VAV and VVT Systems

 

A couple of acronyms in the industry that are often confused. The primary difference is this; Variable Air Volume systems are intrinsically cooling from the air handler. Sometimes they include re-heat on the outlet side of the box. Variable Volume and Temperature systems are modulating zoning with available volume on both heating and cooling cycles. The VVT air handler supplies heated or cooled air according to demand. 

 

VVT systems usually have some sort of voting scheme. More zones calling for cooling puts the system in the cooling mode until their demand is satisfied. Some systems operate on a first call priority. The first zone calling for heat or cool gets that mode before any zone can call for the opposite mode. Feedback from the zones determined what the air handler sends out to the system. The central controller stages heat and cool or economizer and controls static pressure in the ductwork.  

VAV systems, as previously stated are intrinsically cooling. If the space does not need cooling the VAV box shuts down the airflow to minimum position. If the space needs heat, some re-heat equipment supplies it.  Reheat can be a hydronic coil in the ductwork, an electric duct heater, or perimeter hydronic baseboard. A menu of control schemes may be applied.  

 

VAV boxes are either pressure dependent with no static pressure measurement at the box or they are pressure independent that do have static pressure sensors. Pressure dependent boxes will position strictly by temperature.  Pressure independent position by temperature and pressure. If there is no fan running they just open. Pneumatic controls on VAV boxed include a reset volume control that receives signals from both thermostat and static pressure pick-ups.  

 

Other categories of boxes include single duct, dual duct, and fan powered. Some boxes might have a bypass or relief opening. The dominant style in our area is the single duct pressure dependent with hydronic reheat.  

 

The single duct pressure dependent with hydronic reheat is the most popular in our trading area. With pneumatic controls, it is very effective yet simple system. Air pressure extends the piston of a pneumatic damper actuator. Maximum pressure is the fully open position. In a direct acting control set the pressure increases as the temperature increases. As the temperature falls, the damper actuator gradually moves toward the closed position. A minimum position is set up mechanically such that zero pressure is minimum airflow. As the pressure continues to fall, the pressure bleeds from the reheat valve allowing the spring to push the valve stem up opening the valve. One air line from the thermostat does this sequence.  

 

Converting this system to a Direct Digital Control system requires different signals for damper and valve actuators.  Smart actuators with the control and actuator combined as an assembly usually have terminals for an analog output to the reheat valve. Many manufacturers use floating signals rather than analog. Floating is most often found in the VVT world rather than VAV. 

 

The pioneers of VVT systems were Carrier and Trane. VVT is actually a trademark of Carrier. The Trane system is Varitrac. VVT systems are modulating zoning dampers modulate in both heating and cooling modes. Reheat is not an issue.    Zonex systems has targeted these systems for retrofit solutions when the original system comes to the end of it’s life. Other automation companies have their version as well. Pelican wireless is able to control existing Trane or Carrier damper actuators with a floating signal. Pelican also has a floating reheat output for systems where the application is a true VAV.  

 

Pelican thermostats are hardwired to the zone dampers.  They are wirelessly connected to the central plant controller. Central plant zone controller controls the heating, cooling and economizer functions as well as static pressure by-pass dampers or fan speed controller. In terms of user accessibility and affordability, Pelican is hands down the winner. 

 

 

In Summary:

VAV and VVT Systems

VAV (Variable Air Volume) and VVT (Variable Volume and Temperature) systems are often confused in the HVAC industry. Here's a breakdown of their key differences and characteristics:

Key Differences

• VAV Systems: Primarily designed for cooling with airflow controlled at the VAV box. Reheat may be added on the outlet side of the box to provide warmth when needed.

• VVT Systems: Use modulating zoning to adjust airflow during both heating and cooling cycles. The air handler supplies heated or cooled air based on system demand.

VVT Systems

• Typically operate with a voting system:

  • Zones requesting cooling will place the system in cooling mode until those demands are satisfied.

  • Some systems prioritize the "first call," meaning the first zone to request heating or cooling will dictate the mode until that need is met.

• Feedback from the zones determines the air handler's output, with a central controller managing heating, cooling, economizer functions, and static pressure in the ductwork.

VAV Systems

• Primarily cooling systems by design. When cooling isn’t needed, the VAV box minimizes airflow to its lowest position.

• If heating is required, reheat equipment provides warmth. Common reheat options include:

  • Hydronic coils in the ductwork

  • Electric duct heaters

  • Perimeter hydronic baseboard

• Control schemes vary, with two main types of VAV boxes:

  • Pressure-dependent: Positions are controlled solely by temperature (no static pressure measurement at the box).

  • Pressure-independent: Positions are controlled by both temperature and static pressure sensors.

VAV Box Types

• Single Duct: Common in the area, often paired with hydronic reheat. This simple yet effective design is popular with pneumatic controls.

• Dual Duct and Fan Powered models also exist, sometimes with bypass or relief openings.

Pneumatic Control in VAV Systems

• Air pressure extends the pneumatic damper actuator's piston. Maximum pressure fully opens the damper.

• In direct-acting controls, pressure increases as temperature rises, closing the damper as the temperature cools.

• A mechanical minimum position is set to ensure a baseline airflow level. If pressure continues to drop, the reheat valve opens as the spring pushes the valve stem upward.

Converting to DDC (Direct Digital Control)

• DDC systems require different signal types for damper and valve actuators.

• Smart actuators combine control and actuation in one assembly and often have analog output terminals for reheat valves.

• VVT systems typically use floating signals rather than analog outputs, which are common in VAV systems.

Notable Systems and Manufacturers

• Carrier pioneered VVT systems ("VVT" is a Carrier trademark).

• Trane offers a similar system called Varitrac.

• Zonex Systems provides retrofit solutions for aging systems.

• Pelican Wireless offers advanced control for Trane or Carrier damper actuators using floating signals. Pelican systems can also provide floating reheat outputs for true VAV systems.

Pelican Wireless System Features

• Pelican thermostats are hardwired to zone dampers and communicate wirelessly with the central plant controller.

• The central controller manages heating, cooling, economizer functions, and static pressure control via bypass dampers or a fan speed controller.

• Pelican systems are recognized for their user-friendliness and cost efficiency, making them a standout choice in the market.