HVAC question... How many vents do I need?

[Scarpozzi]

How many vents should I put in a 600 square foot room on the 2nd floor of my house with a garage downstairs? It's going to be insulated pretty well all around. My floor joists are going to limit where I put vents...I know I can easily get 4 in...I'm just worried about more than that (mostly because of logistics of running the ducts through i-joists).

Originally, I was going to just do a mini-split for the 600 square feet + a bathroom and tie in a second room to my existing unit, but worry I won't get enough pressure 60 feet down the line to get the return air to work out. It also may not cool the stairwell/hallways effectively in the summer due to the increased heat in the attic deadspace... Also, for the cost, it makes more sense to do a 2-ton heat pump and make that wing of the house its own zone.

My existing living space is 1700 square feet one story.....and I'm adding about 1050 to it. (3 rooms)

 

[NutBucket]

Call a pro for a proper heat load calculation.

Or just do a dual-head mini split.

 

[Scarpozzi]

Call a pro for a proper heat load calculation.

Or just do a dual-head mini split.

I might as well have them come in and just do the install...I hate to spend the money, but I might as spend the cash and do it right.

 

[NutBucket]

"anyone" can do the install. From what I gather not every pro will install the right stuff!

 

[Greenman]

Manual J is your friend, it will tell you the exact volume of air flow you need. It accounts for static pressure in the plenum, duct size, duct material, number of elbows in the duct, climate zone the structure is in, insulation in the walls, floor, and ceiling, glazing, direction the building faces and expected number of people in the room.
Anything other than a manual J calculation and a manual D design is a guess. Some guys are pretty good at guessing, some, not so much. Spend the $400 and do it right the first time.