Calculate BTU Per Square Foot

The British Thermal Unit (BTU) is the basic measure of heat energy in the Imperial system. One BTU is defined as the amount of heat necessary to raise 1 pound (0.454 kg) of water 1 degree Fahrenheit (0.55 degrees Celsius). Knowing the number of BTUs required per square foot of space is important in choosing the right size furnace and air conditioning system to heat and cool your house efficiently and effectively. You'll have to determine the amount of space to be heated or cooled and choose the right furnace or air conditioner unit for the job.

Steps

Help Calculating BTU

Doc:Cooling Capacity Chart

Determining How Much to Heat or Cool

  1. Measure-Square-Footage. Measure the square footage of each room you plan to heat or cool and add them together. If you're installing a furnace or central air conditioning system, this typically includes every room. In some homes, the ducts do not connect to the attic or basement.
    • For a rectangular room, multiply the length and width, measured in feet.
    • For a triangular room, multiply the length and width, then divide by two.
    • For a circular room, measure the radius r (the distance from the center to the edge). Calculate πr2, or (3.14)r2 if you do not have a calculator with a π function.
    • For rooms with odd shapes, divide them into smaller pieces and measure each piece separately.
  2. Determine the necessary heating or cooling capacity. The capacity of furnaces and air conditioners is given in BTUs per hour. You need to establish the correct number of BTUs per square foot for your house needed to either heat or cool it. The methods differ for heating and cooling and are described below.
    • Many furnaces and air conditioners will be labeled "BTU," but this is actually shorthand for "BTU per hour" (BTU/h).
    • At a bare minimum, you can use a figure of 20 BTUs per hour per square foot and multiply this by the total square footage to be heated or cooled, but this does not take into account climate, the amount of insulation you have, or the way you live.

Determining the Correct Cooling Capacity

  1. Figure the total square footage to be cooled. If you're buying a room air conditioner, you only need to figure the room whose window the air conditioner will be installed in; if you're buying a central air conditioner or heat pump, you need to figure the entire square footage of the house served by ductwork, as described under "Determining How Much to Heat or Cool."
  2. Determine the necessary capacity. One method for determining capacity is refer to the estimates on this index.[1] Note that air conditioners are rated in BTUs per hour, but the label may abbreviate this to just "BTU."
    • To cool 100–150 ft2, you need ~5,000 BTUs per hour.
    • To cool 150–250 ft2, you need ~6,000 BTUs per hour.
    • To cool 250–300 ft2, you need ~7,000 BTUs per hour.
    • To cool 300–350 ft2, you need ~8,000 BTUs per hour.
    • To cool 350–400 ft2, you need ~9,000 BTUs per hour.
    • To cool 400–450 ft2, you need ~10,000 BTUs per hour.
    • To cool 450–550 ft2, you need ~12,000 BTUs per hour.
    • To cool 550–700 ft2, you need ~14,000 BTUs per hour.
    • To cool 700–1,000 ft2, you need ~18,000 BTUs per hour.
    • To cool 1,000–1,200 ft2, you need ~21,000 BTUs per hour.
    • To cool 1,200–1,400 ft2, you need ~23,000 BTUs per hour.
    • To cool 1,400–1,500 ft2, you need ~24,000 BTUs per hour.
    • To cool 1,500–2,000 ft2, you need ~30,000 BTUs per hour.
    • To cool 2,000–2,500 ft2, you need ~34,000 BTUs per hour.
  3. Adjust for special conditions. If you're buying a room air conditioner instead of a central unit, you can adjust your needs according to the following factors:
    • Reduce the necessary capacity by 10 percent if the room to be cooled is heavily shaded. However, if the room is very sunny, increase the necessary capacity by 10 percent.
    • If the room regularly hosts more than 2 people, add 600 BTUs/hour for each person after the second.
    • If the room to be cooled is a kitchen, increase the necessary capacity by 4,000 BTUs/hour.
  4. Look at the efficiency rating. While furnaces are rated by their effectiveness in delivering the heat they generate, air conditioners are rated by how efficiently they use electricity over the course of a typical operating year. One such rating is the Seasonal Energy Efficiency Ratio (SEER) rating, created by the Air Conditioning, Heating, and Refrigeration Institute, which is the ratio of the unit's cooling output in BTUs divided by the energy in watt-hours needed to run it for the entire operating year. (Note that 1 kilowatt-hour equals 1,000 watt hours.)
    • For example, take a 4,000 BTU/hour air conditioner run for 1,000 hours during an operating year using 400,000 watt-hours of electric power. This air conditioner would have a SEER rating of 10, since 4,000 x 1,000 / 400,000 = 10.
    • To find the average power consumption divide the unit's power in BTUs per hour by the SEER rating. Since the SEER rating is in units of BTU per Watt-hour, your answer will be in terms of watts. In the example above, (4,000 BTU/h) / (10 BTU/Wh) = 400 W.
    • To find the cost per hour of operation, multiply the average power consumption in kilowatts by the cost of your electricity per kilowatt-hour. In our example, the unit's consumption is (400 W) / (1,000 W/kW) = 0.4kW. If your electricity costs 5 cents per kWh, this unit costs 0.4kW x 5 ¢/kWh = 2 ¢/h (cents per hour).
    • Central air conditioners manufactured in the United States since January 2006 are required to have a SEER rating of at least 13, or 14 to be Energy Star qualified. Room air conditioners are currently exempt from this requirement; many have SEER ratings closer to 10.

