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TECHNICAL TRAINING INSTITUTE

By Peggy Walker,2014-09-25 19:14
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TECHNICAL TRAINING INSTITUTETECH

    Gas Water Heater BTU’s

    Natural gas, propane and fuel oil are common fuels utilized to heat potable water for human use. Natural gas is utilized extensively in urban areas of the country. Rural areas utilize propane gas due to unavailability of natural gas. In certain regions of the United States, fuel oil is utilized. Convenience or associated costs of available fuel are the determining factors in the type of fuel used.

    The heating process of fuel opposed to the use of an electric water heater produces a greater amount of tempered water based on gallons per hour (recovery rate). Rate of recovery is the primary reason fuel fired water heaters are used extensively in the commercial and industrial market.

    Measurement of heat is calculated by the use of a British Thermal Unit (BTU). A BTU is the amount of heat it takes to raise the temperature of water, measured in pounds per hour. One BTU equals the amount of heat it takes to raise one pound of water one degree in 1 hour.

    1 BTU = the amount of heat required to raise 1 lb. H2O 1 degree in 1 hour

    One gallon of water weighs 8.33 pounds. This is an important factor in the sizing of a water heater for any application. The gallon per hour (GPH) recovery rate is a method the plumbing industry rates the output of a heater or boiler. The weight of water must be known to calculate the BTU’s required to produce a desired amount of tempered water per hour.

    1 gallon of H2O = 8.33 lbs.

With the above information, we can now begin to calculate the BTU’s required to size a heater

    for any application. If one BTU raises one pound of water one degree in one hour. It requires

    8.33 BTU’s to raise one gallon of water one degree in one hour.

    1 gallon of H2O = 8.33 lbs. X 1 BTU = 8.33 BTU’s to heat 1 degree in 1 hour

    The next factor required varies with each application. The per hour usage requirements must be established. The usage requirement dictates how many gallons per hour must be heated. Let’s use a certain tank capacity to help you properly understand the correct use of BTU determination. If an 80-gallon heater is to be used, the gallon capacity must be multiplied by the weight of each gallon water (x 8.33).

    80 gallons of H2O X 8.33 = 666.4 lbs. of water

    The next factor required is to determine the temperature rise (TR). The temperature difference of the incoming and outgoing water is known as the temperature rise. The incoming water can vary with the outside air (ambient) temperature, depth the piping is buried, and/or the use of a well or city water; all play a role in the temperature of the incoming water. The average temperature of incoming water used in the calculation process is typically 50 degrees. The outgoing water also varies depending on each application of the heater used. For an example, let’s assume the outgoing temperature desired is 140 degrees. The difference in degrees of the incoming water and the outgoing water is 90 degrees (140 - 50 = 90). This again is known as the temperature rise. The temperature rise is required to calculate the number of BTU’s it takes to heat the water

    in one hour to the desired temperature.

     incoming temperature minus outgoing temperature = temperature rise

     50 degrees minus 140 degrees = 90 degree T/R

    2 Joyce Company, Inc. “School of Trade” All rights reserved 2004

    Gas Water Heater BTU’s

Let’s determine the BTU’s required to achieve the desired temperature previously discussed.

     a) heater capacity is 80 gallons

     b) temperature rise is 90 degrees

; 80 gallons of water weigh 666.4 pounds

    ; it takes 666.4 BTU’s to raise 80 gallons one degree in one hour

    ; it takes 59,976 BTU’s to raise 80 gallons 90 degrees in one hour

; 80 gallons x 8.33 lbs. = 666.4 lbs. (weight of the water in a full tank)

    ; 666.4 lbs. x 1 BTU = 666.4 BTU’s (to raise the water 1 degree in 1 hour)

    ; 666.4 BTU’s x 90 degrees = 59,976 BTU’s (to raise the water 90 degrees in one hour)

    Basically, multiply the gallon capacity by the weight per gallon, then multiply the answer by the temperature rise and the answer is the amount of BTU’s required for a specific heater to perform

    the task desired in one hour. You then have the output of 80 gallons per hour of desired tempered water. The above example is based on a no use situation. Use of water during the first hour of the heat cycle would lower the gallon per hour output of the heater. An 80-gallon heater with a BTU rating of 59,976 does not yield 80 gallons of desired tempered water per hour.

