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  1. #1
    Join Date
    Jul 2008
    Posts
    12

    help with manual j results

    home was built in 1980
    location: baton rouge, louisiana
    built with 3 ton central a/c with electric heat.

    did manual j and need help determining what to replace with

    sq footage 1662

    results
    sensible gain btuh:259334
    laten gain btuh: 6462
    total heat gain btuh: 32396 (2.5 ton)
    total heat loss btuh: 36360

    what size do I need?
    a/c with electric heat?=
    heat pump with b/u heat =
    geothermal heat pump =

    thanks
    james

  2. #2
    Join Date
    May 2007
    Location
    DC Metro Area (MD)
    Posts
    3,371
    Are you sure you input all of the data correctly? A load calculation is only as accurate as the data input into the program. What design temperatures did you use?

    Did the 3 ton cool and dehumidify well before? If it did, then I'd stick with 3 tons. 3 tons isn't oversized because the next size down wouldn't handle the load at design conditions (30,000 BTU/H = 2.5 tons - nominally).

    Are you considering going with a two-stage air conditioner or heat pump? I wouldn't consider using electric heat as backup when you can heat more cost effectively with a heat pump. There are two-stage heat pumps that run at a lower capacity in first stage. Many manufacturers offer such systems. You can use electric heat strips as backup with the heat pump. Generally you size the heat strips to match the entire heat loss figure incase the heat pump breaks. In that case, you'd need 10-15 kW of backup heat strips. This also depends on how much power you have to begin with. How many kW did you have before?

    Have you gotten any estimates yet? You could call some companies out for estimates and share your results with them to get their input. More goes into a replacement. You'll also need to make sure your ductwork is sized and laid out properly for the system to perform well.

  3. #3
    Join Date
    Jul 2008
    Posts
    12
    Hey Ryan Hughes

    i used hvac calculator residential version.

    summer: 93
    winter: 29
    grains: 116
    daily temp: medium

    the 3 ton did ok as far as I can remmeber, it has been out for several years. I question the size because the house was originally smaller, i enclosed the carport which adds about 360 sq feet more than original, perhaps the original ac was oversized. I believe the original heat was 10KW.

    I can't seem to get any good techs in my area, non mentioned a manual j calculations, just went off my old system and got a recommendation of 4 tons from one tech and 3 1/2 tons from another, they just looked at what I had before and added my additional carport I enclosed.

    I am looking at going with a heat pump, maybe a water source geothermal heat pump also. would you recommend a 3 ton on these.

    thanks
    james

  4. #4
    Join Date
    May 2007
    Location
    DC Metro Area (MD)
    Posts
    3,371
    If you go with an electric heat pump, which I would recommend, then I'd go by your load calculation results assuming they are accurate. I'm not sure about geothermal sizing so I won't answer that question (someone else can I'm sure). Geothermal isn't cheap.

    Oversizing isn't that uncommon unfortunately. If they didn't size the system correctly, then can you be sure that they sized the ductwork correctly? I hope the person who recommended a 4 ton looked at the ductwork.

  5. #5
    Join Date
    Jul 2008
    Posts
    12

    3 ton

    the 4 ton tech did not look at the ducts, I will look into making sure they are sized correctly. I think I will probably go with a 3 ton heat pump.


    thanks for the input
    james

  6. #6
    Join Date
    May 2007
    Location
    DC Metro Area (MD)
    Posts
    3,371
    I would meet with the contractors (or new contractors) and show them your load calc results and also explain your concern with ductwork sizing (making sure you have enough supply and return, ductwork sizes, etc.). Then get their input and post back here to go over the estimates. Find a good, reputable dealer who is familiar with such calculations.

  7. #7
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    Is your carport conversion better insulated then the original home?

    What indoor design temp did you use and is that what you want to maintain ?

  8. #8
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,174
    Your sensible load puts a 3 ton border line if your ID design temp is less the 75°F.
    Your latent load seems low to me.
    What infiltration rate did you use.

    As Dash alludes to, the addition may not be adding much load to the house.
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  9. #9
    Join Date
    Jul 2008
    Posts
    12
    carport is same or better insulations
    used 68 summmer and 72 winter

    thanks
    james

  10. #10
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,174
    68 ID temp is a bit low for a summer temp.

    See what is says at a more reasonble temp, say 72, and 74.
    What infiltration rate did you use.
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  11. #11
    Join Date
    Jul 2008
    Posts
    12
    infiltration
    summer .42
    winter .85

  12. #12
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    Quote Originally Posted by toireht View Post
    carport is same or better insulations
    used 68 summmer and 72 winter

    thanks
    james
    Three tons won't cool it to 68.

    The standard btu equip. ratings are at 80 indoors,large deduct of sensible btu's ,to run it at 68F.

  13. #13
    Join Date
    Jul 2008
    Posts
    12
    Quote Originally Posted by dash View Post
    Three tons won't cool it to 68.

    The standard btu equip. ratings are at 80 indoors,large deduct of sensible btu's ,to run it at 68F.
    dash,
    don't understand what you mean. can you explain please?

    thanks
    james

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