Capping Duct

I pretty much only work on kitchen ducts.
My son's bedroom furnace/AC register is only about 4' from the furnace.
The wife tried to seal it off with a magnetic cover. Poor to horrible results.

What's the best way to seal off the round duct in the attic? The duct is flexible insulated.

 

Are you talking about covering a return grill in the ceiling ?
If so..... you mentioned furnace.... how will that work if covered ?

 

I want to cut off supply to a wall register, both heat and AC. The room gets too cold with AC and too hot with heat.
I don't want to close off the register, I want to close off the round supply duct.
I don't know where's the best place to cut and cap it.
Near the plenum? Cut it midway, about 2' from furnace?

 

have you been in the attic and looked for a damper?

 

No dampers. Can I put one in-line with no condensation problems?

Simply trying to block the register with some kind of cover causes bad condensation at the register.

 

This is because of too much air flow. Install a balancing damper as close to the source (plenum) as possible and then throttle down the air flow. Start with the damper about 1/2 open and adjust open or closed as needed. It will likely take a fair amount of small adjustments to tune it best. Depending on the size of the entire system as well as the duct take-off points you may have an interaction with other registers.

Note well, a butterfly damper will be near maximum flow at about 60% open so adjustment is best in the range less than 60% open. Note also that noise level will increase as the damper is closed and this is more pronounced the closer the damper is to the outlet terminal.

 

Airman.1994, could you empty your PM folder. It's full!