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  1. #1
    Join Date
    Mar 2008
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    Tampa, FL
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    54

    Confused Flex duct sizing in mobile homes. Why?

    This is Eddie in Tampa. I would like to know if anyone knows why all these mobile and manufactured homes run nothing larger then 14" FLEX . 3 - 4 TON UNITS AND THEY ARE RUNNING OFF 12 OR 14 FLEX. Then I try to order 16 and 18 mobile home flex from Baker and they can't even get it- like it doesn't even exist. What gives, is there some other realm of ductsizing I never heard of.
    Like what actually happens when you run these package units at a .2 or higher static. Ever looked under a trailer? sure it's short, but come on. It's bad enough they get away with selling them with a 50 foot run of 6x12 floor duct and 4x8 grills. How they do I do not understand either.
    Anyway, anyone got any answers?

  2. #2
    Join Date
    Nov 2008
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    S.C.
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    931
    I could be wrong but I believe the fan is a hi static fan that comes with a mobile home. Single wide or double wide. Double wide would be the cross over

  3. #3
    Join Date
    Jan 2004
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    PA
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    68,789
    I'm gonna guess. You think the .2" Friction Rate of a ductulator, is static pressure.

    Its not.
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  4. #4
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
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    18,836
    Some newer ones are better.

    High static reduces cfms,SEER,capacity and compressor life.

    The fixes are;

    Use a 14 (short piece) to a metal wye,then two 14s.

    Tap each trunkline twice,25% in distance from each end.

    Filters in floor grilles are too small,need a deeper floor box with flex out the side and a larger filter,somtimes add another return.



    Remove cheap bent floor supply grilles and replace wih Hart&Cooley,# 210 style.

    The static can be reduced dramatically,and they are greater then .2 .

  5. #5
    Join Date
    Mar 2008
    Location
    Tampa, FL
    Posts
    54
    dash, thanks for the info. I'm gonna try to go with 16, only problem is that is is not the outdoor stuff. They thought it was then got it and said it wasn't and said they cannot even get anything over 14 mobile home style from they're flex suppliers.
    beenthere. ok. I just know they taught me to size supply duct at .1 on that friction rate, and called it static pressure soooo... I'm listening...

    By the way this site rules!- my old answerman teacher has a competitor now.

  6. #6
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
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    Get your Pro status here and you can learn what you need to know about duct sizing and HVAC in general.

  7. #7
    Join Date
    Jan 2004
    Location
    PA
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    Quote Originally Posted by eddie-boyz1 View Post
    beenthere. ok. I just know they taught me to size supply duct at .1 on that friction rate, and called it static pressure soooo... I'm listening...

    By the way this site rules!- my old answerman teacher has a competitor now.
    As the term implies.
    Friction rate, is the resistance to air flow in the duct.

    Its one of the forces that the blower has to over come, to move the volume of air that we want to move.

    Next time you have an install manual in front of you.
    Look at the blowers CFM rating, its lowest static pressure rating too its highest Static pressure, and then look at the ductulator.
    You'll see they go in opposite directions.

    An 18 X 8 duct may be listed as moving 1900CFM at a FR of .5"
    But, the blower may not be able to move that air against a static pressure of .3"
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  8. #8
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
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    FR is the the static per 100' of duct, this includes the equivalent feet of duct per tables in Manual D for fitting,elbows,take-offs,etc..Your job has several hundred feet duct.

    Get a copy of Man. D from www.acca.org to learn duct design,you can get help here in the Pro sections of this site.

  9. #9
    Join Date
    Nov 2008
    Location
    Biloxi, Ms.
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    176
    Eddie,
    40 years ago I flew into Tampa, yes they did have airplanes 40 years ago, to visit a dealer that was installing mobile home systems by the thousands each year. They were very nice to me and let me ride with an installer all day. He installed a couple system that day with package units. When I questioned him about putting double turning vanes in the duct under the trailer, he said the air gotta go somewhere. It worked, but I decided I wasn't smart enough to do that type of work.

  10. #10
    Join Date
    Mar 2008
    Location
    Tampa, FL
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    54
    I'll keep posting till I get to fifteen and register.
    As for friction rate per 100 feet of duct. is that assuming 100 feet of the particular sizing you are reading at that time or for 100 feet of entire duct system with all the different sizes?? This system is for a trailer so you are only actually looking at aout 80 feet total withe the flex and trunk. I am sizing my trunk with a .1 and if I went with the standard 14" flex (on this 3 ton) then would the f.r. go fom .2 in the flex to a .1 in the duct?? Is the fact the 14" is only actually 10ft (if I used it) reduce the true f.r. and if so how would you figure it?

  11. #11
    Join Date
    Jan 2004
    Location
    PA
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    Its not the linear length.
    Its the total equivalent length.

    A take off could have 10 15, or 35 foot of equivalent length.
    A Y, depending on which type, could be 10, 25, or 35 foot of equivalent length.
    Depending on which type of register boot you use, it could have between 10 and 50 foot of equivalent length.

    The total equivalent length of your duct system. Is the longest run for the supply and the longest run for the return combined.

    An 80' linear supply run plus return run, can easily be 300' Total Equivalent Length.

    Draw out the way duct work will run.
    Then with either manual D, or a fitting chart, find the equivalent length for all fittings.
    Find your longest TEL supply run, and TEL return, and add them together.
    Then subtract your register and air filter pressure drop from your units ESP rating. That give you your ASP.(available static pressure)


    EG: .2" ASP times 100 = 20 divided by 300'TEL = .06 Friction Rate.
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  12. #12
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
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    Beenthere is correct above,I'd add get a copy of Man. D and Pro membership here,and you get the help you need.

  13. #13
    Join Date
    Mar 2008
    Location
    Tampa, FL
    Posts
    54
    duh, ok I'll get the manuel d and read it. thanks

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