Air Conditioning Calculations

How do you do air conditioning calculations on the capacity of air conditioner for your room? This calculation is important because if done wrongly, you will end up installing an oversize or undersize equipment. An oversized air conditioner is not good as the compressor will run and stop regularly and not able to cool the room uniformly.

It will also cause discomfort to the occupants as the dehumidfication of the room is not properly done. On top of that, the electricity bill will be high as the compressor turns on and off too often.

Every time the on/off type of compressor starts to run, its power consumption is 6 times higher than when it is running steadily.

The cycling on and off of the compressor will lead to shorter life span of the compressor besides having to spend more on the unit price and installation cost.

An undersized unit will not be able to cool the room properly and more so if the weather is hot.

Cooling Capacity

Cooling capacity for a room is defined as the heat load in a room that have to be removed in order to achieve a certain room temperature and humidity. The typical design is set to 24°C temperature and 55% Relative Humidity.

Study shows that this combination of temperature and RH is the most conducive for the human body. The unit used to measure heat load is BTU/hr. 1 BTU/hr is the heat energy needed to increase 1 pound of water by 1°F.

When choosing an air conditioner, usually a 1 HP (horse power) equipment is able to remove 9,000 BTU/hr of heat. With better technology, some machines are able to remove 10,000 BTU/hr of heat with the same capacity. The higher the listed BTU/hr, the greater the cooling capacity.

Air Conditioning Calculations - Rule Of Thumb

Calculating the cooling capacity needed for your room is a complicated process as there are many factors to consider. However, there is a simple rule of thumb that you can use to estimate the required cooling capacity for your room. Use this result to compare with the calculation done by the air conditioning contractors for your own checking purposes.

Window Air Conditioner

Step 1

Find the volume of your room in cubic feet. This is done by measuring the length, width and height of the room in feet and multiply all the three dimensions together.

Volume = Width X Length X Height (cubic feet)

Step 2

Multiply this volume by 6.

C1 = Volume X 6

Step 3

Estimate the number of people (N) that will usually occupy this room. Each person produces about 500 BTU/hr of heat for normal office-related activity. Multiply this two figures together.

C2 = N x 500 BTU/hr

Step 4

Add C1 and C2 together and you will get a very simplified cooling capacity needed for the room.

Estimated Cooling Capacity needed = C1 + C2 (BTU/hr)

Air Conditioning Calculations - Other Factors

Other factors that your contractor will consider to determine the sizing of the cooling capacity include the direction of your room. If the room is facing east or west, additional capacity is needed as it will be exposed to the morning and evening sun compared to a room that faces north or south.

If the lighting of the room emits a lot of heat, additional capacity is needed. If electrical appliances that generate heat is used, additional capacity has to be factored in.

The type of material of the room and windows are also important consideration. 

Related Topics 

SEER Calculation 
Learn what is Seasonal Energy Efficiency Ratio(SEER) to help you purchase a better heat pump unit.

HVAC Softwares 
There are many softwares in the market that greatly help designers to reduce the product design cycle time.

Cooling Load 
Find the explanation on the basic of cooling load here.

Heat Pump COP 
Get a simple understanding of HP Coefficient of Performance.

Btu and Btu/hr Definition 
See how to calculate Btu, Btu/hr and SI Metrics equivalent.

Psychrometric Chart 
Basic definition and applications of the Psychrometric Chart in HVAC.

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