Ductwork for VAV system vs CAV system

In our PPD group discussion today we talked about using a VAV system over a CAV system because a CAV system uses more ductwork. I thought the basic system of the two were the same except for a VAV has a terminal box at the end of the supply duct that heat the air before it blows out into the room and is controlled by a thermostat in the room. What additional ductwork does a CAV system require?


If you refer to the book “Mechanical and Electrical Equipment for Buildings,” pages 393-395, which breaks down all of the different all-air HVAC systems. Figure © is the typical VAV system, and Figure (d) is your typical CAV system. CAV requires 2 full branches of ductwork, one which is heated, one which is cooled. Both branches travel all the way down side by side and then the cold/hot air is mixed in the ‘mixing box’ based on thermostat requirements. This offers more ‘comfort’ because the exact temperature can be controlled easier, but is less efficient, consumes more energy, and is more expensive to install. VAV has largely replaced CAV because it only has 1 duct branch with a constant air temperature and air is released at the variable volume box (volume varies, not temperature). VAV systems also essentially halve the amount of ductwork required to run through the building.