Page 1 of 2 12 LastLast
Results 1 to 13 of 21
  1. #1
    Join Date
    Jul 2007
    Location
    North Little Rock, AR
    Posts
    19

    Residential Duct sizing...

    I have always used .1 static (friction per 100ft) pressure when designing a duct system and determining duct size. But what effect does External Static Pressure total have on blower performance if say the total ESP turns out to be .45 or so counting filter, grills, elbows etc.? Is this ESP number different than what my air duct calculator "friction" number shows?

    If I have designed a four ton system that the book says at .1 ESP produces 1645CFM yet after installation ESP total is .45 or greater I will have lost a couple of hundred CFM's right? So velocity will be reduced because I have to much pipe now which in one situation I am seeing is leading to duct sweating.

    How should I design a duct system to anticipate the CFM loss with real ESP totals?


    Thanks,


    S

  2. #2
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    For duct design get ManualD from www.acca.org

    After a couple of reads,and attempts to understand,ask questions in the Pro section on this site,or better yet attend a class.

  3. #3
    Join Date
    Jul 2007
    Location
    North Little Rock, AR
    Posts
    19
    Quote Originally Posted by dash View Post
    For duct design get ManualD from www.acca.org

    After a couple of reads,and attempts to understand,ask questions in the Pro section on this site,or better yet attend a class.
    Gee, thanks dash.



    S

  4. #4
    If you want to make life a lot easier, buy computer design software, this will allow you to do room by room load calculations and duct design with air flow for each room!

  5. #5
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    You are welcome,that was a serious and reasonable offer,not many places like this that you could get that kind of help for free,and learning Man. D is they correct wayto design duct systems.

    But since you didn't like it here's more specific answers;

    1.) "I have always used .1 static (friction per 100ft) pressure when designing a duct system and determining duct size. But what effect does External Static Pressure total have on blower performance if say the total ESP turns out to be .45 or so counting filter, grills, elbows etc.? Is this ESP number different than what my air duct calculator "friction" number shows?"

    ESP and "frction" as you stated ,are only equal if the TEL(total equivalent length) of the duct system is exacty 100'.TEL is the actual feeet of duct plus the equivalent feet of all fitting ,eblows ,grilles ,etc..

    Filters,grilles ,coils added to furnaces,wet coil value,etc.,are all deducted from the static that you chose to design for,leaving you the ASP(available staic pressure ) to determine the FR to use on the ductolator.

    ASP X 100" divided by TEL = FR to use with your ductolator.

    2.) "If I have designed a four ton system that the book says at .1 ESP produces 1645CFM yet after installation ESP total is .45 or greater I will have lost a couple of hundred CFM's right? So velocity will be reduced because I have to much pipe now which in one situation I am seeing is leading to duct sweating. "

    Looking at typical Mfrs. Fan Data sheets,shows that if a 4 ton could deliver 1645 cfm or less at .1 ESP (.1 is so low it's not practiacal to use) on the lowest motor speed,raising the speed to high would deliver more then 1600 cfms ,at .45 ESP.

    Velocity is reduced by a decrease in cfms,in your existing duct system.

    Consider taking the time to get a Man. D,etc.,and become a Pro Member on this site ,lots of valuable help and info., for all of us to learn.

  6. #6
    Join Date
    Jul 2007
    Location
    North Little Rock, AR
    Posts
    19
    Quote Originally Posted by dash View Post
    You are welcome,that was a serious and reasonable offer,not many places like this that you could get that kind of help for free,and learning Man. D is they correct wayto design duct systems.

    But since you didn't like it here's more specific answers;

    1.) "I have always used .1 static (friction per 100ft) pressure when designing a duct system and determining duct size. But what effect does External Static Pressure total have on blower performance if say the total ESP turns out to be .45 or so counting filter, grills, elbows etc.? Is this ESP number different than what my air duct calculator "friction" number shows?"

    ESP and "frction" as you stated ,are only equal if the TEL(total equivalent length) of the duct system is exacty 100'.TEL is the actual feeet of duct plus the equivalent feet of all fitting ,eblows ,grilles ,etc..

