Page 1 of 2 12 LastLast
Results 1 to 13 of 16
  1. #1
    Join Date
    Oct 2012
    Posts
    1

    Poor Heating and Cooling Question

    I have 4" duct running 15 feet off main trunk to a 12x12 bedroom with no return line. Would increasing it to 6" make a huge difference? It would require opening up a drywalled cealing and lots of labor repairing plaster. The other option would be a plug in heater but would not solve my a/c problems during the summer. Temp differences from the living room to bedroom are 70 degrees vs 60 degrees. Half my basement is unfinished and I ran 6" to other rooms with dramatic improvement but those rooms have a return.

  2. #2
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,233
    ____ _ 6" needed + Add returns!

    80 CFM in 6" duct =~ 400 FPM which yields a Good Design..
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  3. #3
    Join Date
    Dec 2005
    Location
    Cincinnati, Oh
    Posts
    4,906
    Quote Originally Posted by powertower View Post
    I have 4" duct running 15 feet off main trunk to a 12x12 bedroom with no return line. Would increasing it to 6" make a huge difference? It would require opening up a drywalled cealing and lots of labor repairing plaster. The other option would be a plug in heater but would not solve my a/c problems during the summer. Temp differences from the living room to bedroom are 70 degrees vs 60 degrees. Half my basement is unfinished and I ran 6" to other rooms with dramatic improvement but those rooms have a return.
    Have someone come out and take a look for ya.
    "Better tell the sandman to stay away, because we're gonna be workin on this one all night."

    "Dude, you need more than 2 wires to a condenser to run a 2 stage heatpump."

    "Just get it done son."

    Dad adjusted

  4. #4
    Join Date
    Oct 2010
    Location
    Anderson, South Carolina, United States
    Posts
    6,338
    Might be easier to abandon the 4" duct and seal it off then run a new 6 so you don't have to pull down Sheetrock

  5. #5
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,233
    Quote Originally Posted by powertower View Post
    I have 4" duct running 15 feet off main trunk to a 12x12 bedroom with no return line. Would increasing it to 6" make a huge difference?
    4" might provide 30 cfm. 6" diameter flexible duct x 15 foot straight would likely be capable of ~80 CFM with a return.
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  6. #6
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    The only time a small 4" duct is appropriate is in small spaces like walk-in closets and small bathrooms. With ductwork, bigger is usually better as long as you select the proper register size for adequate throw (mixing of air). You can always use dampers on a branch to reduce air flow if you have too much.

  7. #7
    Join Date
    Aug 2009
    Location
    Beatrice, NE
    Posts
    1,969
    What is the rest of your system? I have run into houses where every run was 4". They were called "Blendaire" systems if I remember correctly. They worked great with the origional equipment that had a high static blower. Once that equipment was replaced they didn't always work so well.

    Having said that if the whole house is done in 4" then the entire duct system needs to be replace to get it to work with the present equipment, that would include boots and registers.

  8. #8
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    4,850
    Quote Originally Posted by powertower View Post
    I have 4" duct running 15 feet off main trunk to a 12x12 [144-sf] bedroom with no return line. Would increasing it to 6" make a huge difference? It would require opening up a drywalled ceiling and lots of labor repairing plaster.

    The other option would be a plug in heater but would not solve my a/c problems during the summer. Temp differences from the living room to bedroom are 70 degrees vs 60 degrees. Half my basement is unfinished and I ran 6" to other rooms with dramatic improvement but those rooms have a return.
    The amount of CFM airflow required to a room depends on the required heating & A/C Btuh requirements of each individual room.

    Depending on the actual Heating & Cooling Design Btuh load of the room for your individual situation; that could easily range from less than .50% of the sf to slightly 'over' one to one CFM per sf of the room.

    Therefore, you need the room by room Btuh requirements for both heat & A/C. For A/C, you can divide the Btuh requirement by 30 (12,000-Btuh / 400-CFM is 30 for cooling).

    (Example: Room design is 2500-Btuh / 30 = 83-CFM - a 6" metal RD duct; then select a diffuser providing the needed velocity throw & pattern spread)

    Example: you size the duct run to the highest CFM requirement, "with dampers installed at the take-offs."

    Okay; let's say the 144-sf room requires 85% of that sf to cool the room or, 122-CFM. If 60% of the footage, it's 86-CFM; you need to know the Btuh calc required for that individual room.

    A heating & cooling load calc is a requirement for accurate duct sizing & total airflow engineering design work.

    Returns need to equal supplies...

  9. #9
    Join Date
    Jan 2008
    Location
    Texas
    Posts
    1,650
    Udarrell always offers very good info and explains it well.

  10. #10
    Join Date
    May 2008
    Location
    Central Fla.
    Posts
    311
    Just ran some quick/rough calcs. they come out to 7-7.5" duct needed to supply the right amount of air if your ceilings are 8'.

  11. #11
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,233
    Quote Originally Posted by TwincamDave View Post
    Just ran some quick/rough calcs. they come out to 7-7.5" duct needed to supply the right amount of air if your ceilings are 8'.
    OK, I'll take the 7.5" duct.
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  12. #12
    Join Date
    Jul 2012
    Location
    Western KY
    Posts
    1,221
    Hold on every one. If you just go in and start changing, adding, taking away supply runs you are changing the entire system. You'll have your airflow all out of whack. End up chasing hot and cold spots all over the house. A duct system should be designed based on load and equipment. If its not you can't get there from here. I recommend an evaluation of the system to determine what you need and where you need it prior to making any further changes. Fix it once and be done. Otherwise you will end up fixing on it forever. IMO.

  13. #13
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    4,850
    Quote Originally Posted by TwincamDave View Post
    Just ran some quick/rough calcs. they come out to 7-7.5" duct needed to supply the right amount of air if your ceilings are 8'.
    If you're using air changes per hour (ACH) criteria, then I'd use 6-ACH to that room which yields 115.2-CFM using a 7" rd metal duct yields around 432-fpm velocity, using a proper diffuser should be enabled to get proper throw & spread.

    Using a 6" RD metal duct the velocity would be 587-fpm; still under 600-fpm.

    When sizing duct I normally use 460-CFM per ton of cooling or, a factor of 26 with which to divide the cooling Btuh; some in hot dry climates use as high as 500-CFM per ton which is a factor of 24.

Page 1 of 2 12 LastLast

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
Comfortech Show Promo Image