Air-conditioning sizing and tonnage


G&S Mechanical Services of Maryland:

    I Get lots of inquiries about air-conditioning especially is regards to sizing specifically what is the correct size unit I need for my home or business. One would wonder why there are so many different sizes of air conditioning units when one big enough to do the job should be enough. the fact is that unlike a heating unit that only needs to add heat to the room or building the air conditioner must remove moisture as well as cool.
    The problem is that a typical air conditioner will remove a certain amount of moisture over a period of time in relation to how much it lowers the temperature of the air. In humid climates like here on the east coast the main reason for air-conditioning is to remove moisture from the air more than cool it (this is why a small window unit seems to "cool the whole house"). To do this we must size the unit so it runs most of the time on the hottest days. This is why heat pumps never seem to work in the winter because they are usually sized for cooling. A two speed heat pump would solve this problem and greatly improve efficiency. Large commercial systems have compressors that can unload cylinders either by electrical controls or suction pressure.
    The ultimate solution is to make chilled water in a tank and distribute the chilled water to fan coil units or one central unit and slow the fan down to reduce the cooling capacity but still maintain dehumidification. With a conventional cooling system you need the capacity of the duct work and air flow to be at least the capacity of the compressor.
    There are many ways to determine the size of the unit you need. For example there are room charts that have a map of the country and the room square footage and that will tell you what size in BTUs or Tons that you need. There are other methods that take into account the insulation in the walls, ceilings, and floors. As well as the local temperature, humidity and other factors. The most accurate method that I know of is to use what is known as the "ACCA Manual J" this takes into account everything including the electrical appliances that add heat to the room. I believe that there is a program that you can download that uses this protocol called "Quickoads". When I find the website for this product I will post it here.
    When you do a complete calculation you will find that in most cases you will require a smaller system than you may have thought. Including for heating too.  The only exception to this would be for a church, school or office where the room or building is used intermittently throughout the week and the temperature must reach the "occupied setpoint" in a reasonable amount of time and you don't have the luxury of waiting for a "properly sized" system to heat or cool it.
    Back in the early days of heating it was thought that for health reasons you needed to keep the windows open in the wintertime. Therefore heating systems were sized to keep the house at 70 degrees on the coldest days with the wind blowing. This sizing school of thought is finally disappearing although most gas and oil furnaces are still too big today.

To see more pages about airconditioning go to the cooling home page.

We service and repair the following brands:
American Standard, Amana, Arco, Arco-Aire, Bryant, Carrier, Coleman Evcon, Comfortmaker, Day/Night/Payne, Dunham-Bush, Fedders, Fredrich, Goodman, General Electric, Hotpoint, Heil, Intertherm, Janitrol, Kenmore, Lennox (Armstrong, Johnson Air-Ease), Miller, Modine, Nordyne, Rheem/Ruud, Sears, Stewart Warner, Trane, Williams, White-Westinghouse, Whirlpool, Weil Mclain, York, (Frasier Johnson/Borg Warner) and others.


  1. Blower: usually a squirrel cage centrifugal air moving device. Will move large volumes of
    air relatively quiet. Will use less energy with more back pressure.
  2. Fan: a paddle type air moving device used where noise is not a major consideration. Will
    use more energy with more back pressure.
  3. Combustion blower: A blower used on high efficiency gas furnaces or oil burners to move
    combustion air. usually 1/20 to 1/6  horsepower.
  4. OEM: Original equipment manufacturer.
  5. Horsepower: 746 watts
  6. RPM: revolutions per minute.
  7. Service factor: the extent to which a motor can be safely overloaded beyond its name plate
    ratting without over heating.
  8. Air over horsepower: The rating of a motor assuming air flow through the windings usually
    as a result of the air moving device.
  9. SAE: Society of Automotive Engineers.
 10. KW Kilowatt (1000 watts) or 3400 BTUs per hour .
 11. High efficiency furnace: Furnace that uses over 85% of the energy in the gas.
 12. Condensing furnace: Gas furnace that uses over 92% of the energy in the gas and condenses
    the gas into liquid condensate and hot air. If your furnace has PVC pipe venting it then you
    have a condensing furnace.
 13. Evaporator. The part of an air conditioner or refrigeration system that gets cold due to
    evaporating refrigerant.
 14. Condenser. The part of an air conditioner or refrigeration system that gives off heat by
    condensing the refrigerant.
 15. Indoor coil or unit. On a heat pump unit we call the inside section "indoor" so as not to
    confuse it with the evaporator on an air conditioning system.
    Outdoor coil or unit. On a heat pump we call the outside section "outdoor" so as not to
    confuse it with the condenser on an air conditioner.
16. Ton 12,000 BTUs per hour of heating or cooling

Written By:  Scott Meenen N3SJH of:
Specializing in Mechanical, Controls and Electrical Modifications Of
Heating, Air-conditioning, Refrigeration, Cold storage,
Ice Production and Food preservation.
Anything having to do with Heat and Energy.
Serving MD, DC, and Northern VA.
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                 This text written by: Scott Meenen * G & S Mechanical
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