As a knowledgeable professional in the HVAC industry, it’s crucial to provide accurate advice to designers, not only to improve your own credibility but also to enhance their projects and potentially secure more orders. To facilitate this process, we have compiled a set of rules of thumb that can be applied to most general applications. However, it’s imperative to emphasise that while we are well-versed in HVAC principles, we are not designers, and our advice should always be verified by others before installation. This disclaimer must be clearly stated in all written communications if our equipment design is based on these rules of thumb.

Cooling Loads

When considering cooling loads, a general guideline is to allocate 120 – 160 watts per square meter, taking into account factors such as solar gain, people, and lighting. Alternatively, you can estimate the cooling requirement at 1 kilowatt per occupant. Additionally, for each meter per second of fresh air introduced, add approximately 10 kilowatts to the total cooling load. If a designer provides the desired cooling load in kilowatts, a quick crosscheck can be performed using the rule of thumb that suggests a maximum of 20 kilowatts per cubic meter of air. Any value surpassing this threshold will likely necessitate an increase in air volume.

To calculate the cooling load using psychometric charts, the following equation can be applied: Cooling Load (kW) = Enthalpy Difference (total heat content) x Air Volume Specific Volume (m³/kg) at the entering condition

For example, consider the following data: Air Volume: 1.5 m³/s Entering Air Temperature (EAT): 28°C dry bulb / 50% relative humidity (Specific Enthalpy: 59 kJ/kg) Leaving Air Temperature (LAT): 12°C dry bulb / 11.5°C wet bulb (Specific Enthalpy: 33 kJ/kg)

Using the equation, the calculation would be as follows: Cooling Load (kW) = (59 – 33) x 1.5 m³/s / 0.869 m³/kg Therefore, the cooling load amounts to 44.88 kW.

Heating Loads:

For heating loads, a general rule of thumb suggests allocating 45 watts per cubic meter, considering the volume of the space. Additionally, when accounting for fresh air load, a standard equation can be used: kW = AT°C × 1.207 (constant) × M.

It’s important to note that this guideline is applicable when heating the building using the Air Handling Unit (AHU). In cases where heating is provided through alternative means, such as radiators, the equation kW = OT°C × 1.207 × M should be employed for tempered ventilation.


By utilising these rules of thumb, HVAC professionals can assist designers in estimating cooling and heating loads for various applications. However, it is crucial to emphasise that these guidelines serve as initial estimations and should be verified by qualified designers to ensure accuracy and compliance with specific project requirements. Clear communication about our role, lack of design liability insurance, and the need for independent design verification should be included in all written correspondences. By adhering to these principles, we can enhance our credibility, improve collaboration with designers, and deliver superior HVAC solutions.