adding furnace vents in basement

Hey all.
Got a question that doens’t pertain to inspections but the more i think about it it does!
I am helping my buddy finish his basement. How do i tie into the existing ductwork to heat the below level? Can i just cut in and add right there or do i need to do a tee or something. It is a rectangle shaped with like 3 or 4 runs we want to tie into. About 700 sq ft. Help!
Appreciate it.

It depends on the existing ductwork–its’ size, length, etc.

How long do the runs need to be?

Best to enlist the aid of an HVAC tech.

Something disappeared from that post–

Is the ductwork rectangular? And will it allow for additional take-offs?

What kind of furnace is in there now?

Etc., etc.

I’m no expert, Andy, and there’s no easy answer. If, for example, the existing system is using up all the static pressure that the furnace blower is developing then any additional vents will rob air from other areas. A competent tech can tell you if your system has enough oompph to add additional vents. He will also be able to tell you how many feet of duct can be added and how many elbows,etc.

I am not a HVAC tech but I have worked with them enough. Your initial cut-in should be to the main run and as close to the heater possible. The size of the duct is dependent on the number of vents that will be hung off of it. If your going to guess err to the smaller side so you don’t steal too much heat from the main living spaces. I am sure you can find some tables on the web that discuss sizing, so you may want to do a search on some of the HVAC manufacturer sites.


Good luck!

I think you are all right. I don’t think stubbing one in would hurt but 3 or four may. I agree it is all about static pressure. The house is like 2-3 years old. I will recommend an HVAC guy to him because like you all said, it depends on the furnace and the runs and such.
Thanks all.
Andy B