Dust collector return air with duct fan

[Paul F Franklin]

The air is going to take the path of least resistance. Since there is a little resistance in your plenum, and since the door opens easily, it's not surprising it blows open. Just put a latch on it as you say, you are not damaging the motor.

[Tom Bender]

The "Fan Laws" (laws of physics) are counterintuitive. They determine that restricting the airflow thru a fan will reduce the work that the motor must do. So latching your shop door will reduce the airflow and the motor load by a small amount, extending your motor life a tiny amount.

There is a caveat; if the cooling air thru the motor is reduced it will be bad for the motor. This should not apply in your case.

[Bob Michaels1]

Thanks for the info. I guess I'll just put a barrel bolt on the door and move on to the next project.