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 can't 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

Forgot to attach photo, could not figure out how to attach it in original post.

Thanks,
Steve

Forgot to attach photo, could not figure out how to attach it in original post.

Thanks,
Steve

Anyone have response to my original post?

Thanks,
Steve

Are you sure that is your supply, not your return? Where is the insulation? If it is not insulated, you are spending a ton of money heating and cooling the space and not the house. This must be a fairly new house since I see I Joist. Doesn't require insulation where you live? Is this a heat pump? 3.5-4ton is refrigeration, another reason why I question if this is a supply or return. Chicago and no floor insulation either? What kind of heat bill do you have?

It is a supply, that is the main truck that has numerous 6" supply lines going throughout the house. The return is in a different location.

I do not have the main trunk line insulated (may do that in future), I insulated my basement walls, and will put in fiberglass insulation once the stud walls are framed.

Right now, there are only 2 vents on the underside of the trunk, I want to know how many to add for when I finish the basement. No it is not a ton of money for heating/cooling bill.

Yes fairly new construction, built 2009. I just gave the 3.5 or 4 ton to give the approximate size of the unit. I do not want to add too many lines in basement to take away from the air flow in the rest of the house.

My whole house is insulated, the insulation I am talking about is the basement. I insulated the concrete basement walls, and insulated the basement concrete floor slab. There is not insulation in the 1st floor joists. Very little temperature differential between basement and 1st floor, so pointless to insulate, since you do not have a major heat transfer, since the temp difference is minimal, at least that was my understanding from research I have done, that floor insulation is more for noise barrier, rather than temperature.

Basically my basement was unfinished, rest of house is built normal, insulation and all. I am now finishing my basement, and want to know how many supply runs to install. There are currently 2 vents on the underside of the main trunk, I was thinking of adding 2 more supply lines to the rest of the basement, along with adding a return. See my original post for the questions I was asking.

Did I answer your questions on what you were looking for?

Thanks,
Steve

Without doing a heat gain/loss calculation (Manual J) no one can tell you how much air you need to supply to a given space. There are numerous load calculation programs available online.

It's hard to give you any specific answer for your question. We don't know the house volume or what CFM the fan is producing. You are correct in noting the temp differential -delta T -if the same between floors, insulation is not required by the mechanical codes. That said, I have not seen an UNinsulated supply in GA. Insulating a duct keeps the air in the duct hot or cold so you have less stratification in the house. I will assume we are looking at the end of the main duct not the beginning. Ideally a Manual J should be completed, as Grady said, or at least a Test and Balance. The key is to make sure adequate airflow is maintained and that the BTU output is sufficient for the increased volume. Linear Feet per minute airflow X duct cross section = CFM so you need some measurements. CFM should be 1-3 times room volume. Most of the time a couple of added ducts and return should not throw a system off that much. But, if you will be conditioning a 50 % more volume you may find poor distribution and temp control in more remote areas. Also remember that the closer to the fan, the noisier the air flow, a common occurrence in basements.

Live in Chicago area, and I have not seen an insulated trunk line in any of the houses we looked at, when looking for our new place. I plan on eventually insulating it. In the picture, the furnace is right behind me, so at the top left of the picture, is basically the beginning of the trunk, and the end of it is at the far wall in the picture.