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  1. #27
    Join Date
    Jan 2009
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    386
    Quote Originally Posted by beenthere View Post
    That is the formula in the spread sheet.

    I'm at rough 460 foot above sea level, so I'm safe with out correcting for density.
    Your formula in the spreadsheet accounts for air density (that would be a nice spreadsheet, where you could just plug in zipcode and get the air density multiplier )? You may not take altitude into consideration where you live, but it should be taken into consideration when quoting CFM and heat rise for others around the country. I was just pointing out that the 1.08 multiplier is not a one-size-fits-all.

  2. #28
    Join Date
    Feb 2009
    Posts
    110
    Thanks for all the input. I have downloaded the HVAC Calc program and will try to take measurements this weekend.

    I'm sure I'll have some questions. I'll need to do two load calcs I guess. One for the main floor system and one for the upper level system in the attic.

    Clearly closing off the duct will help.

  3. #29
    Join Date
    Jan 2004
    Location
    PA
    Posts
    68,753
    The 1.08 for seal level, and the .77 for 9000 foot is for density.

    When someone says their at 5000 foot, or some other altitude, I'll do the correction.

    Till then, I'll just use sea level.
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  4. #30
    Join Date
    Jan 2009
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    386
    Quote Originally Posted by beenthere View Post
    The 1.08 for seal level, and the .77 for 9000 foot is for density.

    When someone says their at 5000 foot, or some other altitude, I'll do the correction.

    Till then, I'll just use sea level.
    I know that it's for air density, that's what I've been saying. So you'll give them inaccurate info, unless they specify their altitude? Why not just say it's dependent on altitude? I'm not trying to argue here, I'm just not understanding why this is something that should be glazed over on a technical site such as this.

  5. #31
    Join Date
    Jan 2004
    Location
    PA
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    68,753
    Few home owners are going to be taking the temp rise reading where they should be taking it.
    Nor are they going to be taking enough readings for it to be an accurate reading.
    Plus, they aren't going to be able to verify BTU input, or output, to need to be exact.

    A ball park rise is enough for the average home owner, for the type of questions we are allowed to answer here.

    Not allowed to be too technical in the open forums.

    Sea level to 2000 foot is less then 8% inaccuratecy.

    Taking the temp reading at the return grille and supply register, is more inaccurate then not correcting for density at altitudes below 2000'.
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  6. #32
    Join Date
    Jan 2009
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    386
    Quote Originally Posted by beenthere View Post
    Few home owners are going to be taking the temp rise reading where they should be taking it.
    Nor are they going to be taking enough readings for it to be an accurate reading.
    Plus, they aren't going to be able to verify BTU input, or output, to need to be exact.

    A ball park rise is enough for the average home owner, for the type of questions we are allowed to answer here.

    Not allowed to be too technical in the open forums.

    Sea level to 2000 foot is less then 8% inaccuratecy.

    Taking the temp reading at the return grille and supply register, is more inaccurate then not correcting for density at altitudes below 2000'.
    Fair enough. I don't considering correcting for air density in heat rise / airflow calculations to be too technical. But, rules are rules.

  7. #33
    Join Date
    Sep 2006
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    1,253
    Quote Originally Posted by noobs View Post
    Thanks for all the input. I have downloaded the HVAC Calc program and will try to take measurements this weekend.

    I'm sure I'll have some questions. I'll need to do two load calcs I guess. One for the main floor system and one for the upper level system in the attic.

    Clearly closing off the duct will help.
    The program will allow you to do the whole house or break it down into smaller components. I would do a room by room calculation which will then also provide you with the required air flow CFM for each room. Then you can group the rooms together for the first floor and second floor and the software will then give you the total gain/loss for each floor.

    The infiltration rate will be the number you will have to guess at. You may want to consider having a blower door test done before you install the HVAV-Calc program so that you have the infiltration number and any other big issues known in advance. Once you install HVAC-Calc you will only have 30 days of use.

  8. #34
    Join Date
    Jul 2006
    Location
    Virginia
    Posts
    4,861
    Quote Originally Posted by noobs View Post
    OK, that seems to make sense. There is general reluctance in the industry (at least in my area) to do Manual J calculations. My current set up is as follows:

    York
    Condenser: H1RA048S06D
    Coil: G1UA048S17B
    Furnace: P4UB16N06401A

    In the summer, the AC runs almost constantly during the hottest and will not keep up in the afternoon if there are many people in the house. Temperature creeps up.

    In the winter, on cold, windy days, the furnace runs non stop and it “feels” a little chilly. Recovery time from a set back is almost not possible. Maintenance of temperature only. I have to turn the gas fireplace on as a supplement.

    One contractor commented that it was odd to have a 64,000 BTU furnace matched with a 1600 cfm blower and 4 ton coil and AC. I was also told that with older furnaces, the 64,000 was output spec and that input is 80,000 BTU at 80% AFUE.

    So as a starting point, is this far off?

    4 ton 2 stage AC with matching coil
    100,000 BTU (input) modulating furnace

    Since the modulating furnace draws combustion air directly from outside, I was also planning to close the 6” fresh air intake that goes into the cold air return.

    I’m still working on finding someone to do the manual J and will try to buy the HVAC calc program to do it myself as a check.

    Would those specs be that out of whack?
    how about adding more insulation and maybe solar screens

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