BTU stands for the British Thermal Unit. A BTU is the amount of heat energy that is required to raise the temperature of one pound (equivalent to 0.454 Kg) of water by one degree Fahrenheit (equivalent to 0.55 degrees Celsius). The amount of energy contained in one BTU is equivalent to 1055 joules. The joule is part of the metric system and is the more popular of the two units when measuring heat energy in most other applications. It is equivalent to the energy required to raise a mass of 1Kg by a height of 1 meter.

However, in regard to determining the amount of energy needed to warm or cool a given amount of space, the British BTU is the preferred unit. It is therefore not surprising to note that one BTU has been equated to the amount of heat energy evolved when a four inch match stick combusts completely. It is important to note that when calculating the BTU of a room, only the length and width of the room are considered. The height of the room is not factored into the calculation. This is because it is assumed that since a room is warmed or cooled through the use of a continuous vertically oriented column of air, factoring in the height would result in an overly large BTU value. By taking the length and width to be fixed variables, it becomes easier to come up with a more reliable BTU value.

By determining the amount of BTU required to heat up a room, it becomes possible to determine the amount of power an air conditioning system consumes. This is done by factoring in the BTU output rating of the aircon. Rightly so, it is possible to determine what percentage of the monthly power bill can be directly attributed to the air conditioning system. Furthermore, by knowing the number of BTUs needed per unit time for a particular room in a home or office, it becomes possible to choose the most ideal air conditioning system to purchase.

How to calculate the BTU for an office

There are basically three methods that are commonly used in calculating the air conditioning BTUs needed for an office or room. Each of the three methods is broken down into a series of steps. Below is a brief description of the two methods:

A. Method One

The first method is defined by the steps below:

i. Determine the total floor space of the room or rooms to be heated. In the case of an office building, it involves determining the floor space of each and every office and then adding the spaces to find the final floor space. In the case of a home, it involves determining the floor of all the rooms and adding up the space from all the rooms. For improved accuracy the floor space of each room is determined in the following three ways:

• The length and width of a room are multiplied together in instances where the room has a floor that is either a simple rectangle or square.

• For rooms that have a floor that is polyhedron in nature, the floor is sub-divided into squares, rectangles and triangles that fit over the entire floor. Calculating the area of each square, triangle and rectangle and then adding the areas together give the floor space of the room.

• Remember to include the floor space of the basement, attics and even the stairs. For the stair, determine the area of each rectangular or square step and then multiply by the total number of stairs.

ii. Once the total floor space of the office building or house is determined, the total value is then multiplied by the average number of BTUs needed to heat one square foot of floor space. This average value is assumed to be 20 BTUs per square foot. Note that to use this value, the total floor space must be in square feet. For example if the total floor space is determined to be 700 sq ft, then the necessary BTUs are 700×20=14000 BTUs.

B. Method Two

The second method factors in the age of an office building or home and the climate in which the house or office building is located. The first step in this second method is to determine the actual total floor space of the office building or home in question. The approach to doing this is no different from the approach to the first method. The main difference comes in when determining the average number of BTUs needed for every foot of floor space. There are three guidelines to doing this:

• One must factor in the climate of the locality. For office buildings located in temperate regions i.e. regions that experience the four seasons of the year which are summer, autumn, winter and spring, the local climate is said to be cool. For office buildings located in tropical regions i.e. region which experienced only two distinct seasons all year it summer and rainy season, the local climate is said to be warm.

• The number of BTUs required per square foot to maintain ambient temperature lie between 50 and 60 BTU. In warmer climates the number of BTUs required per square foot to maintain ambient temperature lie between 30 and 35 BTU. These values are on the lower temperatures in cooler climates and relatively more favourable temperatures in warmer climates.

• Older buildings tend to require more BTUs to maintain ambient temperature than newer ones. The number of BTUs required per square inch is also dependent on temperature. Older office building located in a region that has a warm climate will only require 35 BTUS per square inch to maintain ambient temperature. The same old building in a cooler climate will require 60 BTUs per square inch. On the other hand a new office building in an area with a warm climate will require only 30 BTU per square inch to maintain ambient temperature. The same new building in a cooler climate will require 50 BTUs to maintain ambient room temperature.

It is important to note that the range of BTUs needed per square inch given for different climatic regions agrees perfectly with what has been given for the BTUs needed to by either an old or new house. Using the above two techniques it should be possible to calculate the total BTUs needed for an office and even by an entire office building.

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