Installing Registers in Plenum's?

BIG1RED9

Our 12 year old home has the furnace / air handler in what was once a garage. The condensing unit is on a pad outside. In about the middle of the garage a wall was put up creating a storage space on the garage door side and a home office with access to the house on the side that has the furnace. Neither is conditioned. I would like to condition the office space and thought maybe the easiest way to do it would be to cut into the plenum's for supply and return. Knowing the easiest way to do things sometimes is the worst approach, I am requesting feedback from those that know more than I.

Thank you,
BR

scottmcd9999

If the system is sized to handle the house, then adding additional footage could impact the overall comfort of the home. A HVAC system moves a specific amount of air, and produces a specific amount of cooling. Adding ducts will change the dynamic of the system, and could rob other areas of airflow and cooling/heating.

With that said, you could run ducts from the plenum, just be sure to use dampered takeoffs. This will allow you to shut the new runs off in the event that you have troubles with other parts of the system.

BIG1RED9

I understand it will impact the balance and if it did in a dramatic way, close it back off.

I was planning to cut directly into the supply plenum (sheet metal box on top of furnace / fan unit) for the supply and directly into the return plenum (sheet metal box mounted to raised wood platform) which feeds the underside of the unit. Still OK to cut?

Thank you for your help!
BR

scottmcd9999

Sure - as long as you install the takeoff (the piece that connects the round duct to your flat plenum) with a damper. Personally I'd do this for both supply and return.