CFM through an 8"x8" vent hole cut into bottom of 8" x 20" return trunk?

Users who are viewing this thread

BigRedd

New Member
Messages
2
Reaction score
0
Points
1
Location
Boston area
Title says it all but as a bit of background, I'm looking to reconfigure one of the return trunks in my basement (incl. three branches) to gain more headroom, which requires shortening the trunk by ~12' and reconfiguring three of the four vents. I'm not running new Manual J, D, and S on the whole system because it's been in place for 20+ years (furnace replaced ~11 years ago) and works fine. Instead I'm taking a "do no harm" approach - I just want to make sure that at least as much return air is supplied after the changes as before (currently). There are 3 branches off of the trunk that are easy to calculate CFM for. Where I’m hung up is estimating the CFM supplied by the 8” x 8” vent hole cut directly into the bottom of the trunk itself (close to the end of the run at the far end from furnace) which serves as the only return in the basement zone. I can’t find any guidance on the CFM for that.

If anybody is willing to help and would like more specifics, here goes:
  • Three zone system: basement, first floor, and second floor
  • The return trunk in question is 8” high by 20” wide and total current length is 24’9” in one straight shot. There is an 8” x 20” drop out of the bottom of the trunk at the furnace (which is the start end of the trunk).
  • There are 4 vents served by the trunk:
    • (a) The first (in order of closeness to the furnace) is not being touched. It is a 6” flex duct, top takeoff, that comes in right above one edge of the drop. It runs straight, perpendicular to the trunk for 52” terminating at a standard end boot. Serves a small first floor office.
    • (b) The next branch/vent is about 10’ down the trunk and is either a 6” or 7” oval (hard to measure due to access) oriented with the long way vertical (and a “broadway” 90 degree top takeoff) that runs perpendicular to the trunk 85” to a rectangular boot that is 20.5’ long by 6” wide by 11.5” deep. There is a baseboard register over that boot with a 16” long by 2 1/4 “ hole cut in the top of the boot. I’m using 6” round duct CFM as estimate for this branch. This branch services the first floor dining room that is in an open floor plan first floor that includes the next vent.
    • (c) Third branch/vent is roughly 4’ further down the trunk. It is an 8” by 10” rectangular duct that sits on top of the trunk and runs perpendicular to it for roughly 9’. The vent served by this branch is just a 5.5” by 13” hole cut in the floor of the first floor living room (and the top of the duct). Unsealed. This is the main return for the open floor plan first floor space. Duct goes about 16” past the hole.
    • (d) The final vent on the trunk is the 8” by 8” hole cut in the bottom of the trunk itself. This vent is the only return vent in a ~ 1,500 sf previously finished basement (that I’ve mostly torn out). This zone is rarely turned on. It never needs AC and we only heat it when we’re using it.
My present plan is to shorten the trunk by about 12’ and replace both (b) and (c) with a single 10” x 12” duct branch that runs a short distance (2-3’) and has a sealed 10” by 12” vent cover cut into the floor of the first floor hallway. This should comfortably supply as much return air as (b) + (c).

The plan for the basement vent is trickier. I would like to replace (d) with a standard branch duct that runs down to the floor of the basement (much preferred since we only ever use heat in the basement). My fear is that the 8” square vent cut into the trunk might supply “make up” air for the first floor zone and if I replace it with a standard branch duct the furnace will get diminished airflow compared to what it gets currently. So it’s important to accurately estimate the CFM possibly supplied by that vent hole (even though it’s technically in a separate zone). Hope that makes sense.

Thank you to anybody who read this far. Happy holidays.
 

Fitter30

Well-Known Member
Messages
5,169
Reaction score
1,023
Points
113
Location
Peace valley missouri
Furnace or air handler have cooling? Design for cooling is 400 cfm per ton. Without a velometer or with design specs of furnace know what hp motor and static pressure. Most residential returns are short of air meaning blower is working harder and return is noisy. A grill will restrict air flow up to 50%.
 

Breplum

Licensed plumbing contractor
Messages
2,405
Reaction score
1,025
Points
113
Location
San Francisco Bay Area
We target max. 500 FPM for return air (RA). At that max., 8" round will supply 170 CFM and square will supply 200 CFM.
When you 'finish', check your delta T between supply and return and make sure you are in range.
 
Top
Hey, wait a minute.

This is awkward, but...

It looks like you're using an ad blocker. We get it, but (1) terrylove.com can't live without ads, and (2) ad blockers can cause issues with videos and comments. If you'd like to support the site, please allow ads.

If any particular ad is your REASON for blocking ads, please let us know. We might be able to do something about it. Thanks.
I've Disabled AdBlock    No Thanks