Revit MEP CourseIntroduction

Have you ever wondered about the invisible factors that determine whether a room feels just right or uncomfortably stuffy? The answer lies in the intricate world of Heating, Ventilation, and Air Conditioning (HVAC) systems, where the quest for human comfort begins with the very air we breathe.  We’ve had a look at How Air Conditioners Work in Summer and How Air Conditioners Work in Winter and now we need to find out exactly where the blance is struck by HVAC systems between these two extremes of operation conditions known as the human comfort zone.  In this exploration of the science behind creating the perfect indoor environment, we delve into the vital role of oxygen supply and its profound connection to achieving the coveted “comfort zone.”

Human Comfort Zone as shown on a psychrometric chart.
Human Comfort Zone as shown on a psychrometric chart. Source: Springer

The human comfort zone, in the context of environmental conditions, refers to a range of thermal, humidity, and air quality parameters within which individuals experience a sense of physical and psychological well-being. It is defined by standards and guidelines set by organizations like the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE). The comfort zone typically includes a specified range of indoor air temperatures, relative humidity levels, air motion characteristics, and air purity conditions that collectively aim to ensure occupants feel comfortable and maintain optimal productivity. Deviations from this defined zone may result in discomfort, impacting an individual’s overall satisfaction and well-being in a given indoor environment.  With that understanding, let’s settle in to understanding the 5 ways HVAC systems keep you comfortable.

Want to use the best BIM Software for HVAC design in 2024? Click Here to Get Revit 2024 Today

1. Oxygen Supply – Breathing Life into Comfort

The importance of maintaining optimal oxygen levels ties directly to HVAC systems achieving human comfort. Adequate oxygen supply is not only essential for the body’s combustion processes but also plays a crucial role in sustaining a comfortable indoor environment. HVAC systems, designed with precision, ensure proper air circulation to meet the oxygen requirements, contributing to a space where occupants can live and work satisfactorily.

Optimal air and oxygen supply can be designed for using CFD simulations. Click Here to Get Revit 2024 Today


Similar to other machines, the human body demands a sufficient oxygen supply to sustain combustion (food digestion). This process transforms chemical energy into work, releasing carbon dioxide as exhaust gas. Each individual needs approximately 0.65 m³ of oxygen per hour in normal conditions and produces 0.2 m³ of carbon dioxide. Monitoring the rise in CO2 concentration serves as an indicator of oxygen consumption.

The atmospheric CO2 level is around 0.03% by volume, crucial for the proper functioning of the respiratory system. When CO2 surpasses 2%, the partial pressure of oxygen decreases, making breathing challenging. Extreme discomfort arises at 6%, and unconsciousness can occur at 10% CO2.  Proper air-supply in air-conditioned spaces is vital to prevent CO2 levels from exceeding the minimum threshold.

2. Heat Removal – The Art and Science of Temperature Control

The human body operates as an engine, converting thermal energy into mechanical work with a thermal efficiency of 20%. The remaining heat is dissipated into the atmosphere. Even when not engaged in external activities, internal work such as blood circulation and respiratory muscle function still occurs.

As a practical example; should an individual be allocated a 6 m³ space without the exchange of heat and air from external sources, the temperature within the space would elevate by 0.136°C for every kilojoule of added heat. Consequently, the temperature would surge by 43°C per hour, given that the human body expels 320 kJ of heat within the same timeframe.  This because in the given scenario, the space’s temperature rise is directly proportional to the heat added to it, following the principles of energy conservation. The calculation considers the specific heat capacity and volume of the space, providing insights into the temperature change resulting from the dissipation of heat by the human body.  This is shown mathematically below:

Q = mlΔt = lv

In the equation:

  • m: Mass of air (in kg)
  • Δt: Change in temperature (in °C)
  • l: Latent heat of vaporization (in kJ/kg)
  • v: Volume of space (in m³)

The objective of the ventilation system is to ensure adequate air circulation, preventing excessive temperature rise in air-conditioned spaces. This creates an atmosphere in which occupants can live and work comfortably.  In HVAC systems, the removal of heat is a fundamental process crucial for maintaining optimal indoor conditions. Whether it’s expelling excess heat to cool a space or adding heat to counterbalance losses, these systems play a pivotal role in creating a comfortable and controlled environment.

