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Old 07-08-2013, 11:24 PM   #1
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New Duct supply runs for basement finishing project


Hello,

Sorry for the long post, but want to try and include as much info as I can.
I am in the process of finishing my basement. I had a few questions with regards to installing a few supply lines for the basement. Currently, my trunk is 10"x20" wide for most of the basement, and then about 3/4th of the way down the line, the trunk reduces to 8"x14" wide. I believe I have a 3.5 ton or 4 ton furnace, main levels of house is about 2300 sf. The furnace sits on the east side of my house, and the trunk runs pretty much across the full length of the basement to the west. Through out the length of the trunk there are numerous 6" diameter supply lines which branch off the top to deliver air to the rest of the house. On the underside of the trunk, there are two openings for vents (see photo). One is about 1/4th of the way down, on the 10x20 size trunk section, the other is near the end of the main trunk, in the 8x14 section of trunk, (you can see first vent at top right corner of picture, other vent opening is towards end of line, hard to see).

I live in Chicago area, so hot summers, cold winters. I have insulated the foundation walls with XPS foam board, and will also install fiberlgass in stud wall (will have total of about R-23 for walls). Subfloor is also insulated, temperature is already much nicer compared to before insulation was installed. I am not sure how much air I will actually need to keep the temperature nice, I figured the air lines would be more for to keep the air moving, rather than having stationary/stale air, which is what I always thought led to having the musty smell in basements (am I incorrect on this?). I will most likely also be adding a dehumidifier.

The finished square footage of basement will be about 800 sf. I dont want to add too many lines, and take away from the air flow to the rest of the house.

My questions were as follows:
1) How many extra lines do you think I need ( I was thinking 2 lines, on top of the existing 2 vents on the bottom of the main trunk line).
2) What size vent lines can I use, do I need to use 6" diameter lines, or can I use smaller, like 3" or 4" diameter lines?
3) Do I need to install return line, and tap into existing return? (Was most likely going to do this, but just want to verify). If needed, then what size line? Only have a 2x4 stud wall for the return line, so cant go more than 3" diameter, unless use box shape. Was going to install the return near the floor.
4) I was going to install the lines between joists, and then have the vent come out of the drop ceiling, or would it be better to have the vent come out of the wall, near the top of the wall, close to the drop ceiling?

Let me know if there are any other questions you need me to answer, or any other info you need me to provide. I can also provide pictures of the area if needed.

Thanks in advance,
Steve
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New Duct supply runs for basement finishing project-main-trunk.jpg  


Last edited by svaic00; 07-08-2013 at 11:28 PM.
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Old 07-09-2013, 12:10 PM   #2
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New Duct supply runs for basement finishing project


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Old 07-09-2013, 10:09 PM   #3
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New Duct supply runs for basement finishing project


Stick with 6" lines simply because it will make it much easier to buy the material.

Return needed? Absolutely.

How many supplies and returns should be determined by doing a heat load calculation on the space to figure out the CFM of air needed to supply the needed Btu's. There are online programs to do this with yourself.

If you want to proceed without this information you do/will have the benefit of having a dropped ceiling which would allow you to modify if necessary.
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Old 07-09-2013, 10:48 PM   #4
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New Duct supply runs for basement finishing project


Yes I always use the rectangular "wall stack" for the returns near the floor. It comes in sizes of 3 1/4X10, 12, 14, and 16 and fit nicely in a wall cavity. You can even put two side by side and have a 6X36" return grill
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Old 07-10-2013, 04:40 AM   #5
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New Duct supply runs for basement finishing project


Is it going to be 1 large open space. Or multiple rooms with closeable doors.
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Old 07-10-2013, 01:34 PM   #6
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New Duct supply runs for basement finishing project


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Is it going to be 1 large open space. Or multiple rooms with closeable doors.
One large open room
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Old 07-10-2013, 04:58 PM   #7
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New Duct supply runs for basement finishing project


In reality. As far as conditioning the room goes then, you would be better off with a mini split. Or zoning the system. A bit expensive either way.

What often happens in the late spring and early fall. is the first floor is warmed by the sun, and the thermostat doesn't call for heat. The coolest air from that floor drops into the basement and keeps it feeling a bit too cool. Then when the A/C runs in the summer, it tends to over cool the basement since there is very little heat gain in it.

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