Splitting a duct will reduce the efficiency of conditioning that space. You will need to know if the HVAC system has enough CFM Flow for the addition to begin with. Ideally it would be best to run a line direct from the main box coming from the HVAC unit.

Can you run two vents one duct?

Well, you can’t! You’d often blow air from one bathroom into the other, and local building inspectors wouldn’t approve it. But while you can’t have two fans with one vent, you can make one fan and one vent serve two bathrooms. … A grille in each bathroom attaches to ducts, which then fasten to a “Y” connector at the fan.

Can you split flex duct?

If the ducts are over 8 feet long, splitting the flex ducts can save you money. You can be equally benefited if you have two or more 90-degree elbows in the system. You can also enjoy the facility in two rooms from a space of one flex duct.