More Opportunity for Higher-Occupancy Building Types

Office, education, and health care buildings are ideal for re-tuning because they are designed with multi-zone systems, which can adjust their operation to match the varying heating, ventilation, and air conditioning (HVAC) requirements.

The process of re-tuning focuses on energy savings opportunities. Many re-tuning measures aim to adjust the comfort heating and cooling operation to correspond to the occupancy patterns. Because the temperature maintenance at warehouses and storage buildings is not for occupant comfort heating and cooling, re-tuning is less suited for those types of buildings. Warehouses and storage buildings are also typically equipped with unitary equipment with factory-installed controllers that do not allow for program modifications, which limits the re-tuning opportunities.

EUI is a metric used to measure the energy consumption normalized by the building square footage. Energy use by a building is affected by multiple factors, such as climate, weather, building activity, and building construction. The higher the EUI in a given building category and climate zone relative to a benchmark EUI, the greater will be the potential for energy savings opportunities for a re-tuning project. Commercial Buildings Energy Consumption Survey (CBECS) Table C7 through Table C9¹ provide consumption and gross energy intensity by Census division for the sum of major fuels. Each EUI provided in these tables can be referenced as a benchmark EUI.

Another resource by which to gauge the energy savings potential is ENERGY STAR’s Portfolio Manager. Portfolio Manager provides a score between 1 to 100 for each building’s energy performance, taking other factors into account, such as business hours and climate regions. Selecting a set of buildings that have lower scores will increase the energy savings potential. It is important to note that energy consumption savings do not directly correlate to energy cost savings. Electric and other fuel rates vary greatly depending on the region and utility tariffs.

¹ Table C7 provides data for: New England, Middle Atlantic, and East North Central regions

The likelihood of benefiting from and the savings potential of re-tuning is higher for buildings for which a building automation system (BAS) controls HVAC equipment such as variable air volume (VAV) systems, dedicated outdoor air systems (DOASs), and/or heat recovery systems.

An office building larger than 50,000 ft² is more likely to have VAV systems that feature central air handlers, boilers, and/or chillers. A building smaller than 25,000 ft³ is likely to have either residential or small commercial packaged equipment with basic controls. For small buildings, re-tuning opportunities tend to be limited to scheduling and zone setpoint management.

²Table C8 provides data for: West North Central, South Atlantic, and East South Central regions

³Table C9 provides data for: West South Central, Mountain, and Pacific regions

A modern BAS’s control capability is not limited to the air-side HVAC equipment. The BAS may control boilers, chillers, pumps, lighting systems, snowmelt systems, and garage exhaust systems. The more equipment and systems the BAS controls, the greater will be the opportunity for re-tuning.

The ideal air-side systems are multi-zone VAV systems and DOASs. These systems serve multiple terminal VAVs or fan coil units, and the central air handlers are designed with fully modulating damper actuators, coil valve actuators, and variable-frequency drives (VFDs). The process of re-tuning will evaluate the modulation sequence to improve equipment efficiency.

Variable refrigerant flow (VRF) systems may not be ideal because of their proprietary controls, which limit the ability to optimize the system.

Modern direct digital control (DDC) systems, using an open communication protocol, such as BACnet, have become more widely available in the past 20 years. The transition to DDC has allowed equipment controllers to be integrated into a BAS, which increases the visibility and control of ancillary points. This transition enable operators to tune the equipment control with tight parameters. Additionally, DDC enhances equipment monitoring and trending capabilities (See Screening Attribute 6). Legacy control systems, such as pneumatic systems, are not generally Re-tunable. Also, verify that necessary software updates and maintenance have been provided.

Re-tuning is a data-driven process that evaluates the equipment operation via a BAS. HVAC component trend data and/or interval energy meter data can often uncover and determine the source of operational inefficiency. While the availability of historical data, ideally interval data, is not necessary, it makes re-tuning easier and potentially more successful.

For instance, the natural gas interval meter data combined with outside air temperature data can reveal the building heating characteristics. The high natural gas consumption above a building balance point temperature (a temperature at which a building does not require heating or cooling, for example, an outside air temperature of 55°F) may indicate the presence of simultaneous heating and cooling within the building. The same set of data can be used to evaluate the building operation during occupied hours versus unoccupied hours. If the electric energy consumption pattern remains consistent throughout a 24-hour period, it is an indication that the building is not using scheduling and temperature setback capabilities.

The equipment component trend data can be compared to the design drawings and control submittal documents to identify control issues or inefficient sequences of operations.

Many re-tuning measures fine-tune the control setpoints to match the supply of heating, cooling, and ventilation to the instantaneous demand in the building. For instance, the "fan static pressure reset" measure typically resets the duct static pressure setpoint for the air handling unit supply fan VFD, based on the feedback from the downstream VAV damper positions. The functionality of this measure relies on the readings of the duct static pressure sensor, as well as properly functioning VAVs.

Successful implementation of re-tuning measures depends on the overall reliability of control components within the BAS:

• Temperature, pressure, and airflow sensors are calibrated and accurate
• Valves, dampers, actuators, and VFDs can fully modulate and are functioning with no known issues (no leaks, etc.).