    Filters,grilles ,coils added to furnaces,wet coil value,etc.,are all deducted from the static that you chose to design for,leaving you the ASP(available staic pressure ) to determine the FR to use on the ductolator.

    ASP X 100" divided by TEL = FR to use with your ductolator.

    2.) "If I have designed a four ton system that the book says at .1 ESP produces 1645CFM yet after installation ESP total is .45 or greater I will have lost a couple of hundred CFM's right? So velocity will be reduced because I have to much pipe now which in one situation I am seeing is leading to duct sweating. "

    Looking at typical Mfrs. Fan Data sheets,shows that if a 4 ton could deliver 1645 cfm or less at .1 ESP (.1 is so low it's not practiacal to use) on the lowest motor speed,raising the speed to high would deliver more then 1600 cfms ,at .45 ESP.

    Velocity is reduced by a decrease in cfms,in your existing duct system.

    Consider taking the time to get a Man. D,etc.,and become a Pro Member on this site ,lots of valuable help and info., for all of us to learn.

    Thank you very much. I am studying the manual D and my duct sizer but I have been told so many different things by so many different "old timers" and "rule of thumber's". I want to know the truth and more importantly I want to do good work and do it correctly. My main question was what relationship ESP had to static pipe friction pressure and I appreciate your time.


    Thanks,


    S

  7. #7
    Join Date
    Dec 2006
    Location
    Arkansas
    Posts
    292
    SSCoulter - I am in Arkansas so if you have any questions or would like to meet, let me know. I am an engineer that does Residential and Commercial design and I also have a Class B HVACR license.

  8. #8
    Join Date
    Jul 2007
    Location
    North Little Rock, AR
    Posts
    19
    Quote Originally Posted by dcmcm5 View Post
    SSCoulter - I am in Arkansas so if you have any questions or would like to meet, let me know. I am an engineer that does Residential and Commercial design and I also have a Class B HVACR license.
    That would be great, where in Arkansas are you?



    S

  9. #9
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,718
    If your sizing using .1

    Have you taken any ESP readings, and actually had only .45 static.

    Around here, most of those .1 FR designed systems are .75 to .9 static, on medium or high to get the proper ( or workable ) air flow.
    Contractor locator map

    How-to-apply-for-Professional

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

  10. #10
    Join Date
    Mar 2005
    Location
    5,000 ft
    Posts
    2,190
    Quote Originally Posted by beenthere View Post
    Around here, most of those .1 FR designed systems are .75 to .9 static, on medium or high to get the proper ( or workable ) air flow.
    BINGO!

    Except where I'm at (5,000 ft) we never see proper AF. Just to think the .82" system I seen yesterday would be even worse @ sea level.

  11. #11
    Join Date
    Jul 2007
    Location
    North Little Rock, AR
    Posts
    19
    Quote Originally Posted by beenthere View Post
    If your sizing using .1

    Have you taken any ESP readings, and actually had only .45 static.

    Around here, most of those .1 FR designed systems are .75 to .9 static, on medium or high to get the proper ( or workable ) air flow.
    No not yet, I am going Monday to take ESP readings. The job is an hour away and it was a busy week so I have not yet done that. The book on this Rheem RSLA (X-13 motor) only goes up to .7 so I must assume the ESP must be between .1 and .7 according to the manufacturer.

    I'll post my findings when I get them Monday.



    S

  12. #12
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,718
    Just because the install manual only goes to .7 doesn't mean that the static isn't 1.1

    You might be greatly supprised how high it really is.
    Contractor locator map

    How-to-apply-for-Professional

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

  13. #13
    Join Date
    Jul 2007
    Location
    North Little Rock, AR
    Posts
    19
    Quote Originally Posted by beenthere View Post
    Just because the install manual only goes to .7 doesn't mean that the static isn't 1.1

    You might be greatly supprised how high it really is.
    You may be right, if I find the ESP to be higher than .7 then I have no way of knowing what CFM's the unit is truly delivering. Also, shouldn't the ESP always be between the min and max that the manufacturer recomends?



    S

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

Related Forums

Plumbing Talks | Contractor Magazine
Forums | Electrical Construction & Maintenance (EC&M) Magazine
Comfortech365 Virtual Event