Results 1 to 8 of 8
  1. #1
    Join Date
    May 2001
    Location
    Abilene,texas
    Posts
    246

    Confused

    Ok well after a fairly good class on heat load calculations and getting my manual j8 AE i come to this simple conclusion...... it is a numbers thing , lol ok look i have a 1200 sqft house when i do a load calc it says i need a 1.5 ton unit in phx az.where we have a avg day of 110 deg now lets look at this for just a sec ok that unit will produce approx.600-750 cfm of air ok cool and roughly 18k btus of cooling this is a 3bd 2 ba house with a 160sqft living room now if( I F ) my duct work is sized right and if... my home is well shaded and well insulated i should get away with it ...... lol right I know for a fact it has been tried using the load calc via man.j8 and well nope not realistic those units had to come out after 6 weeks and we went back to a v-spd 3tor a reg 13 seer 3t and no other complaints , 1 and most common complaint i cant feel the air out of the ducts( perception ) 2nd it takes all day to drop 2 deg(6-8 hrs) 3rd most common wont go bellow 75deg.... so it may be doing the job but the customer says (based on perception that its not working) they want it replaced.now i know i belive that in some areas of this great country we will have a system that will work based on the load calcbut in arizona you have to be very sure of what your doing and what your calculating or you will be back.
    Its not what you know its what you can prove.

  2. #2
    Join Date
    Feb 2006
    Location
    MN
    Posts
    27

    Wink

    in MN using a load calc, ( man j ), the calcs. come back pretty close to what you should be installing. At least in my case, installed quite a few with 0 complaints ( yet ). my last house was for a bulider ( his personal house ) approx. 4200fsf, 2 zone, and i installed a 80,000 VS with 3 ton 2 stage AC, he almost would not let me install based on prior experiences he assumed i was way undersized. after all said and done his highest gas bill $200, needless to say i dont even submitt a bid anymore he just faxes me a print and a start date. LOAD CALC DOES WORK TRUST THE NUMBERS ( most of the time )

  3. #3
    Join Date
    Aug 2002
    Posts
    1,647
    acdude,

    You do know that the nominal size of the units are based on 80 degree indoor air and I think 95 degree outdoor air..
    So the unit must be unit must be picked based on Manual J and utilizing manual S which is system selection.. Also, if you are in a dry arrid area,, humidity control may not be as necessary to maintain and you may need more sensible capacity than latent.. You must pick according to your areas needs.. I don't know your area... Each area has its design criteria..

    Check out Manual S.. Your needs are probably in there..

    Goodluck
    J

  4. #4
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,928
    look in your tech guide book for the equipment you install.
    It may have the derates for your outdoor temp, and for your indoor.

    If not then as stated above, you need to use manual S, for equipment selection.

    That 1.5 ton unit may only be removing 11,000 btu's total at 110° outdoor and 75° indoor.



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  5. #5
    Join Date
    Dec 2004
    Location
    Derby City
    Posts
    3,963
    I guess if the same question is asked often enough, then the answer the person posing the question is looking for, will be given, even if it's wrong! There is absolutely, positively no reason to attempt to 're-invent' manual J. Design conditions when factored into the load calculations, whether 110 summer with 70 degree indoor, i.e. (40 degree delta T) or 85 degree summer with 75 degree indoor (10 degree delta T) you follow the friggin program!

    I can tell you that I have never had an issue that could not be resolved with system operation and capacity that I had performed a manual J heat loss - heat gain on, that could not be easily resolved, usually by way of simple explanation or perhaps some balancing. By performing the manual J, I had a definitive basis for making my recommendations. Personally, since a load calculation should be performed on each and every job, AND a permit should be acquired on each and every job (by a licensed contractor) I hope the day comes sooner than later, that a load calculation has to be turned in with the permit application, and until the load calculation appears, no permit is issued.

    It would not be the responsibility of the inspector to check the accuracy, only to verify it had been performed. It would remain the responsibility of the contractor to have performed an accurate load calc. Obviously, if I do not enter correct information, I won't arrive at the correct recommendations.

    Using variable speed equipment also has a major impact on comfort and air distribution, but I will always stand by the fact that a load is a load is a load. If your heat loss is 45,000 btu, and your heat gain is 24,000 btu, whatever you do with equipment will not alter that! The load is what the load is.

    Obviously, if your design conditions are different (which it stands to reason they will be across the country) then when you enter the correct design conditions, then you will achieve the correct sizing recommendations.

    The fact that equipment cycles for longer periods of time is going to become even more commonplace and we need to advise the public accordingly. Trane is introducing a 3-stage gas furnace, and I understand there is a 4-stage on the horizon. Can you say 'continuous heating?' All the best, John.

    p.s. I know we beat this load calc thing into the ground, but why do people insist on trying to tweak a program that works!?
    Everyone has a purpose in life..........even if it's to be a bad example.

    Seek first to understand, before seeking to be understood.

  6. #6
    Join Date
    Nov 2005
    Location
    Kansas
    Posts
    4,413
    I do know from one of the software companies that there where in the early version of Manual J v8 inaccuracies compared to v7 but those should be resolved. V8 is supposed to be more accurate than v7. But as stated above, with outdoor conditions of 110F, you will have to devalue capacity of the equipment as stated in Manual S. It’s not the load calculation that is wrong; it’s the size selection of the equipment corresponding to btu.

  7. #7
    Join Date
    Mar 2004
    Location
    Vancouver, BC
    Posts
    125

    The other side of the coin is...

    Garbage in=garbage out. Any kind of calculation is only as accurate as the input numbers. The more accurate the input numbers are, the more reliable and accurate the output numbers will be. That being said: how many people actually take the time to factor in thermal bridging adjustments to wall and roof thermal resistance values? Do you use the center of glass performance from the glass manufacturer, or do you use the "overall" thermal and solar performance which includes framing? How much effect of thermal mass and thermal storage is taken into account?Infiltration numbers are a wild card- how many HVAC designers actually police the GC when he builds the walls, roof and windows to make sure what's being built matches the plans and the input numbers? I can't tell you how many times an HVAC designer has been taken to task because the HVAC system "doesn't work", and when the building is reviewed you find changed insulation and glass, lack of sealing of the envelope, gaps in the insulation, etc. etc.

  8. #8
    Join Date
    Dec 2004
    Location
    Derby City
    Posts
    3,963
    gmcd: I have found myself in the situation you describe toward the end of your post. problems associated with 'changes' that took place during constuction that were never related to, or factored into the load calcs. That is why records are extremely important and anymore I date stamp any proposals, loads, layouts, etc. to verify when this was performed and based on the criteria at the time. If someone changes the rules in the middle of the game, then it may not be your responsibility. However, in today's world, unfortunately, Documentation is next to Godliness!

    I also agree, we can over-engineer anything! When I referred to not having sizing problems on projects that i had in fact done a load calc, that is the God's honest truth. At the same time, if I am calculating windows, and some of the windows have a crack in them, I don't make any changes. I also never figure shaded glass (hasn't been a problem) and here is the reason why: When the tornado blew through here back in the early '70's, there was a tremendous loss of large mature shade trees. When the homes were re-built, and the hvac systems replaced with same size equipment, now they couldn't keep up! No one could figure it out until they realized they didn't have the benefit of the shade anymore. Realizing that trees may be here today, but gone tomorrow, made me re-think the use of shaded windows versus full sun. I am probably sounding like I am contradicting myself, but I have had no problems in following this process. Al the best, IRISH! (Hey, it's St. Pat's Day!)
    Everyone has a purpose in life..........even if it's to be a bad example.

    Seek first to understand, before seeking to be understood.

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