Trunk Tee

Need advice on how I can successfully tie in to existing HVAC system for my 800 sqft. basement?

How many registers do I need?
What about return air?
Should I always install from main trunk line or can I just use a Tee?
How much does this project typically cost should I decide to hire a contractor?

The type of system you have makes a difference. If your talking heating only - not using air conditioning, and your fuel source is natural gas or oil, you may be able to use the existing system effectively. If you have an all-electric heat pump, or gas heat and electric air conditioning, adding 800 square ft is too much load and will put your system over the balance point (the point at which the system is ineffective). Generally, you need 400 CFM per ton and can average between 400 to 500 square ft per ton. Possibly your additional 800 sq. ft would require 800 CFM air flow. Depending on the length of the duct run (friction loss) and your system requirements for duct static pressure (usually around .07 inches, you would need a 14 inch diameter duct to handle the amount of required air flow. This much air would require that you tie the duct directly ito the plenum, or main duct off of the furnace. The registers you need would depend on how you split-up the air flow. If you need 14 inch total duct for 800 CFM, then splitting 800 CFM into registers the take a 6-inch duct would require around 8 registers fed by 6-inch ducts.

juvenile style 5th in the trunk