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Air-conditioners and BTUs

  1. Mar 25, 2007 #1
    In my room there's a small aircon, it is 12000BTU. I know there are some bigger ones which can be 18000 or 24000 BTU.
    BTU is a unit to measure heat value, which is similar to calorie or joule. So why do they use a heat value unit to show the power of an aircon, but not a power unit such as kW or horsepower.
    Thanks for reading.
  2. jcsd
  3. Mar 25, 2007 #2
    Well, the writing on the airco is wrong, and there is nothing to discuss about that.
    If the correct reading is 12000 btu/h, it would be equivalent to 3517 W.
    This indicates the correct units are probably "but/h".
    This kind of mistakes is very common, many of them can be found in newspapers or on TV everyday.
    People have the biggest problems with electrical energy units: kWh is an abstract concept for most people.
    But physicist can have problem with units too sometimes, for example the spectral density of a random signal in W/Hz a real obstacle for some, not to mention the corresponding field intensity!

    [leave the political commentary out, lalbatros. -Russ]
    Last edited by a moderator: Mar 25, 2007
  4. Mar 25, 2007 #3
    Now that's more than enough.
  5. Mar 25, 2007 #4


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    Science Advisor

    That is correct. Simply stating BTU for the units is incorrect. It is analagous to a lot of people referring to the units of pressure as "pounds." Most people just don't know any better and when you try to point it out and that it does matter, you'll get told to get lost pretty quickly.

    In most applications, A/C units are rated in "tons." This is a throwback to back when A/C involved putting a big block of ice in a room. The unit of "ton" refers to the heat of fusion of a 1 ton block of ice at 0°C. Today that has carried over to 1 ton = 200 BTU/min or 3.52 kW.
  6. Mar 26, 2007 #5
    That conversion does not apply to AC units. That value is dependant on the unit's Energy Efficiency Ratio (EER). EER = Btuh/watts. A unit with an EER of 10 will require 1200 watts/hr to produce 12000 btuh. The higher the EER, the lower the wattage required.

    http://www.engineeringtoolbox.com/air-conditioner-efficiency-d_442.html" [Broken]
    Last edited by a moderator: May 2, 2017
  7. Mar 26, 2007 #6


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    Staff: Mentor

    I think they were talking about converting output heat (rate) to output power, which you can do with the conversion factor. I've worked on a couple of projects in Europe and they express HVAC capacities in watts and energy efficiency ratios in watts per watt. Ie: http://www.toshiba-aircon.co.uk/pdf/residential/high_wall_inverter.pdf [Broken]
    Last edited by a moderator: May 2, 2017
  8. Mar 26, 2007 #7
    Thanks Russ. Yes, I agree that would be correct for output.
    Last edited by a moderator: May 2, 2017
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