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over sizing AC symptoms

Scenario: one fairly large office room, open area about 70' x 40'. With 10-15 people on average. Using the ~400 btu/hr per person figure less than 1 ton of cooling required to deal with just people. I also estimate less than 1/2 ton of cooling for computers/laptops/tv's and one full size refrigerator. And heat load from sun during the day is minimal.

What would happen if this one room, occupied only from 7a-6p and cooled by a single unit, is done with a unit rated at say 30 tons ? I figure 5 tons is needed at most under worst heat load conditions.

Comments

  • ZmanZman Member Posts: 5,330
    What is your climate?
    Has a heatloss be done?
    Are you bringing in outside air? If so, how much?
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • pecmsgpecmsg Member Posts: 954
    Zman said:

    What is your climate?
    Has a heatloss be done?
    Are you bringing in outside air? If so, how much?

    If not, why not?
  • ronron Member Posts: 140
    edited July 25
    Asking the general question of what to expect to happen if massively over sizing an air conditioning unit for a simple single large room.

    Climate: southern ct, april-october, worst is 90° - 100° F this time of year, high humidity most of all throughout summer

    heatloss done: probably, wrong

    outside air: i think so
  • Jamie HallJamie Hall Member Posts: 11,417
    Just a few generalities... no outside air? Indoor air quality will be somewhere between bad to unhealthy to atrocious.

    Seriously oversized? Humidity high, unit will cycle constantly, temperature distribution around space is likely to be wildly uneven.

    Bottom line. Ooooops.
    Br. Jamie, osb

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • pecmsgpecmsg Member Posts: 954
    ron said:

    Scenario: one fairly large office room, open area about 70' x 40'. With 10-15 people on average. Using the ~400 btu/hr per person figure less than 1 ton of cooling required to deal with just people. I also estimate less than 1/2 ton of cooling for computers/laptops/tv's and one full size refrigerator. And heat load from sun during the day is minimal.

    What would happen if this one room, occupied only from 7a-6p and cooled by a single unit, is done with a unit rated at say 30 tons ? I figure 5 tons is needed at most under worst heat load conditions.

    This requires at a minimum 10% outside air so with your guestimate a minimum of 200 CFM during occupied times.
  • pecmsgpecmsg Member Posts: 954
    As much as I dislike most Engineers you will need one for this project.

    That or read the Honeywell Gray Manual.....all 850 pages!
  • EBEBRATT-EdEBEBRATT-Ed Member Posts: 6,101
    Is it really a 30 ton unit dedicated to just that space??

    Office buildings are usually quick checked 250 sq ft /ton which would be a 11 ton unit. This is just a check # ...no substitute for an accurate load calculation.

    a 30 ton unit will cool the space but will not control humidity well, and will short cycle

    A 30 ton unit in this day and age (depending on the unit age will probably have multiple compressors (at least 2) so you could disable some compressors to get better operation. Not the right fix but better
  • ronron Member Posts: 140
    edited July 30
    mistake above, yes there's gotta be outside air. how much i don't know.
  • ratioratio Member Posts: 2,133
    You can download the Grey Manual here, if you want to take a peek through it.
  • mikeg2015mikeg2015 Member Posts: 1,041
    The often forgotten issue.... To support 30 tons you need at minimum 10,000 CFM. That’s a lot of airflow for 2800 square. It will be very drafty. the temp swing will be wide unless you short cycle it. You’ll have minimal humidity removal because it will rarely run long enough to even wet the coil.

    Specifically, at peak load or 5 tons, it would run only 10 minutes every hour. So at 4 cycles per hour, thats a 2.5 minute run time, then 12.5 minutes off. Worse, at 50% load, the 5 ton would run for 10 minutes on/off. This unit would run 1 minute on, 14 minutes off.

    This is also why chilled water is commonly used in commercial systems and why commercial system often have 2 refrigerant circuits, you get got latent capacity at part load as one circuit is fully loaded at 50% capacity. also means that if one system has a leak, you still have some cooling
  • ratioratio Member Posts: 2,133
    Constant volume, or would it be a VAV system? You might be able to almost get to the turndown you need with a VAV system, they should have hot-gas bypass on the lead stage. IIRC the last VAV system I set up was a 20 ton unit with about 4:1 turndown if you followed the mfgr's airflow tables.
  • HVACNUTHVACNUT Member Posts: 2,628
    Theres a typo somewhere.
    70x40 open office space. I think 30 tons was supposed to be 3 tons.
    Is this a replacement system or new building, new equipment?
    Have you given thought to a dedicated IT room with a ductless system?
    A manual J needs to be done.
  • ronron Member Posts: 140
    edited July 30
    I sh1t u not, no typo. one office room, 70 feet x 40 feet approx
    my finger's freeze during the day if I sit in the room

  • HVACNUTHVACNUT Member Posts: 2,628
    That's utter ridiculousness.
  • ratioratio Member Posts: 2,133
    Maybe you can get enough electric reheat in the space...
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