March 25, 2017
Projects aimed at reducing energy consumption in existing HVAC systems are in high demand at hospitals and publicly owned buildings. Inefficient systems severely reduce any efforts to save energy and represent an opportunity for potential savings by modifying or replacing aging equipment and control strategies. Performance Contractors such as Johnson Controls Inc., Siemens, Trane and others work to improve lighting control, HVAC systems and water conservation in existing buildings. Some of these projects are funded by federal bonds and repayment is in the form of energy savings. Once complete the utility savings flow back to the customer. Third party testing should also be included in the project to document the changes that were made as well as to record the new operating condition of the systems. This article will explore two common strategies encountered during an energy-saving project and their potential impact to the building’s comfort and performance.
Demand Control Ventilation
According to the U.S. Department of Energy, thirty-six percent of the energy consumed in a commercial office building is from cooling equipment and ventilation. Efforts to conserve energy by reducing the need to condition outside air often includes demand control ventilation. Demand control ventilation (DCV) brings in a variable rate of outside air to a building based on actual occupancy rather than a fixed minimum rate per person. When an increase in carbon dioxide (CO2) is present the outside air damper opens to ventilate the space. During times when the building CO2 levels are below setpoint, the outdoor air damper remains at a minimum position. This provides the minimum amount of outdoor air an HVAC unit heats or cools prior to supplying a building. While this strategy is sufficient in providing only the minimum quantity of outside air to control the carbon dioxide concentration levels, it can lead to infiltration if the current exhaust quantity is not taken into account. Infiltration occurs when exhaust air from restrooms, kitchens and janitor closets exceeds the amount of outdoor air entering through the HVAC system. When this happens the building will become negative relative to the outdoors to the point where outside air enters through door openings and through cracks in the building envelope. Some project DCV designs have allowed for a 75% reduction in outside air supplying a building without consideration to the building’s existing exhaust airflow. When the DCV sequence was initiated on a recent hospital project, a negative pressure was generated and caused temperature complaints and dust and debris around the exterior doors due to the infiltration of outside air. Space pressures were also impacted throughout the hospital. Regardless of the percentage of outside air that is being reduced, pre-testing should be performed to determine the current amount of outside air and exhaust supplying the building. Building pressures relative to the outdoors should also be measured and recorded. In critical areas, such as an operating room suite in a hospital, space pressures should also be measured and recorded. After the DCV plan is implemented another test should be performed to determine the impact of the reduction of outside air. Upgrading Terminal Unit Controls Old, outdated pneumatic controls that haven’t been maintained are typically not calibrated and often not functional. Existing pneumatic terminal unit controls are frequently being replaced by digital controls and the proper calibration of these devices is critical to achieve the desired effect. On most of the energy retrofit projects we have been involved in, the airflow at the terminal unit is the most critical to achieving the optimum system performance. By calibrating each DDC flow controller, we can be sure that “at least” the terminal unit is controlling to the actual airflow setpoint. Whether or not the new DDC controllers are calibrated can mean the difference between the central air handling unit operating out-of control or maintaining the minimum speed required. On a recent energy retrofit project each air handling unit was initially found operating near 100% speed in an attempt to maintain duct static pressure setpoint. On nearly every terminal unit we balanced a calibration adjustment was required, and by the time we completed the systems we noticed that the variable frequency drives were operating at a much lower speed resulting in a substantial decrease in energy consumption. By implementing test and balance into the project to verify actual airflow, water flow and temperatures at the terminal units, additional energy savings and improved occupant comfort can be attained. Comfort issues such as overcooling the space will happen if too much air is passing across a VAV reheat coil when the controls “think” they have the sufficient airflow during heating mode. This will translate into the hot water system having to supply more hot water than required to potentially hundreds on terminal units. This will also render the chilled water plant less efficient by having to supply additional chilled water to the air handling units that are forced to supply excessive airflow to the terminal units. Eventually the maintenance and operating staff may have to increase the water temperature setpoint during nonpeak conditions to satisfy the setpoints… totally undoing the project fundamentals. Arguably, having the correct amount of airflow at the terminal unit is the most important component when modifying an HVAC system.
Building pressure and space pressure considerations should be taken into account when adjusting the minimum outside air to implement a DCV strategy, especially in a healthcare environment. The following should be considered when testing or implementing a DCV strategy in an existing building:
- What are the current outside air and exhaust air quantities? What is the current building pressure relative to the outdoors?
- Specified outside air ventilation rates are required for surgery and critical care in a healthcare environment. Prior to any changes in the outdoor air quantity, measure and document the current outside air change rate and calculate how much will be present during the minimum DCV mode?
- What are the current space pressures? A positive space pressure relative to adjacent areas is required for surgery and protective environment rooms.
- How will the outside air be controlled in a variable volume system? If airflow measuring stations are utilized to measure and control the outside air, each station’s calibration should be checked against a duct traverse.
- During all modes (minimum, maximum and economizer), the building pressure should be maintained and the outside air should deliver the proper quantity to make-up the building exhaust air. Space pressures in sensitive areas such as surgery rooms and protective environments should also be checked during all modes.
- All data should be recorded on the proper test sheets.
The following should be considered before modifying the terminal units or terminal unit controls:
- Is adequate flow and pressure available from the central air handling unit? Boxes should have enough flow to obtain the maximum airflow setpoint. Adjustments should be made so that the AHU is providing adequate airflow to each terminal unit.
- What is the current maximum, minimum and heating airflow setpoint of the terminal unit? If the controls are pneumatic, existing design documents such as schedules, submittals or TAB reports will need to be researched. Existing DDC controls should be programmed; however, they should be verified to be correct. If the terminal unit supplies a space that requires a minimum amount of air changes, refer to the current FGI standards for compliance.
- Who will measure and calibrate each terminal unit controller? This should involve measuring the airflow at each terminal unit using a capture hood or Pitot tube traverse and calibrating the controller. There may be additional components that need to be tested and adjusted on the terminal unit such as series or parallel fan controls and electric or hot water re-heat coils.
- How will deficiencies be addressed on existing components such as fans and dampers on existing boxes?
- Record all data on the proper test data forms.
