When considering how to choose and manage your HVAC system in buildings with human occupants, an understanding of the anatomy and physiology of molecules is both interesting and helpful.
The Mother of all energy
Almost all of earth’s energy originates from atomic forces in molecules in the surface cloud of the sun. The sun — a huge, hot ball of hydrogen and helium — has a strong gravitational field that continually pulls the hydrogen atoms toward its center. This pull, like an irresistible dating game, forces hydrogen atom electrons and protons to “squish” together and form another molecule: helium. The creation of helium through this nuclear fusion, a process humans have not yet mastered for energy production, releases huge amounts of energy which is stored as potential energy in electric charges, which then radiates outward at the speed of light as electromagnetic radiation (EMR).
Part of EMR energy, approximately 63,000,000 W/sq-m, is intercepted by the earth and drives processes such as climate, the seasons, ecosystem changes, hydrologic systems, and plant photosynthesis that creates fossil fuel in a few millions years.
Of all the EMR that reaches the earth, our eyes can only detect a narrow range of wavelengths that we perceive as either colors or simply visible light.
Wavelengths For HVAC
Approximately 43% of the total EMR energy emitted from the sun is visible to the human eye. Of the remainder, 49% is in the infrared (heat) range, 7% in the ultraviolet section, and the remainder is categorized as x-rays, gamma waves, and radio waves.
Once EMR reaches the surface of the earth, the materials it reaches absorb the energy. This influx of energy makes the atoms of the materials move faster, transforming the EMR energy into heat.
Introduce human physiology into the equation
Warm-blooded animals such as humans depend on a constant body temperature within the narrow range of 35-39.5°C. To sustain life, humans also metabolize energy from food and respiratory oxygen into mechanical work. Of the mechanical energy created, 60% is released as heat which has to be dissipated into the environment to avoid turning into a baked-human. The factors that influence heat dissipation, and conversely warming, in people include the following.
- characteristics of the human skin
- indoor air temperature
- air velocity
- mean radiant temperature to which the body is exposed
- heat transfer through the building envelope
What is the most energy efficient way to comfortably and safely heat and cool a building? Heat transport within a building is usually through convection or radiation. With convection, the “vector,” or carrier of thermal energy, is the air. With radiation, the rate of heat transfer is determined by the thermal coefficient of the materials with different temperatures.
Differences in these processes should be considered when choosing your HVAC system. For example, radiant heat warms objects in the room rather than the air. This type of system, used in incubators for neonatal infants and young animals, creates body warmth when the mean radiant temperature and air temperature are increased and the air velocity is reduced.
Radiant heat is often used in buildings designed for people with asthma, allergies, and chemical sensitivities because fewer dust particles are stirred up. Humidification is frequently unnecessary with a radiant system because the indoor air moisture content is not altered by combustion or increased infiltration of cold, dry outside air.
Conventional heaters may allow the heat to rise and collect unevenly in places where it can escape such as the ceiling and along walls.
These are some of the many variables that must be considered when choosing the best HVAC systems for building temperature and humidity maintenance.