3. Moisture Control – Conquering the Stickiness Challenge

In the realm of HVAC engineering, meticulous attention is devoted to moisture control as a pivotal aspect of ensuring human comfort within enclosed spaces. The human body constantly undergoes moisture exchange, releasing approximately 50 grams of moisture per hour when at rest. HVAC systems play a crucial role in managing this moisture by regulating the relative humidity of the air. As the air’s humidity increases, the body’s capacity to expel heat through evaporation diminishes. This phenomenon not only creates an uncomfortable environment but also poses challenges in maintaining a sense of freshness within enclosed spaces.

The stickiness you feel on your skin is caused by excess humidity levels.
The stickiness you feel on your skin is caused by excess humidity levels. Source: Weather & Radar

Consider a scenario where the air’s humidity is on the higher side. In such conditions, occupants may experience a palpable stickiness on their skin. This sensation arises from the reduced effectiveness of moisture evaporation, leading to a perception of dampness and discomfort. HVAC systems address this issue by actively controlling the relative humidity, ensuring it stays below the 70% threshold. Through advanced technology, these systems regulate the moisture content in the air, creating an environment where occupants experience a pleasant, non-sticky sensation. Achieving optimal moisture control is a testament to the comprehensive capabilities of HVAC systems in enhancing human comfort and well-being.



4. Air Motion – HVAC Systems Like to Move It Move It

Increased air velocity enhances heat transfer from the body by reducing the thickness of the adjacent air film. This effect leads to increased body heat loss, reducing discomfort in ambient air temperatures lower than the body surface. Conversely, if the air temperature exceeds the body temperature, increased velocity exacerbates discomfort. Moreover, heightened velocity reduces the thickness of the saturated vapor layer near the body, facilitating evaporation. This is particularly advantageous when the dew-point temperature is below 30°C, as the heat loss through evaporation surpasses the heating effect by convection. Recommended air velocity in air-conditioned spaces ranges from 0.04 to 0.12 m/s at 20°C and 0.05 to 0.17 m/s at 22°C.

Proper air distribution, an integral aspect of air conditioning systems, complements air motion by ensuring a uniform supply of air. The combination of controlled air motion and distribution creates a localized cooling sensation known as a draft. This nuanced approach aligns with the requirements of comfort air-conditioning, striving to establish an environment where occupants experience optimal thermal conditions. The interplay between air motion and distribution reflects the commitment of HVAC systems to regulate airflow, prioritizing human comfort through meticulous control of these parameters.  The significance of proper air distribution cannot be overstated, as it complements air motion, creating a localized cooling sensation known as a draft.  To indicate the operating ranges the air velocity and humidity with respect to room air temperature is show in the table below.

Table 4.1 Air Velocity and humidity with respect to room air temperature

Air Velocity and humidity with respect to room air temperature
Room air temp. °C Velocity m/sec R.H.% Minimum R.H.% Maximum
20 0.04 – 0.12 35 65
21 0.04 – 0.14 35 65
22 0.05 – 0.17 35 65
23 0.07 – 0.21 35 65
24 0.09 – 0.24 35 65
25 0.12 – 0.32 35 65
26 0.16 – 0.40 35 65

Regulating air motion is fundamental to HVAC systems, contributing significantly to overall comfort. The systems carefully manage air velocity, striking a balance that enhances heat transfer efficiency without causing discomfort due to excessive airflow. This orchestration, combined with proper air distribution, underscores the commitment of HVAC systems to create an environment aligned with desired comfort parameters, ultimately enhancing the well-being of individuals in the conditioned space.

5. Air Purity – HVAC’s Breath of Fresh Air

The composition of air plays a pivotal role in determining its purity. Elements such as odor, dust, toxic gases, and bacteria are key indicators of air quality. The release of odor through body surface evaporation and the presence of smoke pose significant concerns due to their adverse effects on respiratory organs. Efficiently managing and eliminating toxic gases is crucial to prevent associated irritations. Emphasizing the importance of controlling bacteria, sterilization becomes a paramount measure to safeguard human health in indoor environments.

Air Purifier for Whole House Duct Uv Light Sanitizer Hvac Ac Germicidal Filter UV-C Air Cleaner