Since no HVAC expert is involved and my son in law insists on tapping the existing system to provide heat in the basement, I thought of doing it this way. Would like to know if the following makes SOME sense.
Instead of tapping directly off the main plenum, I would tap the six inch ducts leading to the first floor. I would put a tee in the six inch lines and reduce to 3″ or 4″ so the ducts would run down to the floor between the studs. I would like to get 3 ducts into the basement. I would put a 6″ x 4″ in the return plenum at the ceiling. At the tees I would put a damper that could be accessed above the suspended ceiling. Basement area = 700 ‘ft.
Son in law agrees that if this method does not make work , installing a Mr. Slim unit (my first choice) would be necessary.
Replies
I think it's better to tap off the plenum, preferably with 6" branches, but with balancing dampers installed right at or near the point of connection. This will give you control over a wide range of airflow, and the dampers should be accessible if they are in the furnace room where the supply plenum is.
Tapping off the existing 6" lines with a 3" or 4" could result in actually having the small lines suck air if there is high velocity in the 6" lines. (Venturi effect) The plenum will have lower velocity along with access to the full amount of supply air.
The 6 x 4 return seems small, but can be enlarged if needed, since you apparently will always have access to the return plenum, or trunk.
Pay your money
take your choice ... DIY with half baked theories can end up w/ less than favorable results ... so if you're willing to live with that ... fine. I'm not saying your approach is or is not half baked ... you may not know that right now.
Most duct systems in residences aren't really designed ... so whether there is enough air in your existing duct to the first floor may not be known. Branching off existing duct is probably what most would do, though. Putting in a balancing damper after the T split would be good to get things how you want ... as you will not likely need the air flow in the basement you need upstairs.
However, I'd encourage rather than a T a Y to ensure better air flow.
Your return sounds way small ... maybe more return in the basement than upstairs to encourage sucking out the cooler basement air and recirculating it through the furnace?
Not sure what a Mr. Slim unit is ... a wall furnace is my guess.
More ...
And to answer your lead in question ... of course you will rob heat. Your assumption is that the furnace is fairly substantially oversized ... which is not necessarily a stretch of a statement. One of the biggest problems in the energy industry is contractors who sell units way to big for the application using rules of thumb and/or just trying to ensure they don't get a comfort call back.
Assuming your unit is oversized for the PEAK load by say 20-50%, you're likely good to go ... but again, you don't pay for expert help, you may get what you pay for (and no, I'm not trying to sell myself ... I'm not directly in the trade). As with anything ... there is a reason there are people with the skills in the trade ... to do this kind of thing the right way.
There are plenty of unkowns here that could potentially could turn your good intentions into 'something less than desireable' ... Hope you get some more tips on this topic .... take them all in and sift them for your application ... good luck!