VAV Reheat Terminal Unit Controls, Part 2
When it comes to cost and commissioning, a standard operating sequence can make all the difference.
Last month, this column addressed the fact that VAV terminal units are substantially more “variable” than even their name implies. Not only does the air flowing through them vary, there are a variety of control sequences that can be applied to them. This month, I want to address the long-term benefits to the owner of having a standard VAV terminal unit control sequence for their building or buildings.
As noted in April 2017, in the interests of energy conservation and occupant comfort, some building owners and operators have put a lot of effort into standardizing their terminal unit controls and want things done a very specific way. In addition to energy-conscious operations, the value of standardization cannot be overstated when the quantity of VAVs is in the scores, hundreds, or thousands for a single owner.
The value of standard terminal unit sequences of operation includes the following.
Reduced First Cost
If a building owner is in a position to develop and maintain VAV terminal unit control standards, she may be managing a continuous stream of capital projects requiring new terminal units. The owner may also have only one or two building automation vendors performing the terminal unit controls work. As such, if the VAV box controls are always the same, the controls contractors do not need to reinvent the wheel for every project. They can have standard submittals, standard program language, and standard startup checklists for the terminal units. This should result in lower control system costs for each project.
Standardized Commissioning & Recommissioning Procedures
Each time a new terminal unit is installed as part of a renovation or new construction project, the owner will want to commission it. If the control sequence is always the same, the functional performance test procedure can also be the same. As such, the cost of commissioning is reduced because the commissioning provider does not need to develop a new test procedure for the terminal units on every new project.
In addition, if the owner’s facilities staff has the availability, they can be trained to perform the terminal unit functional performance testing in-house instead of outsourcing the testing to the commissioning professional.
This in-house knowledge and capability can then be transferred to periodic recommissioning procedures. These procedures can be integrated into the owner’s preventive maintenance program to ensure that every terminal unit is given a thorough performance evaluation every few years, or more frequently, depending on the criticality of the spaces served. If O&M staff have the same expectations and acceptance criteria for every terminal unit of a certain type, this work can be very objective, efficient, and effective.
Ability to Automate Performance Diagnostics
Taking the ongoing performance challenge a step further, with common sequences of operation for all VAV units, the owner can use the BAS to look for and alarm common failure modes. If units are all programmed the same, the clues to failed performance will be similar. For example, for a simple single-duct hot water reheat VAV box:
If the reheat valve is commanded fully closed and the VAV discharge temperature is 5°F higher than the AHU supply air temperature.
If the terminal unit damper is commanded fully open and the set point airflow is not achieved within 30 minutes.
If the terminal unit damper is commanded fully closed and the measured airflow is 20% greater than the set point airflow after 30 minutes.
If the terminal unit damper is commanded fully open continuously for more than 24 hours.
All of these conditions can generate an alarm and/or workorder for maintenance to visit the terminal unit, identify the root cause of the problem, and correct it. This process can find and fix performance and energy conservation problems as proactively as practical.
There are many reasons for a building owner to take the time to define standard VAV terminal unit operating sequences and enforce their design and implementation in capital projects. Perhaps the most compelling reasons are the long-term performance and energy benefits of easier troubleshooting and maintenance in the many years after construction.