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  1. #27
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    Quote Originally Posted by roccol View Post
    Furnace: Lennox GV32V5-100-5

    Dash the return duct is not 8 x 30, it is 26 x 8.
    My bad.
    8X26 ,800 cfms ,.04 FR ,600 fpm,still okay.However I think your cfms for high stage heat will be much graeter and may be in the unaccectable range.Maybe someone can post the required cfms for the furnace.

  2. #28
    Join Date
    Mar 2005
    Location
    Suppy NC
    Posts
    4,513
    i have to ask
    2ton system with a 2000 plus cfm furnace why?

    part of your problem may not be just your duct work. That furnace is capable of 5 tons cooling and 100,000 btu of heating.

    It may need to have blower speed ajustments made. hopefuly he furnace wont have over heating problems


    that furnace is designed for about 2000 to 3000sf

  3. #29
    Join Date
    Jun 2002
    Posts
    108
    My first floor is almost 2000 Sqft. The furnace has never failed in 5 years, so I assume the blower speed was adjusted. My home is easy to cool, hard to heat. Sounds crazy, but I have a stone house with a slate roof.

  4. #30
    Join Date
    Jun 2002
    Posts
    108
    sorry my mistake. My Condenser is 3 ton. I guess that makes more sense:

    Lennox HS21-036P(2 stage 16 seer) with a 4 ton coil.

    I have a suspicion much of my duct work is undersized. How can I roughly calculate how many CFM my supply trunks can carry?

    18 x 8 metal with one take off
    14 x 8 metal with one take off

    Also is cfm per sqft roughly 1:1 ratio? I know this is not a proper calculation, I just want to see if I am in the ball park.

  5. #31
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    4,897
    Quote Originally Posted by roccol View Post
    sorry my mistake. My Condenser is 3 ton. I guess that makes more sense:
    Lennox HS21-036P with a 4 ton coil.
    I wondered initially if you had the correct tonnage at 2-Ton.
    I like to size for 450-CFM per ton so it will be adequate for a possible later heat pump install.

    I would use an 18" round duct or a 254-sq.in duct for SA & RA mains.
    My software shows (3-Ton AC) @ 1350-cfm, @ 750-fpm velocity & two 90 turns, the friction rate is a mere 0.02". (I left out that it was figured at a 20-foot duct run.)

    Adjust the large air handler's blower speed to fit the application requirement!

    For proper ductwork functional design you have to solve for Available Static Pressure (ASP) and Total Equivalent Length (TEL), keeping with a proper low Friction Rate (FR) design.
    http://www.udarrell.com/proper_cfm_b...g_systems.html
    udarrell - Darrell
    Last edited by udarrell; 02-11-2008 at 11:58 AM. Reason: Left out - figured at a 20-foot duct run.

  6. #32
    Join Date
    Jun 2002
    Posts
    108
    Quote Originally Posted by udarrell View Post
    I wondered initially if you had the correct tonnage at 2-Ton.
    I like to size for 450-CFM per ton so it will be adequate for a possible later heat pump install.

    I would use an 18" round duct or a 254-sq.in duct for SA & RA mains.
    My software shows (3-Ton AC) @ 1350-cfm, @ 750-fpm velocity & two 90 turns, the friction rate is a mere 0.02".

    Adjust the large air handler's blower speed to fit the application requirement!

    For proper ductwork functional design you have to solve for Available Static Pressure (ASP) and Total Equivalent Length (TEL), keeping with a proper low Friction Rate (FR) design.
    http://www.udarrell.com/proper_cfm_b...g_systems.html
    udarrell - Darrell
    Sorry what is SA and RA ? Supply and Return? I have 2 supply trunks, together they roughly equal 254 sq inch.

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