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  1. #1
    Join Date
    Oct 2007
    Location
    Bucks County, PA
    Posts
    86

    Help with load calc results

    I have recently moved into a 23 year old house with the original 3 ton heatpump. I am going to be getting quotes for a new system and get it replaced before it fails. Thinking of either a Lennox XP19 or a Carrier Infinity. I suspect my current system is oversized due to the fact that duct system is much loader than heatpump in my older home and it heats much better and very rarely does aux. heat come on.

    Here are results of load calc that I have done:

    3 bedroom Cape Cod on Slab with a lot of glass
    2100 sq. ft.

    Outside Design Conditions: 88/14 105 grains of moisture
    Inside Design Conditions 72/72 50% Humidity

    Total Heat Gain 25,904 BTUH
    Sensible 21,895 BTUH
    Latent 4,009 BTUH

    Heat Loss 39,748 BTUH

    Questions
    1. I know there is a correction because of using 72 deg. indoor temp but not sure what it is.

    2. Even with correction seems like 3 ton would be way oversized. Would probably run on first stage all the time. Does anyone make a 2 1/2 2 stage heat pump?

  2. #2
    Join Date
    Feb 2008
    Location
    Western NC
    Posts
    2,504
    There are several 2 1/2 2 stage heat pumps out there. As far as the load calc....Can't do it over a computer. Gotta be there to see it.

    Trust your contractor, thats why theres a warranty.

  3. #3
    Join Date
    Dec 2007
    Location
    oregon
    Posts
    885
    Maybe your last home had a flex duct and this one has an all metal one.
    Genius = The guy who can do anything...except make a living!

  4. #4
    Join Date
    Dec 2007
    Location
    oregon
    Posts
    885
    Call for bids, see who does a load calc and measures your home. Narrow down your list of contactors to those that measure up for a load calc. Trane makes a 2.5 ton dual capacity heatpump.
    Genius = The guy who can do anything...except make a living!

  5. #5
    Join Date
    Oct 2007
    Location
    Bucks County, PA
    Posts
    86
    Old house had duct board main plenums with 6" round metal branch's.
    New home also has a single return vent, 15 1/2" x 25"

  6. #6
    Join Date
    Dec 2007
    Location
    oregon
    Posts
    885
    Quote Originally Posted by Old Lennox View Post
    Old house had duct board main plenums with 6" round metal branch's.
    New home also has a single return vent, 15 1/2" x 25"
    Duct board jobs are pretty quiet due to the sound dampening properties of the duct board.
    Genius = The guy who can do anything...except make a living!

  7. #7
    Join Date
    Dec 2007
    Location
    oregon
    Posts
    885
    Just by changing your grilles on your return, and the registers on your supplies, you can increase air move movment and lower noise levels. Ask your contractord about the commercial line up of grilles and registers from shoemaker and hart and cooley.
    Genius = The guy who can do anything...except make a living!

  8. #8
    Join Date
    Oct 2007
    Location
    Bucks County, PA
    Posts
    86
    Can someone tell me how many btus to add because of 72 deg indoor temp?
    Thanks

  9. #9
    Join Date
    Nov 2007
    Location
    Livermore, Ca
    Posts
    163
    I'm reading the opening post with the same interest I have ( had ) with our home. At first, 3 tons seemed too large for our house. Did five load calcs and came up with a comfortable average number. In this case it was 25k of cooling. 31k of heat.

    The 3 ton is good for 36k. But I'm told and have read here that sizing should be done to the units sensible cooling. Again in my case, when I checked out Rheems site and dug around, it ends up that it's sensible cooling matches my needs. I don't know how to figure in latent cooling yet, if it's even needed to be figured in. The heat is over my needs but maybe that means the aux may not have to come on given my design temps.

    Again as noted in the opening post, they ask about raising the indoor temp input. In my case, I feel at ease that with a little overage on the heat capacity of our unit, jacking up the T-Stat tp 80 shouldn't be a problem.

