Many older buildings were designed with dual-duct constant-volume HVAC systems. The air handler runs at a constant speed and delivers the same volume of air to every zone at all times. The temperature of the air is varied by mixing hot and cold air streams to either cool or warm the space, as called for by the thermostat.
The problem with such a system is that there is a lot of reheat; e.g., even during the cooling season the air is cooled and warmed repeatedly. Although this kind of system can be good for humidity control, it’s wasteful of both the chiller and boiler energy.
A commonplace retrofit is to modify the system to a variable air volume (VAV) arrangement. Each constant-volume box can be either rebuilt into a variable-volume box with a special kit or replaced with a new VAV box. Both hot and cold air are still delivered to each mixing box, but the VAV box varies the airflow to maintain space conditions. This reduces the system reheat and continues to provide a comfortable environment in each zone served.
Further savings are possible for air handlers that serve either all interior or all exterior zones, i.e., areas that do not ever need cooling and heating simultaneously. This air handler can be modified to a single-duct VAV system that can reduce wasteful reheat to an absolute minimum. In either situation, the air handler no longer delivers a constant volume of air at all times since the VAV boxes all vary the airflow to each zone. Consequently, the airflow must be modified, with either inlet vanes or a variable-speed drive, controlled by a static pressure sensor in the supply duct. Both arrangements are configured to maintain a fixed, constant static pressure in the ductwork a few feet downstream of the air handler, which ensures that every VAV box has sufficient air delivered to it.
With inlet vanes, the fan still runs at full speed all the time, while the vanes modulate to control the airflow. The variable-speed drive, on the other hand, actually varies the fan motor speed to satisfy the static pressure sensor setpoint. This can be a real energy saver, especially for larger motors on air handlers that operate for long hours.
A less costly alternative is a combination of these two methods. Installing a new high-efficiency two-speed motor with existing inlet vanes can save nearly as much as a VFD system. The low load conditions can be handled at the low motor speed, using the vanes to modulate airflow. As the load increases and the static pressure drops below setpoint with the vanes full open, the motor engages the fan in high gear, and the vanes continue to modulate airflow to achieve the setpoint.
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Excerpt from Energy Conservation Projects.