pressure?

[michael26]

May be a dumb question but what is the difference between pressure dependent configuration and pressure independent? What does this mean?

[crab master]

Pressure Dependent would be like a VVT system, where typically you do not have airflow sensors and the damper is controlled based on how far from setpoint you are. The further from setpoint the more the damper will open up to max position. The closer to setpoint the damper will will go to minimum position. Both can also be based on supply air temp from the air handler as well if applicable.
Pressure Independent has an airflow sensor and controls the damper position based on the airflow measured and the airflow setpoint in the controller.

[gbfromsd]

Pressure dependent - static damper positions, damper does not modulate as duct static raises or lowers. Volume of air delivered depends upon the static pressure.

Pressure independent - modulating damper. Damper moves to maintain a specified air volume regardless of duct static pressure. Volume of air delivered is independent of the static pressure.

Turn the supply fan off on a Pressure Independent VAV box and the damper will open wide trying to make setpoint. A pressure dependent box will just sit there.

[shifferbrains]

all pretty close answers.....

Actualy the answer is in the name.

Pressure Dependent...... Correct CFM at the VAV box is "Dependent" on a specific static pressure at the supply duct. (If the static pressure drifts or the setpoint has been changed the CFM at the VAV box will be effected.)

Pressure Independent..... The VAV box will maintain correct CFM "Independent" of the static pressure. Static pressure can drift or the setpoint changed with little or no effect on CFM.)