Whenever we design an HVAC system for a residential or commercial customer, we have to take into account a wide variety of factors. These factors include the size of the building and the number of windows. Other factors are also the level of insulation, the size and position of rooms, and many other things.
But one of the most important factors that we have to take into account is the people who will occupy those buildings. And that doesn’t mean just what temperatures and humidity levels will keep them most comfortable. It also doesn’t mean how we’ll remove indoor air pollutants to improve the quality of the air they breathe.
When we talk about the human factor here, we’re talking about how the human beings themselves will affect it. This is the temperature, humidity, and air quality of the buildings that they occupy. We must design a system that takes that human factor into account. Otherwise, we won’t be able to effectively and efficiently control the comfort level or the air quality of the space.
See Also: The Science of Comfort
How Human Occupants Affect the Air
All living things generate heat. Human beings are no exception. Our metabolic reactions result in us giving off about 250 BTU/hour when we’re at rest.
When we up our activity level, the amount of heat we generate increases too. You know this if you’ve ever gone out running on a cold morning. At first, you’re freezing. As you continue with your intense activity, you get warmer and warmer. Eventually, you may be sweating even though it’s below freezing outside.
So human beings generate heat, which obviously matters for HVAC design. We also release moisture. Every time we breathe, the air we exhale carries water vapor from our bodies. And if we’re sweating and the sweat evaporates, that’s even more moisture going into the air.
In addition to affecting the temperature and humidity levels, humans also affect the air quality. Have you ever spent time in a poorly ventilated locker room? Then you know that humans can produce some seriously unpleasant odors.
But human occupants also affect the air quality in another way. During our process of respiration, we inhale oxygen and exhale carbon dioxide (CO2). This means that in an enclosed space, breathing humans will, over time, decrease the oxygen level while increasing the level of CO2.
If you’ve ever seen the movie Apollo 13, you know that human beings can’t survive in an environment with too much CO2. Here on Earth, it’s uncommon for a building to be sealed so air-tight that you’re in serious danger of suffocating due to human respiration. It can happen, especially in spaces like gyms and locker rooms where people are packed in tight and breathing heavily.
But even if you don’t suffocate, breathing air that’s too high in carbon dioxide isn’t good for you. Overly high CO2 levels can make people feel lethargic and drowsy, and like the air is “stale.” That may not be deadly, but it’s certainly unpleasant!
Designing for the Human Factor
So how do we design HVAC systems to take this human factor into account? First of all, when we perform a load calculation to determine how much cooling capacity a space needs. We include the number of occupants who will regularly be using that space.
When we do residential load calculations, the standard rule is to assume one occupant per bedroom, plus one. For example, for a three bedroom house, we’d calculate the load based on four occupants.
Commercial spaces tend to have more variation in their occupancy and require more complex calculations. For instance, a church might regularly have an occupancy of 500 people on Sunday mornings and Wednesday nights. It then have fewer than 50 people inside at any given time during the rest of the week.
Likewise, in an office building, some conference rooms may contain 30 people for a few hours a week, but be empty the rest of the time. We have to design a system that can cool those spaces effectively when they’re at full capacity, while trying to keep the system from being over-sized and short-cycling the rest of the time.
Activity Level of Humans
Additionally, in commercial spaces we need to consider the activity level of the occupants. At a gym, the active humans will be generating hundreds more BTUs each per hour compared to an office where the occupants spend most of their time sitting at a desk.
ASHRAE releases standards that spell out exactly how much sensible heat (temperature) and latent heat (humidity) will be released by human occupants per hour depending on their specific activity level.
For instance, an average adult seated in a movie theater produces approximately 225 BTU/hr of sensible heat and 105 BTU/hr of latent heat. On the other hand, the average adult who is bowling produces 580 BTU/hr of sensible heat and 870 BTU/hr of latent heat. That’s a big difference in terms of the air conditioning capacity you’ll need!
See Also: The Size of Your HVAC Does Matter
Finally, we also need to take human-produced carbon dioxide levels into account when designing HVAC systems. This is especially if we’re dealing with spaces with a high density of occupants, like classrooms and locker rooms. This can also be an issue with highly-efficient, airtight homes.
These spaces must be properly ventilated with outside air in order to keep CO2 levels within acceptable ranges. ASHRAE also provides standards for how much outside air is needed in different types of spaces.
ASHRAE says that a classroom should get 15 cubic feet of outside air per minute (cfm) for each person in that room. So a classroom with nine students and a teacher would need 150 cfm of outside air. That same classroom with thirty people in it would need 450 cfm.
To help ensure CO2 levels are kept in an acceptable range, some newer buildings have ventilation dampers fitted with CO2 monitors. This is so that they open automatically whenever the level gets too high. Older buildings rely on ventilation systems that bring in outside air constantly at the calculated rate.