Determining the Correct Heating Capacity

  1. Consider your climate. In warmer climates, you may need only 30 to 35 BTU per hour per square foot to heat your house adequately. In colder climates, you may need up to 50 to 60 BTU/h/ft2. In general, the further from the equator you live, the greater the number of BTU/h/ft2 you need, but weather patterns and geography modify this somewhat. A map of climate zones for the United States and Canada can be found at http://www.acdirect.com/systemsize.php.
    • Many product labels abbreviate "BTU per hour" (BTU/h) as "BTU." You do not need to do any calculations to turn this into BTU/h.
  2. Consider age and insulation. Newer homes tend to be better insulated than older homes because of revisions to building codes over the years. Better insulated homes need fewer BTUs per hour per square foot than older homes. If your home is new or well insulated, you can use the lower of the two numbers for your climate zone; if it is older or poorly insulated, use the higher number of the range.
  3. Multiply the total square footage to be heated by the heating factor. If you live in a 2,500 square foot home and a climate zone with a heating factor of 40 BTUs per square foot, you need a furnace with an output of 100,000 BTUs per hour.
  4. Take the efficiency rating of the furnace into account. Furnaces are rated not by the actual BTU output you receive but by the amount of heat they generate. How much of the heat a furnace generates (input heat) that actually reaches you (output heat) is a measure of how efficient the furnace is. The efficiency is expressed in percentage as a ratio of the output to input heat. Most modern furnaces are rated as either 80 or 90 percent efficient.
    • In the example above, a 100,000 BTU/h input furnace would not be enough to heat a home needing an output of 100,000 BTUs per hour. An 80% efficient furnace would deliver an output of only 80,000 BTU/h (100,000 x 0.8). To find an 80% efficient furnace that does provide enough power, divide the goal by 0.8. In our example, 100,000 BTU/h ÷ 0.8 = 125,000 BTU/h, so you'd need a furnace rated to 125,000 BTU/h input.

Tips

  • If you're mounting a room air conditioner in a corner window, look for one that can send airflow throughout the room, not into the wall.
  • Central air conditioning units frequently have the capacity coded into the model number; for example, model RDR36 would be a 36,000 BTU unit. They may also report their size as tonnage, with 12,000 BTUs equal to 1 ton (the amount of power needed to melt 1 ton of ice in 24 hours); thus the model RDR36 would be a 3-ton unit.
  • A poorly insulated home will lose a lot of heat to drafts. Insulate-Your-Home may be a cheaper solution than installing a furnace.

Warnings

  • Don't buy either a room or central air conditioner with a greater capacity than you need. If you do, the unit will cool the room too rapidly to also take the humidity out of the air. An over-sized central air conditioner will also turn on and off more frequently, costing more to run and having a shorter operating life than a properly sized unit.

Sources and Citations

  1. ↑ http://www.energystar.gov/index.cfm?c=roomac.pr_properly_sized
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