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A/C Questions

of Fundamentals is a great place to start. A tad dry as it is practically as much a reference as a textbook.

I would check for used copies or go to directly for the latest.

There are two principal methods used, CLTD or Cooling Load Temperature Difference and RTS or Radiant Time Series.

I was initially trained on the former; the latter was established in the 1990's as an alternate system taking into account the radiant output of people, equipment and the march of heat through walls over time.

There is also a chapter on residental AC, more simplified.

What you will enjoy with this book is that your "language and vocabulary" will be the same as most engineers (I say that like it is a good thing, right?? :)

Everywhere you go, you can be on the same page as the ASHRAE method is probably the most widely accepted.
"If you do not know the answer, say, "I do not know the answer", and you will be correct!"

-Ernie White, my Dad


  • Kevin A Gerrity_2
    Kevin A Gerrity_2 Member Posts: 27
    A/C Design

    When designing A/C for a function room can you have multiply units that can stage on and off to meet the rooms needs? If there are only 10 people in the room or if there are 120 people in the room.
  • Eugene Silberstein
    Eugene Silberstein Member Posts: 1,380
    Most definitely

    You have the opion of using multiple units, or you may select a larger system that has the ability to "unload" for lower system capacity.
  • Brad White
    Brad White Member Posts: 2,393

    make sure that your outside air per person is being met. CO2 monitoring may be worth it on larger staged systems, less economical on multiple smaller units.

    Point being, regardless, code-mandated ventilation rates have to be met. Some function halls rely on higher ventilation rates as make-up for adjacent kitchens too so take a look at the larger picture.

    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
  • Kevin A Gerrity_2
    Kevin A Gerrity_2 Member Posts: 27
    Where to go

    I ask that question because I bid on a job that really bothered me. The room in question was 1500 sq' with two out side walls of glass facing the south west. This room is going to be used for wedding. The heat load called for 3.5 tons empty, but 9 tons when full (120 people). I was afraid that when only 30 people where in the room that it would over shoot the t-stat and the windows would condensate on very humid days. My problem is that my supply house reps are not convinced of sizing properly and I don't know where to find info on sizing A/C systems for this type of rooms. Any help would be appreciated.
  • Brad White
    Brad White Member Posts: 2,393

    That, Kevin, is forever the conundrum, changing loads. Seems you have your load profile in hand.

    Does the 3.5 tons cover the exterior and interior base loads as I assume it does?

    I have an open concern about your 9-ton load as it relates to ventilation. Unless you are relying on operable windows for ventilation, (doubt it) you have to introduce it mechanically and condition it via the equipment.

    For group assembly, ASHRAE recommends 15 cfm of outside air per person but allows you to cut that proportionately by 50% if the assembly period is three hours or less and allows one such assembly per day. The theory here is that pre and post occupancy will allow flushing and "storage" of outside air in anticipation of a shorter term occupancy. By allowing one period of occupancy per day means that this would not be used in movie theaters for example with back to back performances. Whew.

    Your 120 people would require 1,800 cfm which alone is worth 10 tons to condtion it. If the 3-hour rule can be invoked (I would see if that would apply), cut that in half.

    Some weddings and receptions do indeed last longer, sometimes longer than the actual marriage so be warned... :)


    If the base load is 3.5 tons, the 120 people add another 5.5 tons for their sensible and latent heat (seems about right especially if some are dancing) that will get you to 9.0 tons. On top of that you will need 10 tons and up to 1,800 cfm of outside air to make the worst case work. Call it 18-20 tons.

    I would install at least one 3.5-ton unit as a base load unit and stage up the rest in 3.5 to 5-ton increments.

    The issue you will find with packaged equipment is that their ability to introduce more than 20% OA is limited. A more specialized unit such as by Aaon may be a good fit and can allow hot gas reheat to alleviate your condensation/over-cooling issues.
    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"

    -Ernie White, my Dad
  • mtfallsmikey
    mtfallsmikey Member Posts: 765
    What is currently being used

    To calculate BTU load for people? I remember 800 BTU/person at rest being a standard (???)back in the day.
  • Brad White
    Brad White Member Posts: 2,393
    Depends on their activity level.

    Normally seated doing office work a person emits about 245 BTUH sensible heat and about 155 BTUH latent (breathing mostly). Thus the total heat of the person is about 400 in an office.

    At the other end of the dozen-plus categories, the extreme is a gymnasium, 710 BTUH sensible and 1,090 BTUH latent (note the switch in latent over sensible now). Total heat of 1,800 BTUH.

    Remember these are averages of men and women. Women and children would tend toward lower values, men of course higher with larger body mass and surface area.

    The larger point is, you have to know the activity level in addition to the number of people. Sometimes you need to select equipment with higher latent load capacity (lower sensible heat ratio) in order to dehumdify.
    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"

    -Ernie White, my Dad
  • Kevin A Gerrity_2
    Kevin A Gerrity_2 Member Posts: 27

    Are there any books that I can read that can help explain proper A/C sizing? I mainly reline on my soft ware, some articals in trade magazines and my supply house sales rep. I have been installing A/C systems for about 5 years now, mostly residentail and rely on my supply house rep (They're real helpful but they are old school). BUT since I have started reading about properly sizing heating systems I felt I should learn about proper A/C sizing. Just so you know I did not win the bid process because I could not explain my A/C design to the customer. So now I want to learn more to help me explain my designs.
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