If the commissioning professional receives the first TAB report, and it is clear that systems are not ready for functional performance testing, there can be a very annoying (to the entire project team and owner) delay while the TAB issues are worked out. This is not good for anyone. Next month’s column will address some ideas on how to avoid this situation.