By Kathleen Matthews, Graduate of California State Polytechnic University
With all of the new, high-efficiency, or “green” technologies for HVAC systems in the marketplace today, it is tempting to want to scrap an existing system all together or at the very least replace it piece by piece. The truth is, equipment is only part of the puzzle. The other parts are maintenance and operation. The importance of maintenance is not to be overlooked, but proper operation is key. A less efficient system operated well will perform better than the most efficient system operated poorly. Great energy saving opportunities exists in improving the control of your existing system.
A Good Place to Start
Changes in system operation over time are common and even necessary. Sometimes these changes improve operations, but often they are made to solve short term issues. As the changes accrue, the overall performance of building systems can suffer. To ensure that building systems are operating in accordance with operational needs and design intent, start by implementing a commissioning program that periodically verifies system performance and accounts for needed modifications. This will establish a performance baseline.
Once you have a good baseline for where your building stands operationally, review the equipment operating schedules to see if run-time hours can be cut back. Running an HVAC unit even one hour less per day can save up to 260 hours of run time per year. Evaluating occupied and unoccupied thermostat setpoints is also important. Using a setpoint of one degree lower in heating mode or one degree higher in cooling mode can save 1-3% of heating and cooling costs1.
Control Strategies
Making changes in the way the building is controlled can be a complicated process. Some existing control systems will have sensors and feedback already in place that will make the following sequences simply a matter of programming. However, other systems will require new sensors which can mean a significant up-front cost. To evaluate what it will take to implement these ideas in your building, it is a good idea to talk to your controls contractor or design engineer. They can help you determine projected costs and energy savings to determine the payback time in your building. They may have other suggestions specific to your building as well. All buildings and climates are unique; what works well in one location may not in another.
VAV Hot Water Reheat
The majority of existing buildings with VAV reheat systems employ a “single maximum” control strategy. In this control sequence, at peak cooling the airflow setpoint is the maximum amount of air the VAV box is set to deliver. As cooling requirements decrease, airflow dwindles until it reaches its minimum setpoint. This setpoint will be based on the airflow needed at design heating and is typically 30% to 50% of maximum cooling airflow2. When it reaches this minimum, the system is in its deadband and is neither heating nor cooling. As the system moves into heating mode, the airflow rate remains at the minimum setpoint and the reheat valve modulates open until the system load is met. This approach keeps VAV airflow minimums higher than they need to be in deadband, as well as the times when heating and cooling loads are not at design conditions. In all of these cases, energy is wasted.
Figure 1. “Single Maximum” Control Strategy
There are different control strategies to try to prevent these energy losses. Recent energy codes including California Title 24 Energy Standards now require that VAV systems with hot water reheat employ the “dual maximum” 2,3 strategy. In this control sequence, at peak cooling the airflow setpoint is the maximum amount of air the VAV box is set to deliver. As cooling requirements decrease, airflow decreases until it reaches its minimum setpoint. This setpoint will be based on the minimum controllable level of the VAV box or the required ventilation rate, whichever is higher. When it reaches this minimum, the system is in its deadband and is neither heating or cooling. Up to this point (cooling and deadband) the sequence looks the same as in “single maximum” logic, although the minimum airflow rate is lower. As the system moves into heating mode, the sequence is broken in the two phases. In the initial phase of heating, the airflow will remain at the minimum airflow rate and the reheat valve will modulate to full open. If additional heating is required, the airflow rate will increase until it reaches a maximum heating airflow setpoint. This maximum heating airflow is the same as the minimum airflow rate that is allowed in the “single maximum” strategy. During both of these phases, the VAV box works to maintain a constant supply air discharge temperature to the room. This requires a sensor that monitors the temperature of the supply air discharge. In the heating phases, the supply air discharge temperature should be approximately 90°F; air temperature should not be allowed to get too hot to prevent stratification and short circuiting of the hot air to the return plenum.
Figure 2. “Dual Maximum” Control Strategy
Static Pressure Reset
In existing buildings, VAV systems are commonly controlled to maintain constant static pressure at a specific point in the system. More recent energy standards and codes including ASHRAE Standard 90.14 and California Title 24 Energy Standards3 now require that VAV systems with DDC controls at each zone employ a “static pressure setpoint reset”. In this strategy, the critical zone changes as loads vary throughout the building. Instead of maintaining a constant setpoint at a specific area, the setpoint must be reset constantly so that it maintains the static pressure required by the critical zone. This strategy allows fans to operate at generally lower static pressures. Studies have shown that 30%-50% of the fan energy can be saved using this techniques.
There are different ways to achieve this setpoint reset, but essentially two things are needed. First, there must be a way to determine the critical point at any given time. This can be done indirectly by identifying the position of the VAV dampers. The damper that is wide open and running at its maximum will act as the critical zone. Secondly, the control system must use logic that continuously checks for dampers that are wide open and need a higher static pressure, and increases static pressure settings accordingly. There are different ways to achieve this—one being “trim and respond” logic. Using this logic, the static pressure is trimmed back in small increments over time until the controller sees that a pressure increase request is required. Upon this request, the system static pressure increases and the system begins to trim again until it receives another request.
Implementing VAV Control Strategies
Both of the control sequences have the potential to save energy and money. Depending on the building, they could have significant up-front costs. Measuring discharge temperature for the “dual maximum” sequence requires a sensor that will cost approximately $250-$500 per point. The static pressure reset sequence requires that you know the position of your dampers. There are a variety of sensors that can serve this function, costing approximately $500-$750 per point.
Using smart strategies to control VAV systems can save energy and money. For more information about these sequences and other ways to save money through improving building system controls, refer to the reference section below.
Kathleen Matthews is a graduate of California State Polytechnic University at San Luis Obispo’s ABET accredited program in Mechanical Engineering with an emphasis in HVAC. Matthews has also participated in energy studies, the development of commissioning plans, evaluations of mechanical systems, performed condition assessments, developed cost of replacements for mechanical and electrical systems, and LEED certification of commercial office buildings. Matthews is a staff engineer in Facility Engineering Associates’ San Francisco, CA office location.