For most of our final customers, an HVAC system is ultimately about comfort. To most building occupants, the HVAC system remains invisible or unnoticed until a problem occurs. The one exception – if one becomes uncomfortable. Comfort is easy to detect, but sometimes difficult to explain. Let’s take a look at the factors that are a part of comfort conditioning.
This is the intuitive measure of comfort for most people. If a space seems uncomfortable, the first statement is usually something like “Hey, it is warm in here, what is the temperature?” Most folks have an idea of the temperature at which they are most comfortable. The fact that most thermostats only have a temperature adjustment reinforces this. Not everyone will choose the same value though, and there are certainly some temperatures that are uncomfortable for all.
The American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) publishes a standard for “Thermal Environmental Conditions for Human Occupancy,” or more simply stated as “comfort.” The standard is quite academian, even comical at times. Strict compliance would require a survey of building occupants and their clothing, down to their underwear, in order to evaluate those results against charts, graphs, and tables. Comfort to the fourth decimal place!
We are going to stick to some simpler descriptions, but it is useful to understand that ASHRAE 55 lists three other variables as important to comfort: radiant temperature, humidity, and air speed. We are going to add one to those – indoor air quality – knowing that ASHRAE likely agrees. In fact, they publish another standard (ASHRAE 62) on the subject.
Well, this sounds like a technical term, and it is, but it may be the simplest way to describe this variable. As a boy, I would hear my mother describe certain homes or buildings as “cold spaces.” Radiant temperature was what she experienced.
If the space temperature is a perfect 72, but one or more walls are cold (or warm), then one is likely to experience discomfort. If the floor is cold, then one will suffer from “cold feet.” If the sun is shining through an unshaded window, then one may still feel too warm.
Heat transfer can occur when a surface of the building is somewhat warmer or cooler than the desired space temperature. Heat radiates from the sun, however it also will radiate from local surfaces or floors.
Radiant heat can often cause discomfort, but it sometimes can create a unique type of comfort. Outside, a bright sunny day can make up for otherwise cold temperatures. A warm floor can also increase comfort. Many of us remember those evening walks on a beach in late Summer or Fall. The sand might be warm to the feet while the air is cool. Often these comfort experiences are among our most memorable.
In the American South, we have an old saying: “It’s not the heat, it’s the humidity!” That expression will live on, because excessive humidity in the environment will make any temperature uncomfortable. Our bodies release heat through perspiration, and evaporation becomes more difficult if the air around us is more humid. Low humidity levels have less impact on comfort, but they can contribute to dry skin, respiratory problems, and static electricity.
HVAC systems must be able to effectively address humidity in some manner, even if it is an indirect method. While this is most important in climates that have humid conditions, it is also important anywhere human occupancy is heavy, like training rooms, theaters, and schools.
Finally, humidity levels outside of normal ranges can contribute to biological growth, such as mildew, mold, and other physiological impacts. These factors begin to overlap with another component of comfort: “Indoor Air Quality.”
Space Air Motion
The air motion around us is important for comfort. If it is too great, we will say a room is “drafty.” If it is inadequate, the we will discuss “stagnant” conditions. As usual, we want things “just right.”
ASHRAE 55 addresses “air speed” but in a way that would confuse a PhD. Better guidance for what designers call “air distributions” exists within the engineering guides of several that manufacture grilles, registers, and the like. The late Harold Straub (Titus) was an early influence on my own understanding.
Essentially, air motion for forced air heating and cooling systems must be designed in a way that provides for, but also limits air motion in a space. The scope of this essay is limited, but it is useful to know that proper air motion is actually not tangibly perceptible (sorry for the big words). If our bodies can detect motion, then it is likely excessive for comfort. There is a range of air motion that is not tangible, but still present. This is the target for comfort.
Indoor Air Quality
If the temperature, humidity , and air motion are all managed correctly, the we are all set – correct? Well, not quite. If it smells in the space, one would probably not profess comfort!
Indoor air quality (IAQ) can become a complex subject. At a fundamental level though, building occupants (people) will dissipate perspiration, odor, and carbon dioxide, usually at imperceptible levels. However, over time these components can build up in concentration, especially if the building air volume is not cleaned or diluted with fresh air. The space condition can become “stale” or worse. Poor dilution of air can cause problems beyond discomfort as well.
All of these factors assume proper occupant conditions, what ASHRAE 55 refers to as “metabolic rate” and “clothes insulation.” Simply stated, if one works out in Miami, only to come into the indoor environment and put a sweatshirt on, there should not be an expectation of comfort.
These factors are good to know when evaluating an existing space, perhaps your own home or office. They are also useful when evaluating architectural design decisions and HVAC system alternatives. The building roof, walls, floor, and shading of glass areas can impact comfort to an extent greater than the HVAC system!