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  1. #1
    Structure:1785 ft home,r-19 walls,r 38 attic,r-19 floor on crawl,all glass accounted for on heatload.
    Design :100 outside,72 inside
    Unit;3 ton bryant hp 410a 12seer
    Duct :hardpipe with 2 in foil in crawl,(new and tight).1 static design
    Return:14 in and 10 in,filters sized at .4 times cfm (state code)Arkansas;
    Subcooling:12-13,(chart specs 12)
    Meter:txv
    Td:17.5
    problem:unit runs constantly, loses ground from noon till nine pm.Set at 75,temp is 79,outdoor is 95 to 101.
    location:central Arkansas

  2. #2

    What was the Gain

    What was the heat gain on the house, and what kind of gain did you get on the windows? sounds like you have a good bit of glass, maybe getting more of a gain than expected thur the glass. Post the numbers on the equipment, sometimes a three ton doesn't quite deliver 36000 btu's , if it's like atlanta the latent load is doing a number on the sensbile load for the last few weeks.

  3. #3
    Join Date
    Feb 2003
    Location
    S.W. PA
    Posts
    3,298
    You will have to see exactly what your capacity is by looking up coil and condenser see what latent and sensible capacities are then see what you need

    Next did you check to see if you have a big t/d from one beginning of trunk to the end and on runs?

    I know you said duct work was new what about home?

    Other than that if it is running right and not cooling my guess would be you missed something in the load calc go over numbers again and see where you are at

  4. #4
    Join Date
    Aug 2002
    Posts
    1,642
    We need more information to properly help you...
    Like Suct pressure, sh, head pressure, idwb, return temp, supply temp,, and any other information you have..

    Also, what is the ESP of the duct system,, this can verify proper airflow.. The .1 static duct design concerns me as that sounds like a thumb job.. Duct system should be designed to manual D..

    Please sign up for Pro status as we can help more in the Pro forum as it is not subject to DIYers or your customers..
    These problems should be posted there not in the homeowners residential section..

    Have a great night
    J

    ps. as hroper stated many units do not provide 36000btus and at 72 indoor design temp the unit usually must be upsized sometimes 1/2 ton as units are designed to maintain higher indoor temps.. Check the spec sheets of the manufacturer for proper match-ups..

  5. #5
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    Sizing at .1 Friction Rate,which is used to size the ducts ,not "static",though they are related ,is just a rule of thumb.If the true FR is .06,the ducts are too small,and cfm will be less then required ,so you have a loss of sensible btus,and it can operate closer to a 2.5 capacity.


    Assuming the Man J ,was done properly and you need a nominal 3 ton ,at 75 indoors,your choice should have been a 3.5 ton ARI rated system.

    ARI ratings are at 80 indoors,since you sized for and want to maintain 75,and adjustment to the capacity needs to be made,to determine capacity at 75.Mfrs specs usually include this info in the footnotes,and it also in Manual S,from ACCA.

    The deduct is 835 btus of sensible capacity per 1000 cfm,for each degree below 80 that you desire.

    So,835 X 5(degrees) X 1.2(1000 / 1200 cfms)=5010 btus of sensible capacity loss at 75 indoors,for your 3 ton system.Likely need a 3.5 ton unit,select one and do the calculations to see how it performs at 75 indoors,instead of 80.


    Many other things as mentioned by others could be a problem as well,but the capacity adjustment is most likely the root cause of lack of capacity.Failing to make this adjustment,is likely the reason,many contractors think Man.J undersizes the system.

    If you go to 3.5 tons ,the duct system size needs to be calculated ,per Manula D,to determine the FR ,o deliver the cfms needed.

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