    Now that we have explained BTU’s using the tank capacity in gallons, let’s realistically apply the same method to gallons per hour required. The next step is determining the needs of the user. If the user requires a heater that can provide 150 gallons of water at 140 degrees each hour, then the amount per hour required is the next calculation in the correct size water heater.

     a) temperature rise is 90 degrees

     b) user needs 150 gallons per hour

     c) 150 gallons x 8.33 = 1,249.5 lbs. of water to be heated in 1 hour.

    d) 1,249.5 BTU’s x 90 degrees = 112,455 BTU’s provide 150 GPH at 140 degrees

    The actual gallon capacity of the tank is determined by the manufacturer. Heaters can be the same capacity with different BTU ratings. The BTU rating of the heater determines the GPH it produces at a certain temperature rise. Manufacturers offer various size heaters to perform the tasks required and selection is based on the offered sizes. Selection and performance charts are used to determine the actual tank capacity in gallons and hourly capabilities.

    If a heater is in use while the heating process is occurring, cold water entering the tank mixes with the previously heated water. This forces the heater to remain in a heating mode longer to maintain the GPH requirements. This occurrence is known as the “recovery rate”. The rate of recovery is the amount of time taken for a heater to produce the GPH required.

    Joyce Company, Inc. “School of Trade” All rights reserved 2004

    Gas Water Heater BTU’s Exam 3

    Name: Answer Key Date: 2000

    1. What three factors must be known to find the BTU rating of a heater?

     Weight of a gallon of water a. __________________________________________________

     Gallons of water required b. __________________________________________________

    Temperature rise of the water c. __________________________________________________

     British Thermal Unit 2. What does BTU stand for? __________________________________

     To measure heat 3. How is a BTU used? _______________________________________

4. How many BTU’s are required to raise 1 gallon of water 1 degree Fahrenheit in one hour?

     8.33 _____________________________________________________________________________

    5. How do you determine the temperature rise of water to be heated? __________________

     Deduct the incoming water temperature from the outgoing water temperature _____________________________________________________________________________

    6. If the BTU rating of a heater has been established, how do you correctly size the capacity in gallons of the water heater?

    Using a manufacturers data sheet _____________________________________________________________________________

    7. If the incoming water is 50 degrees and the desired water temperature is 125 degrees, what is the total temperature rise?

     75 degrees _____________________________________________________________________________

    8. How many BTU’s are required if the temperature rise is 90 degrees and 120 gallons of water per hour is required? (Show all math below.)

    89,650 _____________________________________________________________________________

    120 gallons per hour

    x 8.33 pounds each gallon

    996 pounds of water per hour

    x 90 degree temperature rise

    89,650 BTU required

    8.33 pounds 9. How much does a gallon of water weigh? _________________________________________

     1 pound 10. How much does a pound of water weight? ________________________________________

    Joyce Company, Inc. “School of Trade” All rights reserved 2004

    Gas Water Heater BTU’s Exam 4

    Name: Date:

    1. What three factors must be known to find the BTU rating of a heater?

     a. __________________________________________________

     b. __________________________________________________

     c. __________________________________________________

2. What does BTU stand for? __________________________________

3. How is a BTU used? _______________________________________

4. How many BTU’s are required to raise 1 gallon of water 1 degree Fahrenheit in one hour?

    _____________________________________________________________________________

    5. How do you determine the temperature rise of water to be heated? __________________

    _____________________________________________________________________________

    6. If the BTU rating of a heater has been established, how do you correctly size the capacity in gallons of the water heater?

    _____________________________________________________________________________

    7. If the incoming water is 50 degrees and the desired water temperature is 125 degrees, what is the total temperature rise?

    _____________________________________________________________________________

    8. How many BTU’s are required if the temperature rise is 90 degrees and 120 gallons of water per hour is required? (Show all math below.)

    _____________________________________________________________________________

    9. How much does a gallon of water weigh? _________________________________________

    10. How much does a pound of water weight? ________________________________________

    Joyce Company, Inc. “School of Trade” All rights reserved 2004 37

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