Duct work adequate size for 3 ton system?
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  1. #1
    Join Date
    Aug 2005
    Posts
    40
    Hello all, this is my first posting so be easy on me. I have approximately 1737 sq ft brick ranch, single story home located in the Charlotte area that was built in 1982. My home originaly had a 2.5 ton heat pump and was replaced in '97 with an Amana high efficiency furnace AC split system. Spec's on the system are as follows:
    GDC090X40B horizontal gas furnace
    RCC36A2B AC unit
    CHH36TCD coil
    The original duct work was kept. I'll try to the best describe the current duct system.
    Supply trunk (one continuous trunk) with:
    8"x8" x 14' section with 3 - 6" supply branch runs
    8"x16" x 25' section with 6 - 6" supply branch runs
    8"x8" x 8' section with 3 - 6" supply branch runs
    So I have 12-6" supplies to my rooms. All duct is hard ducted and insulated to R-5.
    Single return (I know this is bad from what I've read on this site) in hallway cemter of house:
    16" flexible return trunk
    20x25 grille using disposable filter

    Last year I had to have the compressor replaced and here's the numbers the technician wrote on the receipt:
    Outside temp: 88 deg F
    Pressures: Low - 85psi, High - 275psi, return air temp - 80 deg F and supply air temp - 65 deg F, delta temp - 15

    I read enough about HVAC designs to be dangerous, but I ran a duct calculator program I found on the web site and here's what I found:
    Supply: assumed 1200cfm, 750fpm > 17.13in round
    Return: assumed 1200cfm, 500fpm > 20.98in round

    Questions:
    1. Does the duct system design sound like it will function ok for the 3ton system or does it need to be replaced?
    2. What are the recommendations for getting return paths back to the central return? jumper duct or transfer grilles? I am especially concerned about the master bedroom because it has 2-6" supply trunks and 1-6" for master bath.
    Using a transfer grille for the bedroom would mean cutting about a 14" square hole in the wall which would look terrible. Can't put transfer grilles above door due to construction.
    3. From the numbers I got, seems like the return is undersized. Does this sound right?

    Sorry for such a long post, but wanted to give as much detail as possible. By the way, I also had to replace the horizontal coil due to a leak. So I'm frustrated with the whole system, but thats another story. Thanks in advance.

  2. #2
    Join Date
    Sep 2001
    Location
    Vancouver Canada
    Posts
    996
    What is the configuration of the supply duct? Does it start out at 16x8 and then taper down? Is there duct running both ways or all off one take off. Need a little more info. on duct layout to figure it out.
    "Go big or Go Home"

  3. #3
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    Assuming the 3 supply trunk are coming off a plenum independently.

    1.Sounds like branch ducts are okay,supply and return trunks,marginal.No one can tell ,without very detailed info.,or testing the static,and finding the air flow on the mfr. fan data chart.


    2.Jumper ducts,quieter and work just as well.Remember some air can go under the door,depending on how large the gap is.Adding a ducted return from the plenum ,is the best,as it looks like you could use a little more return duct.

    3.Could be ,takes detailed info., or test the ststic to tell.

    Coil if not kept spotless,reduces the air flow,over time.Increasing the return and adding a better filter after cleaning the coil,teting the static,would be a good way to go,at this point.



    Velocities you used are lower then needed,but Static rules the decision, on duct sizes.

  4. #4
    Join Date
    Jul 2005
    Posts
    717
    The 16" return is far too small for the supply total you have. It should be approx 25 % larger

  5. #5
    Join Date
    Aug 2005
    Posts
    40
    Thanks to all that have responded. I drew up a layout of my duct work so you all could see a picture of it but I don't know how to attach it. Any help would be great. I have a .jpg or .bmp file. Anyway I'll try to do a better job of explaining the layout. The furnace sets perpendicular to the main trunk line which has a tee junction off the furnace output supply. The opening from the main furnace is 26x19" and then goes to a transition duct down to 23x8. At the 23x8 duct is where the trunk is tee'd. To the right of the tee is an 8x8x14' run which has 3 branch supply lines to rooms. To the left of the tee, there is an 16x8x17' section with 4 branch supply lines. The trunk is then reduced to a 12x8x8'run which has 2 branch supply lines. The final section is reduced again to 8x8x8' and it has 3 supply branches.

    The furnace fan is set to Low with the following data given from the Amana data sheet.
    ESP 0.1 CFM 1238 rise-64
    ESP 0.2 CFM 1234 rise 64
    ESP 0.3 CFM 1222 rise 64
    ESP 0.4 CFM 1198 rise --
    ESP 0.5 CFM 1173 rise --

    I don't think anyone has done a static test to determine the CFM. The bedroom doors have about 3/4" undercut.

    Dash, I have read about the Return Air Pathway (RIP) that Tamarack Technologies have and was wondering if these are worth the price? I probably would prefer the jumper duct for the simple fact that I wouldn't have to cut a hole in the bedroom wall.

  6. #6
    Join Date
    Sep 2001
    Location
    Vancouver Canada
    Posts
    996
    From the sound of the layout I think you will have adequate duct for a 3 ton. Lot better duct design than a lot we see out there. Have seen a contractor put 14 5" outlets on a 10" round pipe. Wouldn't blow a match out at the registers. Some guys are either to stupid to know the difference or just don't care about customer satisfaction.
    "Go big or Go Home"

  7. #7
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    So .5 ESP to start,less grilles,coil and filter presuure drops,around.186 ESP left for the ducts.


    .186 Available Static,times 1000,dividved by ,260 TEL of duct(an estimate based on your input)+ a .07 zFriction Rate.


    Most all of the ducts are too small to deliver close to 1200 cfm,for a 3 ton unit.In a humid climate,1050 may be all you really want.

    If it's delivering < 925 cfms now,you are likely getting more like 2.5 tons of cooling.Do a load calcualtion,with some added attic insulation and 2.5 tons may be all you need.

    Good news is the duct system may be fine for 2.5 tons,and .5 ton less will save 16% ,over the same SEER 3.0 ton system.

  8. #8
    Join Date
    Aug 2005
    Posts
    40
    I got a copy of the HVAC-Calc program and did a load calculation and it calls for 2.5T for AC and around 34K BTUH heat loss which seems low to me. I bought a hard copy of Manual J and used the CD that comes with it and got about the same for AC but the heat BTUH loss was higher by about 15K BTUH. Sounds like the contractor just oversized my system.
    The current system does do a good job cooling and maintaining an average RH of about 50%. Although I have noticed that sometimes after the AC shuts off, it does come back on in anywhere from 5-10 minutes. It then will run for about the same amount of time (maybe a little longer) and shut off again. I guess this is short cycling?

    I do have one question regarding HVAC-Calc program. For the floor load, which one of "Select floor location" should I chosen? I have a vented crawl space (not conditoned)that has open vents with R-19 insulation between the joists.

    over conditioned space
    over unheated basement
    over enclosed crawl space
    over garage or open crawl space (I used this one)


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