Is Manual-J absolutely necessary? - Page 2
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  1. #14
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    Just out of curiosity, what sizes did everyone else come up with. Jultzya is right its real.

  2. #15
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    Jul 2001
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    Gold Coast of Connecticut
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    The second most importent thing is duct design and static pressure of your system. If the other contractors do not do a load calc I can almost gaurantee the seer rating of the equipment will not matter. A pooly installed and over-sized 17 seer or 90+ will cost more to operate that a properly sized and installed 10 seer (or 78%)system.

    Go with someone who knows and you will be paid back every month for years to come. I know of identical homes to mine which have electric bills that double my bills every month.
    We both have pools ect, my cooling system was installed properly!
    Aire Serv of SW Connecticut- Gas heat, dual fuel and central a/c systems installed and serviced

  3. #16
    Join Date
    Oct 2005
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    88
    There is some truth to the statement that since it was built in 1948 it does not apply. Manual J is only accurate if you use actual data in the calculations. With a home built in 1948 you are going to be guessing on some of the important data
    The silver lining is that the house built in 1948 probably has a long history of actual billed gas usage, and that's really actual data: a working furnace hooked to an accurate gas meter for a few years is a darned good measuring instrument for actual heat loss. IF: 1. you have access to a year or better of gas bills (during which time you didn't make any major changes to the house or furnace), 2. your gas co. shows amt. of gas used, heat value of gas, and number of heating degree days on each bill, 3. you know the efficiency of the existing furnace, and 4. you know the design winter outside temperature for your location, you can come up with a size number that can be a very good sanity check of an estimate from Manual J. It's not hard to do if you liked drawing graphs in math, or have a spreadsheet program that can draw a graph and fit a line to it.

    It's not an officially recognized alternative to Manual J as far as I know, so you might want to be cautious about relying only on this number, especially if it's smaller than the J estimate; it's probably the better number, but if you go with the J estimate no one can say you didn't go by the book. Still, if the numbers are very different it would be a good clue to look again at some of the guesses on the J form.

    And the inexpensive homeowner license for HVAC-Calc (jultzya posted the link) looks like a good way you can make sure you have a Manual J estimate done, and done right, without having to count on a contractor to do it.

  4. #17
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    Dec 2003
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    2,347
    If you have called over a dozen companies you are wasting a lot of folks time. If you call anyone else tell them they are the 15th company youve called and see how many come over for a free quote.

  5. #18
    Thank you again for responding.

    The other companies guessed 4-5 tons, and the one with the Manual-J says 3.5 ton.

    I've decided to go with a 10 seer A/C since I don't plan to use it much -- maybe a few days a year.

    jultzya -- thank you for the link. And chapmanf -- do you know anyone who'd like to review my utilities bills (for pay of course). I'm definitely not the person to do it.


  6. #19
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    Nov 2004
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    SW FL
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    6,225

    Talking Testing, Bldg Envelope Upgrades THEN Analysis

    Originally posted by jacquelynn
    Only one of them considers a Manual-J important -- and that company costs $4K more than the others.

    The others all say they go by square footage; that with a house built in 1948, the Manual-J doesn't apply anyway.

    Since the one company that did the calc came up with 3.5 ton -- what's wrong with asking for that size system from another company and getting the job done for less $?
    As a starter, Blower door test would be more significant for a 1948 house.
    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

  7. #20
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    Feb 2003
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    Huntsville,AL
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    ACCA manualJ, or ASHRAE Fundamentals (used by engineers), apply to houses built whenever! I have used both for 50+yr --
    else someone is guessing | assume = ass\u\me

    without it, one does not know how much conditioned air is needed for each room --
    although, one can install large size pipes with dampers, then adjust these by trial & error -- ( probably by the homeowner)

    another aspect is your knowing just where your biggest loads are comming from --

    re: infiltration: one can make reasonable choices for individual items: eg: door fit, window fit, oversize holes around electrical boxes, ...

    harvest rainwater,make SHADE,R75/50/30= roof/wall/floor, use HVAC mastic,caulk all wall seams!

