Variable Air Volume (VAV)

Each person has a preferred temperature.

In the past people not comfortable with the temperature simply stuck a piece of material over the outlet vent. This action the left a surplus amount of air in the system.

The VAV simply provides for the accommodation of individual requirements.

It also provides for changing weather.

Air conditioning systems utilizing Variable Air Volume devices are very efficient in the use of energy. They are used primarily in commercial air conditioning systems. The VAV unit is a sophsticated damper system containing an adjustable static pressure controller such that as the damper is closed the S.P. is reduced and controlled, reducing the volume of air entering the space. As the VAV units reduce the volume of air circulating, the blower rpm is reduced through the use of a VFD, Variable Frequency Drive, controlled by a static pressure controller located in the main supply air duct. The VFD controls the speed of the electric motor driving the blower, to maintain required system static pressure.

The ratio of the power required is the cube of the ratio of the RPM - (HP1/HP2)^3 = (RPM1/RPM2), thus with 100% power required at design full load, at 70% load the power required would be (0.7)^3 = 34% less slight decrease in efficeincy and small decrease in P.F., usually net power required would be around 40% of design (100 %) power. Example would be a system requiring a 20 HP motor to handle design full load, at 70 % load, the system would only require approximately 8 HP. The motors are standard, but designed for VFD operation.

Both the VAV system and the VFD units went through several years of development with varying degrees of success. These systems now are very reliable and have proven themselves to be real energy savers.

Variable Frequency Drives are being used in hydronic pumping systems and in systems using centrifugal and screw refrigeration machines.

Air conditioning systems generally operate at 80 % full load or below most of the time, but the systems are designed to meet the full design load, required for the other 20% of the time.

In a VAV system the primary air handling unit maintains a constant temperature of 55 degf and at the VAV a reheat coil is in place to heat the air to provide heat when necessary. A VAV system can provide individual control locally.

Everything that everyone has said about the VAV system is true. A VAV system is very efficient and is very good at individual temperature control. The biggest draw back from the system is cost it is a very expensive system to install. I have watched this system evolve over the last 30 years and I don't think that there is a better system for controlling temperature in an efficient manner. If you can afford the installation cost.

The system that VAV replaced was Dual Duct this did an excellent job of controlling temperature, when it was working. But it did not do it in a very efficient manner. And maintenance was VERY high. The other big draw back of a Dual duct system is real estate, they take up a lot of room. With a dual duct system you basically had two AHU one suppling cold air and the other suppling hot air. This was a constant volume system so both fans would be running a max speed all the time. You would then have a damper in each room that would modulate between the hot and cold air giving you a constant air flow in the room. Very good control very expensive to operate. They have improved the control of a dual duct system over the years and made them operate more like a VAV system.

The other system that engineers like because it is so efficient is water to air HP. In this system condenser water loop is run throughout the building with the HP's condenser side connected to the water. As a room needs heat or cooling the HP draws heat out or dumps heat into the condenser loop. The reason that this can be so efficient is because the north side of a building might be cold but the south side is hot and the heat pumps move the heat around the building. The biggest draw back of a HP system is that the HP compressors have a limited life, at some point you will have to replace them. So maintenance is can be very high indeed.

In general in a large commercial building the VAV system has become the primary system of choice.