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  1. #1
    Join Date
    Jun 2010
    Location
    Indianapolis
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    10

    What info do I need to provide contractor to do a load calc?

    I've been doing some reading on this site and others and the consensus is that doing a Manual J load calc is the best thing to do to properly size the equipment.

    What information do I need to give to a contractor so he can do a load calc?

    I'm just in the initial stages of talking to contractors, and had the first one out last night. I asked how he figured out how big of a system I need and he said he went off of existing and without me asking (he probably knew where I Was going with it) he mentioned that an engineered load analysis could be done, but we would need to give him R numbers of what's in the attic, our windows, our roof, etc. Are these things that can be measured, or would I need to find specs? Is this something I need to do or the contractors should be able to do?

    Any help would greatly be appreciated.

  2. #2
    Join Date
    Jan 2010
    Location
    Athens GA
    Posts
    1,234
    Most of what he needs to know he has to messure.
    UIt would help if he knew for sure what insulation you had in the waslls and cieling.
    He should be able to tell what kind of windows you have when he messures each and every one of them.If he is not the Daniel Boone type be prepared to tell him where n-s,,,e-w ,,,,is.He should know about kitchen and bathroom fans if any or what type of fireplace you might have.It might save a little time if you knew what hieghts your cieling were if you have any abnormally high ones.
    Off the top of my head that is a good list.He will get everything else as he measures each room and each window and each door.

  3. #3
    Join Date
    Jun 2006
    Location
    West Palm Beach, FL
    Posts
    50
    Heck I had one guy tell me he needs the house blue-prints! Yup, really.

  4. #4
    Join Date
    May 2008
    Location
    Orlando,FL
    Posts
    377
    Quote Originally Posted by southeflorida View Post
    Heck I had one guy tell me he needs the house blue-prints! Yup, really.
    LOL sorry i see that your in west palm. So id first ask you.
    How big is the house? lol

    Im not gonna come measure some 8000+ sqft house if you have blue prints.

    Also @ OP how big is your house? And how old is it?

    Plus I also like some good gut instinct. I had one who argued with me that the load calc said this and that i was oversize. Low and behold that summer the sucker was runnin 24 hours a day just to stay at 78-79 during the day and just hitting 72 at night right before the sun was about to come up again. Actually i over exaggerated. It probably only ran 22 hours a day. lol

    Im saying dude forget your load calc. I walked into house. My mind is the greatest tool available to you. =)


    IDK i just worked for a company that was a $200 million company and i noticed on many occasions that their load calcs came to mean nothing. There were many instances where they were incorrectly installed.

    You should be focusing more on correct installation and duct size. Im gonna go off the old unit because you cant just increase the ahu without increasing its duct.

  5. #5
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    67,907
    Quote Originally Posted by southeflorida View Post
    Heck I had one guy tell me he needs the house blue-prints! Yup, really.

    Its easier to do it off the blue prints, then by measuring everything yourself.
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  6. #6
    Join Date
    Jun 2009
    Location
    Albuquerque NM
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    You can estimate the R values using factors such as the age of the house, the builder, etc.

    I had two contractors do a Manual J and I did it myself. We all got pretty-much the same results, at least close enough that we all would select the same size equipment.

    Of course our sizing didn't agree with the guy who only asked "how much square foot you got here?", then said "yup, you need 4 tons of cooling".

  7. #7
    Join Date
    Jun 2010
    Location
    Indianapolis
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    10
    Quote Originally Posted by Fatstogey View Post

    Also @ OP how big is your house? And how old is it?

    I have an approx. two story 1970 sf house built in 1989. Existing units are the original.

    Would a load calc likely have been done when the house was built? If so, and the right size equipment were originally installed, would a similar size unit be appropriate if no upgrades or modifications have been done to the home that would change anything. FWIW, the existing units also seem to have done an adequate job at heating/cooling in the 10 years I've lived in this home.

    I'd like a load calc done, but I get a feeling it's going to be difficult to find someone that is willing to do one, based on questions I've asked other locals.

  8. #8
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    One might have been done.
    Or they may have just used the ame one that they used for 100 other homes. To save themselves time.
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  9. #9
    Join Date
    May 2008
    Location
    Orlando,FL
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    Quote Originally Posted by garya505 View Post
    You can estimate the R values using factors such as the age of the house, the builder, etc.

    I had two contractors do a Manual J and I did it myself. We all got pretty-much the same results, at least close enough that we all would select the same size equipment.

    Of course our sizing didn't agree with the guy who only asked "how much square foot you got here?", then said "yup, you need 4 tons of cooling".
    Ive had some do it and come up differnt too though.

