How BTU Is Calculated DIY?

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How to 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 (0.45 kg) of water 1F.
Furnaces are rated based upon how much thermal
heat they can produce per hour (BTU/h), while air
conditioners are rated based upon how much
thermal heat they can remove per hour (BTU/h).
Choosing a furnace or air conditioning system
with the right number of BTUs per square foot of
space can help you heat or cool your home
efficiently and cost-effectively.
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• Measure the square footage of your entire home.
  If you're installing a furnace, find the square
  footage of each room in your home and add them
  together. This will give you the total square
  footage of your space so you can choose a furnace
  that will adequately heat your home.
• For a rectangular room, multiply the length and
  width, measured in feet.
• For a triangular room, multiply the length and
  width, then divide by 2.
• For a circular room, measure the radius (r
  which is the distance from the center to the
  edge). Plug the radius into the following
  equation, using 3.14 for p pr2
• For rooms with odd shapes, divide them into
  regular shapes and measure each shape separately.
• If the room is not rectangular, you can try to
  break it up into rectangles so that you can
  calculate several lengths times widths.
• For example, if your room is L shaped, break that
  up into two non-overlapping rectangles. Then
  calculate length times width for both rectangles
  and add them up.

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2.Find out which climate zone you live in to
determine the heating factor. Look for a
climate zone map online and figure out which zone
you live in to help you determine the heating
factor, or the number of BTUs you need per square
foot to adequately heat your home. In general,
the further from the equator you live, the
greater the number of BTUs youll need.
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• 3. Multiply your square footage by the heating
  factor for your zone.
• All you have to do to find out what capacity
  furnace is best for your home based on its
  location is to multiply the square footage of the
  space by the heating factor.
• For instance, if you live in Zone 2 and have a
  1,200-square-foot home, multiply 1,200 by 35-40
  to get a BTU range of 42,000-48,000.

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• 4. Use the lower end of the range if your home is
  well insulated or the higher end if its not.
• 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 2 numbers for your climate zone if it is
  older or poorly insulated, use the higher number
  of the range.
• Say that you live in a brand new home in Zone 1.
  Multiply your square footage by 30 BTUs to find
  out what capacity furnace you need.
  Alternatively, if you live in an older home in
  Zone 6, multiply your square footage by 60 BTUs
  to ensure you purchase a furnace with enough
  capacity to heat the space.
• Note that newer homes tend to be better insulated
  than older homes because of revisions to building
  codes over the years.

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• 5. 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 efficient.
• For example, 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 BTU/h
  rating you need by 0.8. So, 100,000 BTU/h 0.8
  125,000 BTU/h, meaning you'd need a furnace rated
  to 125,000 BTU/h input.

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• 1. Measure the square footage of the space you
  plan to cool.
• If you want to install a central air-conditioning
  unit, measure the square footage of each room and
  add all of the numbers together to find the total
  square footage of your home. Alternatively, if
  youre only going to install an A/C unit to cool
  a single room, find the square footage of that
  room.
• Multiply the length by the width, in feet, to
  find the square footage of a rectangular room.
• Multiply the length and width of a triangular
  room, then divide that number by 2 for a
  triangular room.
• Measure the radius of a circular room (r which
  is the distance from the center to the edge),
  square the number, then multiply that by p (3.14)
  (the formula is pr2).
• Divide rooms with odd shapes or alcoves into
  regular shapes and measure each shape separately.

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• 2. Multiply your square footage by 20 BTUs to
  get a general estimate.
• Typically, plan to purchase a heating or cooling
  unit with 20 BTUs for every square foot of space
  you have. However, keep in mind that factors such
  as your climate zone, sun exposure, and the
  number of people living in your home may require
  you to adjust this figure.
• For instance, if you live in a home thats 800
  square feet, youll need a unit with 16,000 BTUs.
  On the other hand, if you live in a
  5,000-square-foot home, get a unit with 100,000
  BTUs.

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• 3. Select an A/C unit with a lower capacity if
  you dont have a lot of sun exposure.
• The position of your home also affects how many
  BTUs you need to properly heat or cool it. Reduce
  the capacity by 10 if your home or room gets a
  lot of shade, or increase the capacity by 10 if
  your home or room is usually in the sunshine.
• Take a look at your sun exposure during the
  middle of the day in the summer season to help
  you gauge this accurately.

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• 4. Increase the capacity if more than 2 people
  live in your home
• If you have a lot of family members, youll need
  an air conditioner that has a higher capacity.
  Add 600 BTUs per person for a family of more than
  2.
• For instance, if there are 6 people living in
  your home, multiply 600 by 4 to get 2,400. Add
  2,400 BTUs to the number you calculated by
  multiplying your square footage by 20 to ensure
  your unit will cool your space well.

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5. Get a system with more BTUs if it will be in
the kitchen Kitchens have lots of appliances
that give off heat, including stoves and
dishwashers. If youre installing an A/C unit in
your kitchen, select one with 4,000 more BTUs
than you need based on the square footage alone.
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• 6. Factor in the units 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.
• 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.

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