by Kevin Departhy, P.E.
Like the name suggests, HVAC systems are responsible for the heating, cooling, and filtering of the air in our residential, commercial, and industrial facilities. However, they also largely impact the temperature, moisture levels, and air pressure within these spaces, contributing to not only the perceived comfort in our homes and businesses but also the stability of environments where continuous human occupancy is not expected. The proper functioning of HVAC systems can significantly impact the longevity of the mechanical and electrical equipment operating in spaces such as equipment rooms, server rooms, and nuclear power plants which require unique and controlled conditions for optimal and safe operations. Understanding the fundamental science behind how these systems operate will help to explain why HVAC systems can be so complex.
We have all experienced the cool blast of air conditioning as we retreat indoors to escape the blistering heat of a summer day, but what if I told you that HVAC systems are actually responsible for the movement of heat? Let’s break down the principles of thermodynamics and physics which govern our HVAC systems. A fundamental principle of thermodynamics is that heat is transferred from higher-temperature bodies to lower-temperature bodies. This movement occurs spontaneously, meaning that heat moves in a direction from “high” to “low” on its own and without assistance. For example, when you hold a carton of ice cream in your hand and it feels “cold,” what you are feeling is the heat leaving your skin and flowing to that carton of ice cream. The comfort of our indoor environments is dependent on this principle of heat transfer. ASHRAE’s Fundamentals handbook indicates that acceptable human comfort zones can vary depending on factors like season and clothing, but generally speaking, most people prefer indoor air temperatures to be between 68°F and 80°F.
Our comfort is not only dependent on the “heat” in our environment, but also the amount of water in the air. Psychrometrics is the study of the interaction and effects of moisture, or humidity, in the air and is often studied by engineers seeking to improve our HVAC systems. The fascinating relationship between humidity and temperature can lead to an interesting event in which water can change phases from liquid to vapor without a change in temperature in the environment. An example of this relationship can be seen in how we cool our body temperature through perspiration. As we sweat, the water on our skin evaporates into the air, meaning that there is a phase change from liquid to vapor. This phase change releases heat which reduces our body temperature but increases the amount of water in the air. You may have noticed that it feels “hotter” in environments with higher humidity. That is because high humidity means there is already a high percentage of water in the air and the sweat on your skin cannot evaporate as readily. Our HVAC systems need to account for this relationship between humidity and temperature to maintain a balance in our indoor environments. Failures in these systems can cause vital mechanical systems to overheat or be destroyed by condensation and can result in human death or injury.
External conditions and weather events can negatively impact the operations of our HVAC systems. In late December, 2022, there was a prolonged cold front that reduced daily temperatures on the East Coast to be below 0°F for a long period of time. This resulted in the freezing of pipes and the failures of numerous piping systems across the state of North Carolina. My investigations into the failures of these systems were centered around determining whether the pipes were adequately protected from cold weather prior to the cold front by evaluating things like wall thickness, insulation R-value, temperature setting on the thermostat, outdoor air dampers, and a plethora of other protections. When investigating control system failures at industrial plants, a failure to regulate the temperature of the water that was being circulated in plastic piping systems led to the melting of the plastic pipes, flooding the entire plant and causing significant damages and delays to the operation.
Every year, residential spaces suffer from moist conditions and biological growth. If the HVAC unit is not sized correctly, the equipment may not be able to remove an adequate amount of humidity. This is a major function of residential equipment during the warmer seasons in the Southeast. Traditional thermostats sense temperature only and during the shoulder seasons like spring and autumn, indoor humidity can build while indoor temperatures stay moderate, preventing the thermostat from turning the system on. We have seen this scenario result in biological growth and rot on interior and exterior walls.
How many times have you tried to open a door to a restaurant, but the door pulls back and doesn’t want to open easily? The resistive door can be connected to an imbalance in airflow at the kitchen hoods that remove the grease-laden vapors coming off the cooking equipment. You may have also noticed a sewer smell, deterring you from finishing your meal or having a pleasant experience. Air exhausted out must be replaced, and when that doesn’t occur, negative pressure can build, pulling sewer gases through dried floor drains and ultimately pulling back on that entrance door and creating resistance. This failure can occur in aging or deteriorating HVAC systems and can be a much greater issue in places like healthcare and nuclear facilities where hazardous or contaminated air can pose a serious risk.
HVAC system performance can change depending on the time of year because most air-sourced equipment (e.g. chillers, condensers, economizers, etc.) operate as a function of the outdoor air temperature and humidity. Collecting data on the performance of an HVAC system may not be a one-site-visit affair and HVAC performance issues can sometimes warrant a more elaborate study of the conditions. This can be done by placing dataloggers in the equipment, and gathering data over time and multiple seasons.
In conclusion, HVAC systems are more complicated than they might appear. The proper functioning of these systems is crucial for the comfort and safety of human occupants, as well as the longevity of our infrastructure of mechanical equipment. Understanding the science behind these systems can help us appreciate their complexities and the importance of their maintenance and upkeep.
