I have one vent in my house that doesn't blow any cool air out of it. I went underneath the house expecting to see the duct had fallen off or something, but it was all properly secured and wrapped in insulation. I removed the insulation and unscrewed the duct and it blows perfect when it is disconnected and hanging into the crawl space. When i connect it, there is zero airflow through the vent.

I could hold the two sections apart so that there is only an inch or so gap between them, and airflow is normal up through the vent. When i join the sections together, it immediately stops.

Could there be some sort of vacuum or negative pressure (or whatever it might be called) that occurs when the duct is connected to the house, that cancels out the airflow up through the vent?

Thanks for any help!

 

There should be no airflow restriction for that short of distance. I know it is elementary, but are the vent slats open?

 

Hey chandler, actually i completely removed the register and had the person up above stick their hand down and into the duct where i was holding it separated. As long as there was a gap, even as small as an inch or two, they could feel normal airflow. As soon as i joined the sections together, the airflow stopped.

Here's some more information i forgot to add:

The entire run to that register is only about 20 feet, and it is a straight shot that does a bend up to the floor. It is the first outlet from the main AC ducting when it enters the crawlspace. There are no other registers serviced by this duct... it's one straight shot out to this single register.

In this room there are two other registers served by a separate duct run, and they blow air normally. The return duct for the house is only about 10 feet outside of this room, and the non-blowing register is closer to it than the ones that are blowing properly.

 

I'm assuming this is flex duct? Can you see the entire length of it? It sounds like it may be pinched or collapsed somewhere along the route so when you pull on it to connect it to the trunk it is closing off at the pinch point. You could test this theory by taking some thin cardboard and wrapping it around the duct and take-off to form a makeshift coupler that will join the duct without pulling on it at all....

If you get normal air flow when you do that, the easiest thing to do might be to extend the duct a few inches.

 

Paul's test would confirm this as well but I was thinking maybe pulling it tight causes the duct to come apart somewhere further back.

 

A couple of tests.
1. Check to see if opening and closing the door to that room has any affect.
2. Seal off the other two supply registers blowing into that room and check for any change.
3. While sealed, open and close the door.

Having two different trunks supplying air to the room raises a concern that one side may be pressurizing the room more than the problem supply can overcome.

Also, make sure that the person who is checking for air flow is noting which direction the air is flowing. When disconnected are the other two supplies blowing air down through that problem register. It would still feel like air flow, but the wrong direction.
If there is no blockage, then something has to be nullifying the pressure across that short direct path.

Bud

 

I'm going to go back under in a bit and do some more testing with the things you guys have recommended.

I can say, Bud, that when i have the duct disconnected and the other two registers are blowing, a good amount of air is flowing down the disconnected register in the wrong direction. It's enough that i can feel it, and see dust/debris fluttering in the opposite direction where the register duct exits into the crawl space.

 

Bud had the right idea. Closed the other 2 registers in the room, and airflow became normal through the non-working register. Opened one of the other registers, and airflow was reduced to a tiny tiny amount... opened both, and airflow stopped entirely.

Any way to remedy this? If not, i guess it isn't a big deal as we still have 2 working registers in the area (1 is in the bathroom connected to the room)

My mom and I are trying to remember if this vent ever worked... but it sure seems like we would have noticed it at some point in the past 30 years if it didn't... but cant say for sure. Could the AC unit have lost power or something in recent years, and no longer has the ability to pump air out of all 3 registers or something? The unit is around 25 years old, i believe.

Anyway, thanks for all the feedback... glad we solved the mystery.

 

Very good, now I'll have to figure out why. When you covered the other two supplies, was the door closed, I'm assuming there is a door. If a door is there a return path under the door and how much.

Your solution might be to add a return duct to that room., if you need a solution.

Bud