Description:

The retro-commissioning package includes operation and maintenance measures. Estimates for these measures come from the Department of Energy's Advanced Energy Retrofit Guide. These include:

Reduce envelope leakage

Air leakage through the building envelope most often occurs where building envelope elements are connected together. Leakage is typically a result of either improper design or construction, lack of maintenance, or normal degradation over the life of a building. Envelope leakage is most pronounced when the HVAC system is off, i.e., when the building is not mechanically pressurized. Significant night-time air leakage causes the HVAC systems to operate harder upon morning start-up, to bring the building back to setpoint temperature.

Energy savings can be achieved by identifying significant air leaks in the building envelope and sealing them. Specific methods of sealing will vary depending on the component(s) being sealed. In general, large gaps should be sealed with a structural material before applying caulk.

For office buildings, common areas of air leakage include curtain wall structures, soffits, roof-to-wall joints, expansion joints, parapet flashing, and roof penetrations. Infrared cameras can help identify air leaks in buildings.

Air leakage can affect occupant comfort, HVAC system performance, window and door performance, and building energy usage.

Revise air filtration system

Many large built-up air handling systems include both pre-filters and final filters for cleaning the air before it is supplied to the zones. The pre-filters are installed to extend the life of the final filters, yet typically they do not achieve this since it’s easy for dust to pass through and around the pre-filters. In addition, the pre-filters impose significant additional pressure drop, resulting in higher fan energy use and maintenance costs. Extended surface area filters are now available that can replace both the pre-filter and final filter. These extended surface filters have a long life (high dust holding capacity) and low pressure drop characteristics. Both maintenance (labor) and energy savings can be achieved by using these types of filters.

Calibrate air-sensors

HVAC systems rely on input from sensors to determine how to operate. However, these sensors can drift out of calibration over time. Sensors found in typical HVAC systems can include, but aren’t limited to, temperature, pressure, and flow sensors. If these sensors are not calibrated - i.e., if the value being reported by the sensor does not match the actual condition – this could negatively impact equipment performance and occupant comfort, and could result in energy waste due to simultaneous heating and cooling. Calibrated sensors are necessary for automatic control sequences to operate properly, and for accurate diagnoses of system performance.

This measure requires developing and implementing a sensor calibration plan. In general, sensor calibration consists of comparing reported sensor readings (e.g., at the building automation system) with readings from a calibrated device, and taking corrective action where there’s a significant difference between the two readings. Corrective action might include simple offsets or multipoint calibrations to align the readings. If the sensor is significantly out of calibration, replacement may be necessary. For example, if the supply air temperature setpoint is reset based on outside air temperature, it’s important to ensure that the outside air temperature sensor is calibrated and located in a representative location.

Re-enable the supply air temperature setpoint reset

For multi-zone air systems, whether CAV or VAV, automatically changing the supply air temperature setpoint to better match the needs of the zones can result in lower reheat energy use due to reduced amount of zone reheat (simultaneous heating and cooling). The supply air temperature setpoint is typically reset based on either outside air temperature or an indication of zone demand - e.g., average difference between zone temperature and zone temperature setpoint.

Care should be taken when implementing this measure to verify that internal zones receive enough cooling at higher supply air temperatures. While significant reheat energy can be saved by implementing this measure, there is usually a slight increase in fan energy usage due to internal zones requesting more airflow at the higher supply air temperatures to maintain space conditions. This fan energy penalty should be weighed against the reheat energy savings.

Reduce HVAC runtime, close outdoor air dampers during unoccupied periods

The maximum energy savings related to an HVAC system can be achieved by shutting the system off when not in use to minimize run time. This measure reduces the scheduled operating hours of the HVAC system, including fans, pumps, chillers and boilers, to more closely match the occupancy of the building.

In addition, many HVAC fan systems operate with the outside air damper open whenever the fans are operating, even during morning warm-up and cool-down periods, prior to the occupied period. Since ventilation is only required during occupied hours, the outside air dampers can be closed during non-economizer operation in unoccupied hours. This feature eliminates the energy associated with cooling and heating outside air when ventilation is not required.

