Results 1 to 10 of 10
  1. #1
    Join Date
    Sep 2018
    Posts
    3
    Post Likes

    Sizing Return Duct "Rules of Thumb"

    Hello HVAC forum,

    I bought a house where monkeys apparently installed the HVAC (complete gut). For instance, instead of a proper duct underneath the furnace, it consisted of some sheet metal and cardboard in the dirt beneath the garage pad, the plenum was a swiss cheese of holes with miles of bent flex duct, a duct off the end of the trunk, a supply takeoff facing into the blower, leaks you couldn't believe and improper sealing, insulation and hangers (to name a few). I had to remove it all anyway since the plumbing wasn't much better and the so-called supply system was in the way. Fast forward to this past month, I have just completed the new supply side ducting and now need to fix the return side.

    The return system was not much better. They had chopped two large rafter bays open in the ceiling of an end room and had a small return on the second floor (inside a bedroom). I extended and enlarged as much as I could the second floor return to the hall (6 supply vents on second floor so some air will be returned down the stairwell). A small return from the basement rooms (a mother in-laws quarters that does not commune with the main house) has 4 supply vents. And the main floor of the house with 8 supply vents. I am endeavoring to close the giant holes in the ceiling end room in favor of a return in the top of the hallway wall in the center of the main floor. This is where my question arises.

    The supply being a 16" trunk is ~201sq". The return must be that or greater. I have seen, heard and read that trunks must be 10-15% larger all the way to twice that of the supply. I have heard arguments of static pressure, velocity, sound due to whistling, etc. both for and against in the sizing of the return. I can find no definitive code that states the return size versus the supply size either. Can anyone answer this definitely or point to a code or rule that states what it must be? I have included data below which may (or may not) be meaningful to answering - so I included it just the same.

    Details of Home: I have a story and a half house here in Seattle built in 1991 with a daylight basement on a sloped lot. The climate is Marine 4 and quite mild most of the year. The house is well sealed and insulated (R19 walls, R30 floors, R50 attic, all penetrations caulked and foamed, all ducts mastic sealed). The house has gas forced air 67,500 btu furnace (needed only 2 months of the year) and no AC. If we had AC, I would say you would need it for 2-4 weeks a year). The conditioned space is ~2900 square feet. The 2nd floor has 6 supply floor registers and is ~900 square feet. The main floor has 8 supply floor registers and is ~1500 square feet. The ground floor ~500 square feet and is a Mother-in-laws quarters that is seperate from the main house and only has 4 supply floor registers (these four were added later after the house was built).

    Supply Side: the gas furnace resides in the garage in a downward arrangement. There is a large 16" concrete elbow in the pad that exits in the basement through a transition plenum going from square to round in about 2' which attaches to a 16" metal tee running north-south. The trunks reduce from 14>12>10>8 with nine 6" dampered takeoffs on both the north and south round trunks (with correct spacing between takeoffs and reducers). All supply ducts are 6" round metal (no flex) and completely sealed. Six runs leave the trunk and travel to the 2nd floor, 8 runs travel to the rafters above fort the main floor, and 4 runs run down to the floor of the MIL rooms.

    Return Side: The return ducts are within wall or floor bays, sealed and lined with foil bubble wrap (mainly for decreasing sound transmission in the empty spaces). There is a second floor duct (~51sq"), which joins up in a floor bay with the basement return (~42sq"). This total 93sq" which exits above the furnace into a metal mixing box. The new hallway return duct will also enter this metal duct box with the filter below that feeding back into the furnace. The proposed size of the duct is 22.5" x 7.5" (or 10.5" or 14" depending on numerous "rules of thumbs"). The 7.5" duct would provide 169sq". Along with the 93sq" from the upper and lower floors, this would equal 262sq" (or about 30% bigger than the 16" supply trunk). Making the return bigger than this poses some issues due to the metal mixing box is mounted above the hot water heater. If I have to increase more than 10.5", then a fourth return has to be jammed into the mixing box so that it does not interfere with the hot water heater.

    Thanks for all your help and advise on the SIZE of the return VS the supply side.
    Jeff in Seattle

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    PS: The furnace is original to the house (1991). I am not sure about reinstalling due to its age (again not used very much overall). It also had electronic honeywell filters. If I was replacing these, it might be time to consider an AC unit too as the summers continue to get hotter and hotter. Due to the mild temps and the daylight basement, heating and cooling the ground floor is almost a moot point, the main and second floor rarely experience temp swings (even without the furnace for two seasons during this remodel). Sizing calculators of furnaces by BTUs and AC units by tons (based on square footage) seems skewed for this mild clime and the house being so energy efficient.

  2. #2
    Join Date
    Apr 2016
    Location
    Kentucky
    Posts
    62
    Post Likes
    Call a professional and have them complete a manual J on the entire house and design a proper duct system. You are in over your head on this one. I don't say that to poo poo DIYers or be disrespectful, but there is much more to designing an HVAC system (including the ductwork) than one might think. The type an size of duct work is very important, as it the length of runs and types of connections between runs. It sounds like you have a mess on your hands and this just isn't one of those times you want to try to sort it out without some serious help. Best of luck and hope you can get things straightened out.

