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  1. #1
    Join Date
    May 2012
    Posts
    32

    BTU's per square feet

    I know theres alot to consider (light,windows, ect) but in general what would u use?

  2. #2
    Join Date
    Jul 2011
    Location
    East coast USA
    Posts
    1,000
    500 sq.ft. per ton

    2000 sqft house 4 ton unit. In most cases after you do a study you will fall somewhere more or less close to that.

    Did i just hear an ME curse?

  3. #3
    Join Date
    Jun 2006
    Location
    Richmond, VA
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    2,989
    Quote Originally Posted by dlove View Post
    500 sq.ft. per ton

    2000 sqft house 4 ton unit. In most cases after you do a study you will fall somewhere more or less close to that.

    Did i just hear an ME curse?
    Most are nowhere near that. Manual J is the only way to go.

    I live in a 2000sqft house....my 3 ton is too big

  4. #4
    Join Date
    Feb 2004
    Location
    Ontario
    Posts
    700
    I'm in a 2400 sq foot split level with a 35 yr old 8 SEER unit and I'd say it's sized just right.

    By your R of T I'd have a 5 T unit in here which would blow my ductwork to bits.
    Question authority!

  5. #5
    Join Date
    Jul 2007
    Location
    Virginia
    Posts
    5,913
    Quote Originally Posted by beshvac View Post
    Most are nowhere near that. Manual J is the only way to go.

    I live in a 2000sqft house....my 3 ton is too big


    500 sqft/ton is so old schooL.

    I have 3500 sqft. and my 4 tons is just right.

    I can't imagine how humid my place would be with 6.5 tons (or the nearest size - 6 tons ).
    Instead of learning the tricks of the trade, learn the trade.

  6. #6
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,586
    You can take the same house and put it on 3 different lots facing different directions and you'll need 3 different sized A/Cs. Take the same house and move it north or south and you'll change the load some more. A rule of thumb gets you nowhere accurately. Better off standing across the street, holding you hand up and the number of fingers it takes to cover the house is the number of tons it needs

  7. #7
    Join Date
    May 2012
    Posts
    10

    rule of thumb

    my boss lives by rule of thumb, thanks all you more "seasoned" techs for helping us newer ones know the right way. hacks come from bad teachers

  8. #8
    Join Date
    Oct 2009
    Location
    new jersey
    Posts
    256
    We talk about this all the time on change outs where the new unit is not the same size.

  9. #9
    Join Date
    Jul 2011
    Location
    East coast USA
    Posts
    1,000
    Quote Originally Posted by rundawg View Post


    500 sqft/ton is so old schooL.

    I have 3500 sqft. and my 4 tons is just right.

    I can't imagine how humid my place would be with 6.5 tons (or the nearest size - 6 tons ).
    true old school but he said rule of Thumb

    If someone, said hey i got a 2000sqft house, sits on a lot no shade, bad windows etc. built 1980.. I would tell them 4 ton for budget purposes. Of course you will need to do a study. But that was not his question.

    If you told someone a budget # to install a 4 ton unit. regardless if they needed a 3 ton or 4 ton how much cost difference is there? not a whole heck of a lot. If they can't afford the 4 ton they sure as heck cant pay the 3 ton cost. Unless you pay a hack !


    yes I'm old school. But people will waste your time, ask for quotes when they cant afford to install anything. When i give a rule of thumb based on their description and they don't cry at the cost. I then say would say "but lets do a study and see what you really need. could be more or could be less.

    Everyone here has good points, but a rule of thumb has it place and will never go away. Sometimes that's called experience.

    the next time you find yourself asking someone "hey what do you think that would cost" you will get a rule of thumb answer and you all done that!

    when someone comes up to you with a quote to install an AC system you may say , yeah sounds right, or wow sounds high! you just gave your rule of thumb thought...

  10. #10
    While a R of T is simple, it more often than not results in an oversized system which costs the home owner more money to operate and may not control the humidity. Not only does the orientation of the house make a difference, the shape of the house will drive the heat gain. A square house is more efficient than a long rancher. It takes less than 30 minutes to do a simple manual J.

  11. #11
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,586
    Quote Originally Posted by dlove View Post

    If someone, said hey i got a 2000sqft house, sits on a lot no shade, bad windows etc. built 1980.. I would tell them 4 ton for budget purposes. Of course you will need to do a study. But that was not his question.

    If you told someone a budget # to install a 4 ton unit. regardless if they needed a 3 ton or 4 ton how much cost difference is there? not a whole heck of a lot. If they can't afford the 4 ton they sure as heck cant pay the 3 ton cost. Unless you pay a hack !
    Cousin in KY just sent me a drawing of her 2000 sq ft ranch with no shade but decent windows. Using HVAC-Calc, I spent a few minutes and crunched it. Using R11 walls and ceiling, which I told her we need to upgrade the ceiling, my calcs greatly different from what is installed.

    Living room, family room, dining room, kitchen have a 100K 80 and 2.5 ton A/C. Calcs say with this insulation 60K furnace and 1.5 ton A/C

    3 bedrooms and 2 baths, about 700 sq ft with no east or west glass and no doors, have 80K 60% and 2 ton A/C. Calcs say 40K furnace and 1 ton A/C.

    So for this 2000 sq ft, they have 180K heat and 4.5 tons A/C where they need less than 100K heat and 3 tons cooling. With insulation added to ceiling, the whole house is barely over 1.5 tons A/C and they have 4.5 tons!

    Oversized equipment isn't about affording, it's about comfort and efficiency. Short cycling equipment isn't efficient. Oversized A/Cs don't dehumidify and with today's stuff, that's even more of a problem. The 70s relics she has had a low sensible heat ratio and got to a cold coil faster than the high efficiency stuff of today.

  12. #12
    Join Date
    Apr 2011
    Location
    Coastal Maine
    Posts
    883
    He is another old school opinion on rule of thumb.

    I use rule of thumb as a troubleshooting tool. I teach my techs to do the same.

    If they walk into a building and they see a boiler that is short cycling and they suspect oversizing. They know that up here, you can expect 45, 40, 30, or 25 btu/sq ft depending on age of the structure, usage, and insulation. It isn't what you size equipment by, but it gives you a starting point that you can derive on the fly in the field.

    Same with HVAC. The ROT's give you a starting point. If you walk into a Dental Clinic and see a 3 ton unit trying to service 2,500 sq ft with all their equipment and patients, then you know there might be a problem. Just an example.

    It is just that. A Rule of Thumb. Something to start with. If used as they were intended to be used, then you should be ok.

  13. #13
    Join Date
    Jul 2007
    Location
    Virginia
    Posts
    5,913
    Quote Originally Posted by dlove View Post
    true old school but he said rule of Thumb

    If someone, said hey i got a 2000sqft house, sits on a lot no shade, bad windows etc. built 1980.. I would tell them 4 ton for budget purposes. Of course you will need to do a study. But that was not his question.
    No one will argue that rule of thumbs aren't used daily in this business to give people estimates, or ideas of what would work in their home.

    The point I was trying to make, which I failed to do by saying "OLD SCHOOL", was that a statement of "500 sqft/ton" is just as much of a WAG, as is "you have a 3 ton now, lets put in another 3 ton unit", or "from my vast experience we usually install this size unit for this type of home".

    I just feel that giving the OP any answer other then "do a manual J", just encourages these Old School "rules of thumb" for sizing equipment.
    Instead of learning the tricks of the trade, learn the trade.

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