Re-tuning Methodology and Goals
Re-tuning™ is a systematic process aimed at minimizing building energy consumption by identifying and correcting operational problems that plague buildings. The investigative process of Re-tuning can also uncover and diagnose HVAC equipment deficiencies that require corrective actions. Because Re tuning relies on making changes to HVAC equipment operations, the buildings are required to have equipment in good working conditions that allow for fine-tuning.
The goal of Re-tuning is to tune the control setpoints and sequence of HVAC equipment operation to adjust the HVAC system operation to the actual or predicted conditioning needs of the building. Re-tuning measures developed by PNNL are based on the following four basic principles:
- If you don’t need it, turn it off.
- If you don’t need it at full power, turn it down.
- Mitigate simultaneous heating and cooling.
- Reduce infiltration and outdoor air.
“Turn it off” measures limit the equipment operation based on user-defined schedules or ambient conditions. A zone programmable thermostat or basic building automation system (BAS) can typically incorporate these measures. If lighting controls are integrated into the BAS, the lighting schedules should also be reviewed. Other measures to turn down the equipment load, mitigate simultaneous heating and cooling, or reduce infiltration and outdoor air, require feedback from various control components so that adjustments to the control setpoints can be made. These require a more sophisticated BAS as well as equipment that allows for modulating system operation.
Re-tuning does not require HVAC equipment upgrades. However, inaccurate sensors as well as broken control components are common issues that can hinder Re-tuning opportunities. When control component issues are persistent within a building, the building may require recommissioning. Re-tuning also requires an in-depth evaluation of the system operation as well as careful implementation of Re-tuning measures by a control expert. Therefore, the screening process becomes an important step in ensuring that Re-tuning can provide the intended benefits.
Re-tuning Screening Using Building Attributes
Screening a building in accordance with the seven attributes described below can help determine if a building can benefit from Re-tuning.
Screening Attribute 1: Building Types ─ 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 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.
Screening Attribute 2: Energy Use Intensity ─ The Higher the EUI, the Greater the Savings Potential.
Energy use intensity (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 C9 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.
Screening Attribute 3: Building Size ─ Buildings Larger than 50,000 ft2
The likelihood of benefiting from and the savings potential of Re-tuning is higher for buildings for which a 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 ft2 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.
Screening Attribute 4: Controlled Equipment ─ VAV Systems, Boiler Plants, and Chiller Plants Allow for Full Re-tuning
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.
Screening Attribute 5: BAS Age ─ Newer BAS Installed or Upgraded within the Last 20 Years
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.
Screening Attribute 6: Data Availability ─ Trend Data, Interval Energy Meter Data, and Building Design Drawings Will Identify More Re-tuning Opportunities
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 vs. 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.
Screening Attribute 7: Control Components Are in Good Operational Condition and Allow Modulating Controls
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.).
Re-tuning Candidate Checklist
The checklist below provides a summary of the attributes used to score a building’s Re-tuning suitability. Complete the checklist for each of the candidate buildings by marking the appropriate answer in either the green, yellow, or red box for each attribute. A Re-tuning suitability scale is provided below to gauge the overall Re-tuning suitability of your buildings.
Re-tuning suitability Scale:
- Excellent candidate: All attributes check green.
- Very good candidate: Majority of attributes check green with no red.
- Good candidate: Majority of attributes check yellow, no red.
- Potential candidate, needs further evaluation: 1 or 2 attributes check red.
- Poor candidate: More than 2 attributes check red.
Note that building level EUI or an ENERGY STAR score may not be available for buildings on a campus. If this attribute is unknown, then complete the checklist for the other attributes to screen buildings for Re-tuning.
Additional Considerations
Re-tuning is typically not recommended for buildings that are under a term of energy savings guarantee project, such as energy savings performance contracts (ESPCs). For buildings under an ESPC, determine that the changes to operational schedules and setpoints will not interfere with the terms defined in the contract. Also, it is important to verify that proposed Re-tuning measures comply with all code requirements of the local authority that has jurisdiction.
Re-tuning requires the engagement and agreement of multiple individuals/parties: building managers, owners, occupants, and control contractors. The building managers are required to secure the resources, including
- funding qualified control contractors to implement Re-tuning changes,
- allocating staff time for building management and operations staff,
- engaging stakeholders in advance and throughout the process, and
- providing operations staff with upfront guidance on controls changes implementation.
The limitations within the selected building should also be identified before Re-tuning activities take place. For example, limitations might include the following:
- Critical functions: Work with the tenant/space manager to determine if space may be Re-tuned. Can operation in these spaces be isolated from the Re-tuning effort if necessary?
- Spaces that have special HVAC requirements (e.g., mission critical spaces and/or 24/7 occupancy spaces).
Re-tuning Resources
Re-tuning resources are available at no cost. These resources include the following: