VVT System Overview & Benefits in Commercial HVAC Design
When deciding on air zone HVAC systems, the Variable Volume & Temperature (VVT) is an HVAC control system with many advantages. The VVT system overview is a cost eﬀective way for building occupants to maintain and control temperature. This type of system is traditionally for small to mid-sized low-rise commercial office buildings. Read on to learn what they are and if it makes sense for your building.
VVT System Explained
Variable Volume & Temperature is a zone damper terminal air unit system that creates floor zoning paired with rooftop air handling units (RTUs). The design is to create multiple zones within the building and have control from the single RTU via a bypass damper between the supply and return ducts at the RTU. This process relieves excess duct static pressure when zones are throttled back or closed. VVT systems must maintain constant pressure in the system.
The purpose is for each zone to control its temperature to the desired setpoint of the occupant by modulating a damper installed in the ductwork. Airflow in this system is neither measured nor directly controlled at the terminal unit. Each zone has a wall thermostat that controls its respective zone damper but communicates with a master unit that polls all zone thermostats for their heating or cooling requests.
How It Functions
The VVT system begins with the RTU, which energizes heating or cooling. The RTU can only provide heating or cooling at any given moment, not both simultaneously. If the current condition is for heating zones, the dampers of the zones calling for cold are forced shut. RTU uses heating or cooling coils to control the temperature of the air.
When some zones close, the static duct pressure increases, and a transmitter senses the static duct pressure, which modulates the bypass damper to relieve the pressure and maintain the static duct pressure setpoint. The pressure sensor in the central supply duct opens a damper in the bypass duct, directly connecting the supply and returning airflow.
After the calling zones are satisfied, the unit will switch over to the opposite mode to satisfy the remaining zones. The previously open dampers will close, and the bypass damper will continue to modulate to maintain the static duct pressure setpoint. Once all the zones are satisfied via feedback from the thermostat, and there is no call for the other mode (heating or cooling), the compressor or burner can shut off until needed.
Benefits/Advantages of VVT Systems
The benefits of the VVT system are that it has a lower capital cost, and the rooftop units are cheaper than central plants with boilers, chillers, and associated piping and controls. Variable frequency drive (VFD) fan motors cost more than the constant flow fans in the VVT packaged system unit.
These systems require a bypass duct between the supply and return ducts at the unit. For retrofits to existing equipment, there must be headroom available for the bypass duct. They also require ceiling space for terminal boxes and straight lengths of ductwork. For low-rise buildings with flat roofs, rooftop units are more attractive than building a mechanical room for a central plant, making the VVT design a better fit.
The same unit in VVT systems requires a “timeshare” methodology between heating and cooling when both are necessary. This timeshare method is critical during spring and fall seasons for buildings that need interior cooling during the heating season. VVT systems vary in temperature and don’t require reheating of air.
Cost plays a significant role in engineering decisions. The low initial cost of a VVT system makes it the more attractive of the two options for smaller budgets. For retrofits on buildings with existing rooftop units, a VVT system is the best choice.
The Severn Group Can Help
This system typically requires a comprehensive maintenance plan. This is typically due to the system’s frequent switching from heating to cooling. The starts and stops can shorten the equipment’s life. Typical maintenance includes changing balance damper positions, wiring in more straightforward thermostats, blocking off diffusers, and removing damper actuators. You will want regularly scheduled maintenance to extend the life of your VVT system.