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Commissioning: Reconsidering Resets

A current trend can prove troublesome if it takes itself to extremes.

One the most common energy conservation strategies being recommended, specified, and implemented these days is automatic reset of selected system setpoints based on real-time building needs. This is most frequently applied to VAV air-handling systems. With full BAS coverage of all air handler and VAV terminal unit components typical in almost all new projects and in an increasing number of existing buildings, there is clearly an opportunity to control central VAV air handler components based on feedback data from the terminal units.

The terminal unit controls provide information through the BAS regarding what is happening at the individual temperature control zones (e.g., the VAV box damper command); the reheat valve command; whether the space temperature is above or below setpoint; whether the zone airflow is above or below its setpoint. Each of these points can be interpreted with respect to whether the zone is in cooling mode or heating mode, or whether the zone is receiving adequate airflow or is being starved.

To use a military analogy, the VAV terminal units represent the front line in the HVAC system’s effort to achieve the space temperature and IAQ goals for the occupied spaces. The central air-handling system represents the support troops whose job it is to make sure the front line has all of the resources necessary to do their job. For a VAV air-handling system, the air handler is responsible for providing the VAV boxes with appropriate temperature air, a minimum amount of outdoor air, and an adequate total amount of air.

Traditionally, central VAV AHUs have been controlled to supply air at a constant temperature, as calculated by the design engineers to be the temperature needed to cool all of the spaces during the peak cooling time of the year. Similarly, the central supply fan speed has traditionally been controlled to maintain a constant supply duct static pressure to ensure adequate backpressure at each VAV box to allow for maximum design airflow to spaces during the same peak cooling time of the year. 

Seeking Improvement

The traditional control sequences have saved significant energy compared to the constant volume or multizone air-handling systems that used to be commonplace before VAV. However, as we try to squeeze even more energy out of our facilities, it has become apparent that we can improve on the constant supply air temperature/constant duct pressure setpoint sequence.

Most buildings are only likely to experience “peak cooling” conditions once or twice a year and maybe not even then, depending on how the owner has chosen to occupy and use a facility. Therefore, it saves energy during all other times of the year if the air-handling system knows exactly how much cooling is needed at any particular time and adjusts the discharge temperature and/or static pressure setpoints accordingly.

Given the significantly increased data processing capability of central BAS systems available over the past 5 to 10 years, it is now possible to continuously share all key terminal unit parameters with the AHU controller. If any single terminal unit indicates a need for lower temperature air, the air handler’s discharge air temperature setpoint can be reduced to accommodate that need. Similarly, if a terminal unit’s damper is fully open and not achieving its setpoint maximum airflow, the supply duct static pressure setpoint can be increased.

Theory vs. Reality

These are solid energy conservation strategies in theory, but what we invariably find in real building retrocommissioning is that there is at least one terminal control zone which is influencing the entire air-handling system such that the supply air temperature setpoint is always at its lowest value and/or the duct static pressure setpoint is at its highest value. This defeats the entire purpose of programming the reset strategy.

The reasons behind this scenario vary but can include that the VAV box damper, valve, or airflow measuring station has failed in some way; the space temperature sensor is out of calibration; the space temperature setpoint is outside of the original design range; or the space use has changed and its cooling load increased subsequent to design and installation of the VAV box.

To help mitigate this problem, I recommend monitoring how long an individual terminal unit is the “controlling” zone for the air handler setpoints. If it remains the controlling terminal unit for more than 24 (adjustable) continuous hours, I recommend sending an alarm to the central workstation and removing that particular terminal unit from the air handler reset strategy. It can be restored after facilities maintenance and/or engineering address the inevitable terminal zone issue.

It seems that a little more control system programming would help building operators realize potential energy savings and utilize the terminal unit feedback information to proactively identify problem zones requiring maintenance or engineering attention.


Commissioning: Getting It Right

Engineered Systems, April, 2010

Rebecca Ellis, PE, LEED AP, CCP, CPMP, CxA
Questions & Solutions Engineering
1079 Falls Curve
Chaska, MN  55318