    I was also told that overages should be kept to a minimum ( 10-15% ). I may be a little over that, but such is life. It'll give me a reason to build that addition

    Don't mean to highjack this thread but I also have the same question as
    Old Lennox.

  10. #10
    Join Date
    Nov 2007
    Location
    Livermore, Ca
    Posts
    163
    Old Lennox. I, as a stoopid homeowner searching for the truth, would do another calc with your desired temps. My three paid Man J's used 75 on cool and 70 on heat. May be a industry standard.

  11. #11
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,357
    Quote Originally Posted by Old Lennox View Post
    I have recently moved into a 23 year old house with the original 3 ton heatpump. I am going to be getting quotes for a new system and get it replaced before it fails. Thinking of either a Lennox XP19 or a Carrier Infinity. I suspect my current system is oversized due to the fact that duct system is much loader than heatpump in my older home and it heats much better and very rarely does aux. heat come on.

    Here are results of load calc that I have done:

    3 bedroom Cape Cod on Slab with a lot of glass
    2100 sq. ft.

    Outside Design Conditions: 88/14 105 grains of moisture
    Inside Design Conditions 72/72 50% Humidity

    Total Heat Gain 25,904 BTUH
    Sensible 21,895 BTUH
    Latent 4,009 BTUH

    Heat Loss 39,748 BTUH

    Questions
    1. I know there is a correction because of using 72 deg. indoor temp but not sure what it is.

    2. Even with correction seems like 3 ton would be way oversized. Would probably run on first stage all the time. Does anyone make a 2 1/2 2 stage heat pump?
    One thing to remember, three tons is only three tons at ARI testing/rating conditions, which are 95 degrees outdoor ambient temperature, 80 degrees indoor at 67 degrees wet bulb. If the ambient is above 95 or the indoor design is below 80/67, the delivered capacity of the system changes.

    Three tons is also only three tons for design day conditions, on a well installed system with sealed, well flowing ductwork that has no heat gain (not run in attic).

    May I ask why you chose 72 degrees as an indoor design temperature? Are you concerned about adequate dehumidification?

  12. #12
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,323
    Quote Originally Posted by Old Lennox View Post
    Can someone tell me how many btus to add because of 72 deg indoor temp? Thanks
    Equipment is rated at 80'F indoor temperature.
    DERATE capacity per the mfg and model/equipment selected.

    Total Sensible Capacity is 25,120 BTUh at 1,050 CFM, WB entering 63'F and 95'F outdoor temperature.

    SEE PAGE 16

    http://www.docs.hvacpartners.com/idc...25hna9-2pd.pdf
    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

  13. #13
    Join Date
    Oct 2007
    Location
    Bucks County, PA
    Posts
    86
    Quote Originally Posted by shophound View Post
    One thing to remember, three tons is only three tons at ARI testing/rating conditions, which are 95 degrees outdoor ambient temperature, 80 degrees indoor at 67 degrees wet bulb. If the ambient is above 95 or the indoor design is below 80/67, the delivered capacity of the system changes.

    Three tons is also only three tons for design day conditions, on a well installed system with sealed, well flowing ductwork that has no heat gain (not run in attic).

    May I ask why you chose 72 degrees as an indoor design temperature? Are you concerned about adequate dehumidification?
    I have always been more comfortable 72 degrees. 78 and 80 degrres has not been comfortable. Granted humidity has been struggling to get below 55rh in my last home due to being oversized. I want to make sure that I don't have this problem with new system.

    Dan thanks for information. Looks like I would only be slightly oversized with 3 ton carrier. Do you have this info for the Lennox XP19? Probably has nothing to do with it but existing Lennox has lasted 24 years.

    I was shying away from looking at Trane due to their low heat numbers but the 50/50 split XL19 looks intriging. I am concernrd that this unit is only R22. Seems a little behind in technology.

    I am going to start having contractors give me bids. I am just trying to have some idea of what I need before I proceed. I am also fortunate to be in position that current system still works and I am in no hurry.

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