Sizing an air conditioning unit to a house is a matter of calculating the square footage, converting the square footage to British thermal units (BTUs), and adjusting for specific rooms. BTUs is a type of measurement used to determine the amount of energy to heat a space. Choosing the right type of air conditioner unit for a 1,650 square foot house should be done after evaluating the size and BTU needs of each room.
Calculate the Square Footage
Realtors measure square footage based upon living space. When looking for an air conditioner unit, the calculation is a little different. Multiple each room's length by the width to get a room-specific measurement. Include hallways and other areas, such as laundry rooms, in the calculations. Do not include closet spaces or areas that will not be connected to the unit. Sealed off rooms, such as basements or attics, are not measured unless you plan on installing ducts leading into the rooms.
Converting Measurements to BTUs
While BTUs are the measurement of heat generated in a room, air conditioner units reverse that to measure the amount of heat removed from a room. Convert the square footage for each room to BTUs. For example, 100 to 150 square foot rooms equal 5,000 BTUs; 150 to 250 square foot spaces convert to 6,000 BTUs; and 250 to 350 square foot equal 7,000 BTUs. Multiply the square footage of rooms larger than 350 square foot by 25 for the average BTUs. Add 4,000 BTUs for kitchens, and 1,000 BTUs for laundry rooms and bathrooms. Add the BTUs for each room together to determine the total BTU needs for a house.
Converting BTUs to Tonnage
Most split-system air conditioner units, the most common type found in modern homes, measure the output based upon tons. Converting the home's BTUs to tons is a matter of dividing the square footage calculation by 12,000. A 1,650 square foot house, when divided properly, equates to approximately 3.5 tons. The calculation is based upon multiplying 1,650 by 25 to get the approximate BTU needs and dividing the total by 12,000. You should still measure each BTU needs individually to get a more accurate representation. Assuming the house has a kitchen, three bathrooms and a laundry room, the tonnage would actually be 4.5 tons.
Choosing the Right Style Air Conditioner Unit
Window units and ductless units use zone control. Zone control focuses the cooling on specific rooms. By limiting the cooling to specific rooms, the energy bill can be moderately reduced. For a more even temperature throughout the house, split-systems are preferred. A split system, or central heat and air, will cool a house more evenly. Choosing the right size based upon the tonnage will allow you to keep your electric bill lower than with window units. However, installation of a split-system is a substantially higher upfront cost.