VAV static pressure

[comfortmaster1]

Does it take more torque to open a VAV damper than it does to close it?

Have anyone seen an electronic VAV controller that output more torque (in the form of 24VAC) to drive open than to drive close?

I ran into one today. Drive open (voltage output) would turn the motor. Drive close would not. (nothing wrong with the motor)

I put in a (used) slow acting actuator and everything worked. The fast acting actuator did not. I'm thinking: the slower the motor, the less the torque(input) it needs.

[chrisc]

It's going to put more load on the motor when closing against the airflow. Usually you'll notice the motors are rated in stroke time, typically 1.5 min to stroke from 0-100% open. Torque ratings should be typical across the board, depending on the application.

[comfortmaster1]

Thanks for replying. However, I don't understand how the VAV motor/damper is closing AGAINST or WITH the airflow. The air flow (or air pressure) is acting on the top and bottom of the damper evenly. So when one (top or bottom) end of the damper is acting AGAINST airflow, the other is acting WITH airflow.

Theoretically, the pressure will cancel each other out and the net effect is zero - so it does NOT take more torque to open OR close the damper.

However in reality, I do notice that most of the time when the damper shalf is free to spin, it will slam close but not the other way. That tells me the damper wants to close more than open. Conclusion: if you make the damper do want it don't want to, you are going to need more force than the other way around.

Feel free to comment.

[hotrod53]

I have seen problems like you are experiencing only in two cases...The VAV is in such poor repair that it is internally binding, or the static pressure in the duct is too high. A typical a Belimo LF24 or LF24SR under normal instalation conditions can open a 12 or 14" box with no problem.