  3. #3
    Join Date
    Sep 2018
    Posts
    3
    Post Likes
    Thread Starter

    Thanks for the reply...

    Quote Originally Posted by NewtonJones View Post
    Call a professional .......
    The supply system was designed by a pro and the supply side is large enough for the home. It should be noted that since the supply system was within the garage and basement, it was easily corrected. However, the janky returns are tougher since there is limited options to correct these within the finished space of the home. More to the point, the return side "is what it is" for the second and ground floor. And the balance of the second floor can reach the first floor return if it is too small thanks to the stairwell. Even the new hallway return on the first floor makes the total return 30% than the supply.

    I wonder in most situations (on average - I know every home is different), if you yourself size the return larger than the supply side duct - and if so, by how much (on average)? Like I had mentioned that many pros (on this site and others) state anywhere from 10 15 and 100% larger. That's quite a range. Manual J calculates the load and needed supply side (which is completed), but I was looking for a specific code or calculation for sizing the return LARGER than the supply.

    Thoughts?
    Thanks!

  4. #4
    Join Date
    Nov 2006
    Location
    Southeastern Pa
    Posts
    28,918
    Post Likes
    From what you were describing to us, the HVAC system was designed by a contractor...... but I don't think any of us would believe that he was a professional, and that is the type of person we are suggesting you seek.

    We have a contractor map here of our professionals, but in your own area you may have better responses if you talk to three or four friends who have had HVAC work done in their own homes, and ask them which contractors they would use a second time based upon that experience.....and that can help you find someone in your area who can assist you in this undertaking.
    [Avatar photo from a Florida training accident. Everyone walked away.]
    2 Tim 3:16-17

    RSES CMS, HVAC Electrical Specialist
    Member, IAEI

    AOP Forum Rules:







  5. #5
    Join Date
    Aug 2009
    Location
    Beatrice, NE
    Posts
    6,549
    Post Likes
    Technically anyone that gets paid for a job is a professional . . . but that does not mean they are proficient at their work.

    Duct work is probably one of the hardest things to understand in the HVAC world. I say this because it seems that it is messed up more often than any other single item in the industry.

    Find a dealer that everyone loves his duct jobs and use him, he will not be cheap and probably not fast but he will be well worth it.

  6. #6
    Join Date
    Sep 2018
    Posts
    3
    Post Likes
    Thread Starter
    Thanks - I have been looking for someone locally to ask. The HVAC person was a professional and used software for manual J. He designs large datacenter ductwork in his main job but did start his career on the resi side. That was 3 years ago while I was in the midst of this huge rework of the utilities (electrical, telecom and plumbing). He left a detailed plan with parts list. I did the grunt work of installing it 3 years later after all the other work was completed. I work in both IT and gas utilities and understand about flow rates and pressure with compressed gases and liquids. I am confident the supply side is sized correctly, and the return is currently 30% larger despite the monkeys who originally installed the HVAC system in this home (and was by no means even passable - I don't even they you could call them contractors by the look of the work). In any case, my proposed return is 30% larger than the supply. I will keep looking for a pro that can advise or find an actual code or formula that spells out the return size difference to the supply.

    Thanks for you help

  7. #7
    Join Date
    Feb 2015
    Location
    Ohio
    Posts
    829
    Post Likes
    There is not a specific ratio that fits every situation. This is why it is being suggested that someone skilled in duct design and installation be consulted, someone who can come in person and evaluate the entire system. If you keep looking someone will give you an answer eventually, but whether that answer is correct for your system is a gamble. There are many steps involved with proper duct design and something that is best not left to "rule of thumb" or educated guessing. If you trust the design of the guy you used before why not contact him again? Some simple tests would answer a lot of the variables no one here can answer across the interweb. Good luck!
    "If common sense was so common, everyone would have it."

    Do it once, do it right.

    It will take as long as it takes, which will be not as long if you let me do my job.

  8. #8
    Join Date
    Sep 2002
    Location
    Virginia
    Posts
    4,750
    Post Likes
    Im no pro when it comes to airflow , but I can tell you it cannot be TOO BIG on return grills , or ducting to it


    If you can fit a 30x30 filter grill in your wall , go for it

  9. #9
    Join Date
    Jan 2004
    Location
    PA
    Posts
    75,689
    Post Likes
    Moved to AOP.
    Contractor locator map

    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

  10. #10
    Join Date
    Aug 2009
    Location
    Beatrice, NE
    Posts
    6,549
    Post Likes
    Quote Originally Posted by Snapperhead View Post
    Im no pro when it comes to airflow , but I can tell you it cannot be TOO BIG on return grills , or ducting to it


    If you can fit a 30x30 filter grill in your wall , go for it
    Actually I think you can. The first generation of forced air systems had huge return ducts so they had a low velocity of air flow. Those systems always had a mat of dirt, lint, etc that had dropped out of the air and settled in the duct which makes a breading ground for organics. The return needs to have enough velocity to carry particles to the filter where they can be removed. I suppose if you are using all filter grills that may change but I believe it would hold true on systems with the filter at the equipment.

Tags for this Thread

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  

Related Forums

Plumbing Talks | Contractor MagazineThe place where Electrical professionals meet.