    Plus IMHO if you cant walk into a place and know what it needs then you havent been paying attention over the years. Ill say it. Forget a damn load calc. Ive done at least 500 rough ins, and for a company that did proper load calcs, and i changed things all the time. And supers would always come to me "this other model isnt right." Id alwasy tell them "after this one is sold check back with them, i bet its fine."

    Plus they did all these load calcs. Which dont mean much if the duct isnt properly installed. So you paid for a load calc that didnt evne matter.


    Also on a change out i mean what are you guys doing? Selling new duct too? Most people shudder at the price of the unit. I mean im all for doing things correctly but my expertise is what your paying for. And my expertise cant be measured with a formula
    lol write that down

  10. #10
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    Quote Originally Posted by Fatstogey View Post
    Ive had some do it and come up differnt too though.

    Plus IMHO if you cant walk into a place and know what it needs then you havent been paying attention over the years. Ill say it. Forget a damn load calc. Ive done at least 500 rough ins, and for a company that did proper load calcs, and i changed things all the time. And supers would always come to me "this other model isnt right." Id alwasy tell them "after this one is sold check back with them, i bet its fine."

    Plus they did all these load calcs. Which dont mean much if the duct isnt properly installed. So you paid for a load calc that didnt evne matter.


    Also on a change out i mean what are you guys doing? Selling new duct too? Most people shudder at the price of the unit. I mean im all for doing things correctly but my expertise is what your paying for. And my expertise cant be measured with a formula
    lol write that down
    Lots of guys walki n a place and know what size the house needs without measuring or doing a load calc.

    thats why there are so many over sized systems installed.
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  11. #11
    Join Date
    Jun 2006
    Location
    West Palm Beach, FL
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    Quote Originally Posted by beenthere View Post
    Its easier to do it off the blue prints, then by measuring everything yourself.
    I certainly understand that. I didn't get on him about it...just thought it was funny he'd ask for blue prints on a house he knew is 30yrs old. Maybe some folks DO have blue prints that old tho...I dunno. He did say he routinely does Manual J's on all new construction from the blue prints. Just doesn't do them on replacement systems routinely.

    Fatstogey: This isn't a McMansion...2600 total and only about 1200-1500 for the system I'm replacing. Lots of windows but only 2 sizes and their all the same otherwise.

    FYI not one company I've contacted does Manual J's on replacements. No that's not entirely true, one company would do it for $$$, but only after I signed a contract with them for the install.

    There was an addition put on the kitchen 10 yrs or so after original construction, maybe 12x12 with windows covering 3 sides. I would think this would have an impact on unit size. Also the condenser is 2001 and to the best of my knowledge the AH is original (lablels unreadable now and I don't have my previous inspection reports with me here).

    I don't know. I have read here and everywhere else for that matter that it's important to have a Man J. Then I can't find one company that will do one except the one who wants a mortgage payment AND to sign with him first. I've had 3 sizes quoted from different companies, a 2, 2.5 and a 3. Can you see why I'm frustrated?

  12. #12
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    67,907
    Contact dan sw fl. He is a member here.

    If you give him your homes info. he can do one for you. For a fee.

    Or, you can do your own. For 49 bucks. http://hvaccomputer.com/talkref.asp
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  13. #13
    Join Date
    Jun 2009
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    Albuquerque NM
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    Quote Originally Posted by southeflorida View Post
    I certainly understand that. I didn't get on him about it...just thought it was funny he'd ask for blue prints on a house he knew is 30yrs old. Maybe some folks DO have blue prints that old tho...I dunno. He did say he routinely does Manual J's on all new construction from the blue prints. Just doesn't do them on replacement systems routinely.

    Fatstogey: This isn't a McMansion...2600 total and only about 1200-1500 for the system I'm replacing. Lots of windows but only 2 sizes and their all the same otherwise.

    FYI not one company I've contacted does Manual J's on replacements. No that's not entirely true, one company would do it for $$$, but only after I signed a contract with them for the install.

    There was an addition put on the kitchen 10 yrs or so after original construction, maybe 12x12 with windows covering 3 sides. I would think this would have an impact on unit size. Also the condenser is 2001 and to the best of my knowledge the AH is original (lablels unreadable now and I don't have my previous inspection reports with me here).

    I don't know. I have read here and everywhere else for that matter that it's important to have a Man J. Then I can't find one company that will do one except the one who wants a mortgage payment AND to sign with him first. I've had 3 sizes quoted from different companies, a 2, 2.5 and a 3. Can you see why I'm frustrated?
    I've been where you're at. Since none of your 3 proposed sizes was calculated, I would suggest using the hvac-calc software to do it yourself, if you have the time and patience. Or, pay someone to do it.

    If you oversize an AC in Florida, you're going to be uncomfortable. For best comfort, and to mitigate the effects of a slight oversize, you might consider a 2-stage unit if you aren't already.

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