Reduce economizer damper leakage

An airside economizer cycle utilizes outside air for cooling a facility when the outside conditions are cooler than inside conditions. Economizer cycles reduce the amount of mechanical cooling energy necessary for cooling a facility. During integrated airside economizer cycle operation, when the outside temperature is cooler than the indoor temperature yet warmer than the supply air temperature necessary for cooling the space, the outside air dampers are fully open and the return air dampers are fully closed.

If the return dampers are leaky, meaning if they don’t have blade and jamb seals and/or they are not adjusted to close fully when commanded to do so, the effectiveness of the economizer cycle is reduced and more mechanical cooling is required than would be necessary if the dampers leaked less.

Some HVAC systems include a morning warm-up/cool-down cycle. During this cycle, the space is cooled or heated to address the heat gained or lost during the night (unoccupied period). Typically this cycle occurs prior to the start of the occupied period, and the minimum outside air dampers remain closed during this period. If these dampers are leaky, more outside air would be drawn in than is necessary, adding an unnecessary load on the HVAC system.

Minimizing the air leakage through closed outside and return air dampers can reduce HVAC system energy use during integrated economizer operation and morning warm-up/cool-down cycles.

Calibrate water sensors

HVAC systems rely on input from sensors to determine how to operate. However, these sensors often drift out of calibration over time. Typical waterside sensors can include temperature, pressure, and flow sensors, to name a few. If these sensors are not calibrated – i.e., if the value being reported by the sensor does not match the actual condition – this could negatively impact equipment performance and occupant comfort, and could result in energy waste due to simultaneous heating and cooling.

This measure consists of developing and implementing a sensor calibration plan. In general, sensor calibration consists of comparing reported sensor readings (e.g., at the building automation system) with readings from a calibrated device, and taking corrective action where there’s a significant difference between the two readings. For example, if chillers are staged based on measured chilled water flow, then it’s especially important to regularly calibrate the chilled water flow sensor to maintain performance, equipment reliability, and energy efficient operations.

Shut down the cooling plant when there is no cooling load

Facilities often run their cooling plants continuously during occupied periods of the cooling season. This includes the chiller, cooling tower, and related pumps. However, there may be periods during the cooling season when the plant does not need to run, especially when airside economizer cooling is used at the air handlers. If there is a space that requires continuous cooling, even after hours, it may be more efficient to install a small cooling system to serve this space, and turn off the larger plant.

Automatic controls can be added to turn off the entire cooling plant when cooling is not needed in the facility during occupied hours. Two common automatic methods include:

• Outside air temperature or enthalpy-based lockout. These controls would turn off the cooling plant when the temperature or enthalpy drops below a certain value.
• Enable plant based on the cooling demand. These controls would turn off the cooling plant when there is no cooling demand (e.g., all air handler cooling coil valves are closed). Adequate time lags and proper trigger points need to be programmed with this strategy to prevent the plant from cycling on and off excessively.

 

Before this measure is implemented, the retro-commissioning team should ensure that the cooling plant is not operating after hours. Turning the cooling plant off during unoccupied hours is typically easier to implement than cooling plant shutdown during occupied hours, due to greater building operator buy-in.

For facilities with multiple chillers, the chiller staging strategy can have a large impact on the energy consumption of the plant. It’s often beneficial to shut off chillers at low load, to reduce pumping energy consumption and increase chiller efficiency. Each facility is different, and will have its own optimal chiller staging strategy. BAS software overlays are available that will continually optimize the performance of a chilled water plant.

In addition to energy savings, this measure should result in increased equipment life due to less run hours for the cooling plant.

This measure may not apply to smaller buildings, which typically do not have a cooling plant.

See DOE’s Better Buildings Decarbonization Resource Hub for more information on energy efficient HVAC and building envelope technologies that can help to cost-effectively decarbonize buildings.