  8. #21
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    Nov 2004
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    SW FL
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    Design Details

    Originally posted by jacquelynn

    The other companies guessed 4-5 tons, and the one with the Manual-J says 3.5 ton.

    I've decided to go with a 10 seer A/C since I don't plan to use it much -- maybe a few days a year.

    And chapmanf -- do you know anyone who'd like to review my utilities bills (for pay of course). I'm definitely not the person to do it.
    If you need definitive, economical A/C design and heating set-ups, let me know off-line.

    Appropriate and complete descriptions are necessary.
    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

  9. #22
    Join Date
    Jul 2004
    Location
    midwest
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    2,868
    Originally posted by cem-bsee
    ACCA manualJ, or ASHRAE Fundamentals (used by engineers), apply to houses built whenever! I have used both for 50+yr --
    else someone is guessing | assume = ass\u\me

    without it, one does not know how much conditioned air is needed for each room --
    although, one can install large size pipes with dampers, then adjust these by trial & error -- ( probably by the homeowner)

    another aspect is your knowing just where your biggest loads are comming from --

    re: infiltration: one can make reasonable choices for individual items: eg: door fit, window fit, oversize holes around electrical boxes, ...

    One never knows how many CFM are required for every room because it constantly changes and is always a guess. As the sun moves around the home and the wind changes direction so does the required CFM's.

    Reasonable guesses is another way to word your above statement which is the point I was making. On a home built in 1948 you are going to be guessing on several of the important inputs on a manual J and its not going to be as accurate as a new construction home were values should be known. Manual J requires correct information to get a correct result.

  10. #23
    Thanks again everyone, I really do appreciate your time and information.

    bigtime: I called the companies to "screen" them by phone -- regarding whether or not they do a Manual-J. I don't want anyone to waste their time coming out to do a bid -- if I already know they don't do the calc.

    I'm sure I'll be back with more questions...thank you again.

  11. #24
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    Feb 2003
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    Huntsville,AL
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    just what makes a home built in 1948 different from one built in 1998 or in 2004 with respect to knowing about the construction?
    I owned one built in 1947 and one built in 1927 and one built in 1956 and one built in 1974 that I now live in -- what am I not understanding?

    BTW, once again, my highest bill this summer for a/c was $55 for 2133sf, trilevel, or 1.54btu/DD/sf @Huntsville AL -- but, had to wear a sweater, since unit set low to make run more to keep RH% lowest.
    harvest rainwater,make SHADE,R75/50/30= roof/wall/floor, use HVAC mastic,caulk all wall seams!

  12. #25
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    Jul 2004
    Location
    midwest
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    Originally posted by cem-bsee
    just what makes a home built in 1948 different from one built in 1998 or in 2004 with respect to knowing about the construction?
    I owned one built in 1947 and one built in 1927 and one built in 1956 and one built in 1974 that I now live in -- what am I not understanding?

    BTW, once again, my highest bill this summer for a/c was $55 for 2133sf, trilevel, or 1.54btu/DD/sf @Huntsville AL -- but, had to wear a sweater, since unit set low to make run more to keep RH% lowest.
    We would hope that any home being built is using the best insulation and windows available at that time and installing it in the latest recommended way. Your right, there may not be any difference in the homes you owned. That would all depend on the contractors that built them and there interest in seeing that no corners are cut.

    The only point I'm trying to make is Manual J is not 100% correct unless you know 100% how the home is built. Most manual J calculations are done by guessing on certain factors when dealing with an existing home and it gets worse the older the home is to come up with the closest actual inputs.

  13. #26
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    Aug 2003
    Location
    Central Kentucky
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    Originally posted by trane


    The only point I'm trying to make is Manual J is not 100% correct unless you know 100% how the home is built. Most manual J calculations are done by guessing on certain factors when dealing with an existing home and it gets worse the older the home is to come up with the closest actual inputs.

    I agree with you on this trane. Manual J has a tremendous amount of fudge factors built in for these reasons. It is better than the square footage method though.

    Another thing to think about is just because a heat loss/gain is done does not automatically mean the system will perform as designed once installed.

    Designs look good on paper but working as designed in the field is completely different.
    Have you set up a Google alert for Carbon Monoxide yet?
    Click here to find out how.

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