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  1. #1
    Join Date
    Mar 2006
    Posts
    5

    Question

    I am going to be replaceing my 14 year old 3 ton a/c , 50,000 btuh gas package unit.I have had two estimates given ,each said that a 31/2 ton with same btu heat out put as current unit would better suit my home from thier calculations. However one Co. said that I would need a entirely larger duct system while the other said it would only be the return plenum that would need to be larger. duct system contains 20" X 25" central return air filter , return plenum 14" flex duct 35' long ,Supply plenum 10" X 20" rectanguler duct 40' long, with 10 branch 7" flex ducts off the trunk. Who seems to be rite here?

  2. #2
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,189
    My views, 14" flex a max of 2.5 ton. That's what I use on my 2 ton. 20x10 main duct metal, max of 3 ton. 10 7" flex ducts the same. Definitely enlarge the return, should enlarge the supply just a bit too.

    Has the 3 ton been cooling OK all this time? If so, stay there regardless of what their calcs say. Oversized lowers effective SEER and humidity control.

  3. #3
    Join Date
    Mar 2006
    Posts
    5
    I tend to believe that they are correct on up sizeing the tonage Since during the hottest months My a/c operates for 7 hours nonstop until about midnight before house temp falls below 76 F. Been that way as long as I can Remember. Thanks for The Info.

  4. #4
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    Your ducts and filter are minimal size for the tons at best.Could be causing low air flow which reduces capacity and could be the cause of long run times.Can't tell from here.

    If you go 3.5 tons increase the size of all of it.

    Man.J then Man S,then Man. D.sizing ,seletion and duct sizing.

  5. #5
    Join Date
    Mar 2004
    Posts
    1,042
    Longer cycles get you better efficiency, better durability, and better dehumidification. In this field, at least, small is beautiful.

    Unless you want it to be able to keep the house cooler than your old unit did, there's nothing wrong with it running as you describe.

  6. #6
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    Yes if it maintains the temp,you want keep the same size.

    If fact due to the cost of duct work,you should look at adding attic insulation,window tinting etc..,to reduce the load instead of increasing the size.

    3.5 ton will cost more to operate,so insulate and save operating costs,forever.

  7. #7
    you need to change your return air duct to a 16" other wise you will not gain any thing because you will just slow down your blower moter because of static pre.

  8. #8
    Join Date
    Sep 2003
    Posts
    1,311
    Originally posted by pearce heating & air
    ...because you will just slow down your blower moter because of static pre.
    Guess again

  9. #9
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,295

    Hmm Options ?

    Originally posted by rug rat
    ,each said that a 3 1/2 ton with same btu heat out put as current unit would better suit my home from thier calculations.

    ... would only be the return plenum that would need to be larger.
    Duct system contains 20" X 25" central return air filter , return plenum 14" flex duct 35' long ,

    Supply plenum 10" X 20" rectanguler duct 40' long, with 10 branch 7" flex ducts off the trunk.
    Initial impression:
    Add another 12" or 14" Return and
    evaulate performance this summer before upgrading
    (assuming equipment is not in state of imminent failure).

    Location?
    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

  10. #10
    Join Date
    Mar 2006
    Posts
    5
    Location is N. Georgia. On furnace half heat ex. busted and roll out has melted control board according to techs. Could that have been done by draft blower problems? Back to air flow I noticed that the return duct collar flange on the A/C unit is only an 14" round hole will makeing the return flex duct larger even help the air flow?

  11. #11
    Join Date
    Jun 2004
    Posts
    459

    duct sizing

    rug,

    You might ask for a second opinion on the need to increase tonnage. Have another load calculation done for sure if you feel it is necessary to increase size. If it turns out you can stay with 3 ton, your supply is within reason, (not your return) like Bald Loonie said, increase it. If you decide that 3 1/2 tons will be installed, then increase your supply and return.

    Here is a short anlysis of your supply

    3 ton a/c, 1200 cfm of air
    1200 cfm air traveling in a 10 x 20 duct yields .085 in. w.c. pressure loss and has a velocity of 950 fpm.

    3 1/2 ton a/c, 1400 cfm of air
    1400 cfm air traveling in a 10 x 20 duct yields .14 in. w.c pressure loss and has a velocity of 1100 fpm.

    Concerning the return

    1400 cfm of air @ 700 fpm velocity yields 20" round
    1200 cfm of air @ 700 fpm velocity yields 18" round


  12. #12
    Join Date
    Oct 2003
    Location
    Washington
    Posts
    7,405
    Here's an off the wall question guys. And I do believe there was a thread on this before. But, isn't it impossible to have too large of a return, theoretically? I mean, if this guy is borderline on his return he should by all means upsize it, can't hurt anything. At least that's the way I see it. For every 100 instances of problems due to return I would think 99 of them are due to it being too small.

  13. #13
    Join Date
    May 2003
    Location
    chesterfield, michigan
    Posts
    93
    Originally posted by sadlier
    Originally posted by pearce heating & air
    ...because you will just slow down your blower moter because of static pre.
    Guess again
    Absolutely true, by restricting you will actually increase your airflow...duct sizing is based on fan curve and noise criteria, the fan will deliver a particular amount of air based on static pressure, but there is a fine line with pressure and the cavitating effect of the fan based on the manufacturer's fan curve.

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