Moving ductwork to accomodate finished basement
We have a 1200 SF two bedroom 1 bath ranch with a full basement that was partially finished by the previous owner into an office space. The walls were fully finished (framing, vapor barrier, insulation, drywall, etc...) but there was never any carpeting and a drop ceiling had been installed. It was a usable space but we would like to turn it into a combination family room and office. The major obstacle to making the space really "feel" finished is the ductwork, which like in many homes, runs off of a central trunk line which is routed down the center of the house with the furnace at one end. In my case it runs perpendicular to my basement stairs so that in its current configuration, once you reach the bottom of the steps the supply and return are directly above your head and clearance is about 6' 1" which is claustrophobic for my wife and I both at 5' 10" and an actual obstacle for some of our family members who are 6' 2" and taller. In any event, having the ductwork there and not being able to enclose it will definitely detract from the finished product, so I am committed to moving it somehow.
First it would be helpful to establish what I have so here are two pictures taken from roughly the center of the basement looking toward the outer walls at the ductwork:
In the above photo I'm standing near the basement steps (they are white and can be seen in the lower left corner of the photo) looking toward the furnace. As you can see in addition to my oil furnace we have a wood add-on furnace which is actually our primary source of heat unless we're away from home for an extended period. I should also add that we have central air. The next photo was taken from the same spot facing the opposite direction toward the end of the run:
The supply is on the right and the return is on the left. Both ducts start out as rectangular 8" x 18" and then reduce (the return reduces before the supply) to 8" x 8". Hot air is carried from the main trunk to the registers by 6" round duct. The supply ducts are simply spaces between the joists enclosed by stapling sheet metal over them to form a conduit for air. This can be seen clearly in the second photo. Because it may be hard to visualize the system in it's entirely I've also enclosed a diagram of the system as it is now (attachment D_1) and a diagram of what I envision as the solution (D_2) to this post.
Basically my idea involves shortening the main trunk lines so that they end just before reaching the basement stairs where they will be capped and hidden by a wall. The difficulty of course is the fact past that point there are four registers, two for the kitchen and two for the living room. Currently there are also two returns in the living room as well, but I am not as concerned about them for a few reasons. First, there is a central 14" x 14" return vent in the hallway leading out of the living room to the bedrooms, and each bedroom has it's own return, all of which would be retained. Second, there are no barriers to air movement between the central return and the living room at all. In fact, the two living room returns are currently blocked by a couch because there is no other good place to put it and heating and cooling do not seem to be affected by this. Of course, the return seems to be the simple part-it's delivery that I'm worried about.
Shortening the delivery duct would mean that the four registers mentioned above (two living room, two kitchen) would need to be fed by ductwork which is detoured to either side of the finished space. My thought was to tap four collars into the supply trunk (two on each side) and run 6" round duct perpendicular out to each wall. Upon reaching the walls the duct would turn 90 degrees and run along the wall to the register. These runs along the walls could then be covered by a chase, leaving most of the area barrier free except for the girder, but since it's only about 5" wide it's not nearly as obtrusive as the ductwork. Again, the "before and after" can be viewed in the two attached files. My main concern is the increase in length for the ductwork that will need to detour the finished areas. For example, as it stands now, the far rear kitchen register is a 12' 8" run from the main trunk. My planned detour would increase the run to 28' with a 90 degree bend in the middle. How this will affect the velocity and temperature of air coming out of the register itself I have no idea-I feel like a degree in fluid dynamics (or at least a few introductory HVAC classes) would be extremely helpful right now.
I would perform this work myself since the process of it is fairly straightforward and I can obtain the needed materials at cost, but I'm worried about the possibility of ending up with a cold living room and kitchen because of the long runs. Am I worrying about nothing? Are there factors I haven't considered? Is there a better way?
Do not modify you supplies or returns. They are sized accordingly as to the cfm vs volume. Imagine a soaker hose for your garden. All the leech points are the same size as you travel away from the supply to the end. But, if you look closely, you will see that there is more water coming out at the begining than at the end. HVAC contractors know how to size your ductwork to keep this from happening. As far as the returns, they are as important as the supplies. It doesn't matter about how close they are to another one as this is also sized accordingly.
I would be fairly safe to estimate that any modifications to you ductwork whether they are supply or return will greatly effect your HVAC system. Most likely for the worse. If mods are necessary, talk to a HVAC contractor.
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