Heat Load Calculations - Do You Need Them?

Why Do We Need Heat Load Calculations?

When you go shopping for clothes you don't buy twice your size in case you need that extra room. It would be a waste of money and material. The same principle applies to sizing an air conditioning system.

But because heat load calculations require time and attention to detail, most air conditioning contractors rely on simple rules of thumb to determine the sizes of the systems they install based on the total M² of the floor area. Yet every house is different and needs specific and accurate calculations.

Most people may think that "bigger is better", but an oversized air conditioning system can result in:

  • A humid house because the system doesn't run for enough time to dehumidify the air making you uncomfortable
  • A shorter lifetime for the system because it turns on and off frequently, known as "short cycling"
  • A needless physically larger, heavier and more expensive to install and operate system costing you money

On the other end of the scale an undersized unit can cause a system that is continually operating and struggling to achieve the set temperature, resulting in higher running costs.

The correct way to size an air conditioning system is with a heat load calculation based on the DA09 Application Manual, provided by the Australian Institute of Refrigeration, Air Conditioning and Heating (AIRAH). This manual is specific to Australian conditions and can determine how much heating and cooling an area actually needs. These days calculations are performed with dedicated computer programs.

After a careful and systematic survey of the area, all the relevant data is entered, such as the home's orientation, ceiling heights, insulation levels of walls and ceiling, window types, glass thickness and sizes, areas of all the surfaces and their materials that gain or lose heat, heat sources such as lights and equipment and more.

All this takes considerable time and effort, which is why contractors developed rules of thumb using simple formula's for average conditions.

When an engineer performs a heat load calculation, they accurately measure and enter all the relevant data, such as the home's orientation, insulation levels, window types, areas of all the surfaces that gain or lose heat, and more. The output of the calculation is how much cooling and heating the area needs.

With the results from an accurate heat load calculation, system designers can select equipment and design a system that will perform at its maximum efficiency.


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