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Duct Sizing


We moved into our house a few years back and after living here for about 4 years I've noticed that the home really never cools down lower than about 75. The unit has a Carrier 2 ton split system. I verified that the condenser and air handler are matched. I have not performed a Manual J. I am just looking for opinions on if the current duct work is sufficient before I dive any deeper into the cooling issue. The home is 1162 sq ft. ranch.

I was looking into the return and supply ducts of the house and found the following.

14" return flex duct with 14x20 filter grille
8" return flex with a 10x10 filter grille
5 - 6" supply flex ducts
1 - 7" supply flex duct

Does this seem adequate for a 2 ton system?



  • GW
    GW Member Posts: 4,702
    Pete it should work just fine. Duct size seems good enough 

    You did not mention where the air handler was, if it’s in the attic, I would be looking for leaks in the return system, well, look for leaks everywhere, but the return leaks will kill you.I am blown away how many times I see flimsy air handler systems in the attic, with the filter slots sucking all sorts of attic air.

    It could be a little bit low on charge. Making the grand assumption that you don’t have HVAC tools, the next best thing is to check the temperature drop. Checking the dry bulb temperature drop is a little old school, check on the wet bulb drop is more precise, and factors in the humidity in the air.

    The good trades guys have tools a little bit more sophisticated, but the check humidity  you can buy something like this

    This tool will tell you with the wet bulb temperature. So if you take the return wet bulb (temp) and subtract the supply wet bulb, we generally just refer to that as “well bulb drop” it’s jargon but it works in our company 

    a 10 degree Wb drop is good. Higher the better. When houses are all heated up in the air conditioning is cranking, what bulb drops can get up to 13 or 14, maybe 15 Degrees. But when the house is stable, normal air-conditioning temperatures, you’re going to be more like 10 or 11 for system operating well.

    Some of the old timers and less skilled technicians look at dry bulb split, which is fine. Just not very accurate. Dry bulb is what the $10 thermometer tells you.

    I hope this helps

    Gary Wilson
    Wilson Services, Inc
    Northampton, MA
    EBEBRATT-Ed Member Posts: 15,629
    2 tons you need to move 800cfm. a 6" supply is good for 100 each so 500 ttl and the 7" is good for 150

    so you have 650 which is a little short. The 8" return is good for 200 and the 14" (I can't remember) maybe 1000 so I think your returns are ok supplies are a little short.

    2 tons may run hard for 1100 square feet or maybe ok. Depends on windows, doors, insulation.

    Duct leakage and duct insulation can be a killer as @GW mentioned.

    Most ac systems are sized for around 74 degrees indoor temp
  • ChrisJ
    ChrisJ Member Posts: 15,784
    edited August 2021
    How long are all of those flex runs and how many turns are in them?  Are they pulled tight?

    Can you share some pictures of the ductwork?

    What's feeding all of those flex runs?
    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
  • Big Ed_4
    Big Ed_4 Member Posts: 2,806
    You only mentioned one temperature , can we assume the reading of the thermostat ?... What temperature is it outside when its 75 inside ? What part of the country is the house being conditioned for ?

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