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Cooling load sizing

I have to size a house for a/c. What is available to help me correctly size each room, and figure our proper register locations etc., other than just going by the square footage of the house. Is there a simple formula/chart/cd/site available to help me accomplish this.
Thanks in advance,


  • Brad White
    Brad White Member Posts: 2,393
    Oh God.... Help this poor soul....

    Nothing simple about it and I would rather not have you wing it. ACCA Manual J worksheets may be available but using these or anything requires some study. (This is not to overcomplicate things, just how things can be.)

    If you took eight identical houses and rotated them to different compass points, each would likely have different cooling needs due to solar peaks. Solar gains and time lags affect cooling loads far more than insulation and infiltration.

    Check out WrightSoft or Elite Software (RHVAC) for some programs. The lower cost programs limit the number of zones. If you are just doing your own system or just one, find a friendly contractor, designer or engineer and pay them to calculate it for you. If this is part of career building, buy the software. Shop around I am sure that there are others. I use Elite CHVAC and Carriers HAP for commercial loads.

    My $0.02

    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"

    -Ernie White, my Dad
  • Ken D.
    Ken D. Member Posts: 836

    Carrier's REZCALC is a very good, easy computer program. Works like a charm.
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928

    I've used HVAC-Calc for heat gain calculations to very good success. Both for systems I've designed, specified and installed as well as systems designed for others.

    By all appearance it compensates for orientation--change the direction of the room and the heat gain changes as well.

    When it comes to forced air it also appears to be designed with A/C as "priority" with regards to duct sizing. Suggested return sizes are larger than suggested supplies.

    While it does give suggested supply/return sizes, it does not however design a ductwork system. If you keep the system compact with a plenum (supply AND return) system that sizes down proportionally as air is taken off for branches and keep all branches close to the same length you won't have any problem. Relatively long branches however require special consideration as you have to size them for equivalent airflow regardless of the length of the branch.

    Coming from an area with an extreme cooling load I can tell you the worst problems and things to avoid. Interesting, most have to do with returns...

    1) Inadequate (both in size and number) of returns is by far the worst problem. EVERY logical space (e.g. a room not open to another by a PERMANENT opening at least 4' side) should have a dedicated return!

    2) Even when the return duct (or chase) is sufficient in size, the return grille is often too small. This results in obnoxious harmonic noises. The typical "round to rectangular" boot usually find the rectangle too small.

    3) Round (usually used for ceiling) supply registers have FAR less resitance to flow than most floor or wall registers--at least they have a built-in and effective damper...

    4) It's nearly impossible to serve a 2-floor home via a single unit in the basement. You can however serve a two-floor home with a single unit in the attic by dumping a HUGE amount of air AND returning a HUGE amount of air from an OPEN stairhall. When doing this the kitchen is almost certain to be under-supplied unless it's VERY open to the stairhall. The kitchen should have its own supply AND return or use a "window shaker".


  • Eugene Silberstein
    Eugene Silberstein Member Posts: 1,380

    Personally, I think it is well worth the time to learn and use Manual J for your heating and cooling calculations.

    At the college, I teach my students to use Manuals J, N and D. Although they gripe a little, okay a lot, they ultimately thank me, especially since I make them do all of the calculations MANUALLY. This way they learn the ins and outs of the process.
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928

    I remember doing my best to compute the heat loss of my upstairs bedroom--VERY complex with knee walls, shed dormer and doghouse dormer--using Audel's books and without a calculator. At least I knew the construction as I'd helped dad with insulation--I could fit in smaller holes than he as I was in elementary school.

    Why? It was NEVER comfortable despite "central heat and air" being added. Only time it was truly nice was when you could open the windows in favorable weather and get cross ventilation.

    Have also sized three central A/C systems using a sizing chart for window units that used to be published in Consumer Reports on a room-by-room basis and then making everything proportional INCLUDING RETURNS! Did though have benefit of a calculator.

    Both experiences gave me a decent understanding of the "meat" and an appreciation of the complexities (AND CONCESSIONS) involved. Fully computerized calculations are wonderful but the garbage-in = garbage-out rule certainly applies and then it still seems prudent to temper the results if they seem contrary to your experience.

    'Tis both an art and a craft...
  • Barrington White
    Barrington White Member Posts: 37
    Reliable Source

    I suggest you listen to Professor Silbersteins advice, it not just him talking its experience talking. I have been listening to him for the last two and a half years. In that time span I have achieved nothing less than rapid success.
  • Brad White
    Brad White Member Posts: 2,393
    Eugene nailed it

    Doing the calculations MANUALLY is the right way to learn the hows and whys of heat loss and gain. I say this in the spirit of learning calculations as part of your career skill set.

    If you are just doing your own house or it is just a few small jobs, the softwares is fine as you may never need the background. But I would not use the software as a career tool without the manual background.
    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"

    -Ernie White, my Dad
    [email protected] Member Posts: 139
    Understanding what you are doing!

    Hi all,

    I attended a class on sizing refrigeration systems. The guy who gave it was "old school" in the best sense. He works for URI in the Kansas City area.

    Told the story how he was waiting for a load calc to be done and called the factory about why it was taking so long? The reply? The computer was down and they did not know how to do it manually!! He laughed and said that is the only way they did it back then, before computers.

    An aside, but still relative to the thread. I told my boss that the school was more about sizing than troubleshooting. I and the other service tech went because he was new to refrigeration. The boss was disappointed, thinking the class a waste of time! I informed him that to be able to troubleshoot something you really ought to know how something was designed and how it works together as a complete system!! But then again I could be wrong?!

    Don in Mo
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