June 2020 — An HVAC system’s primary responsibilities are to add or remove heat, filter air, and control humidity. The HVAC system is also used to control and filter the air to protect equipment. These factors affect human comfort levels. Comfort helps people enjoy their surroundings and improves their productivity.
To meet human comfort needs, an effective HVAC system manages the temperature, humidity, and airflow, and, to some extent, the air quality of the interior environment. The interaction between people and their environment is complex, as comfort is a subjective feeling and is impossible to measure; the ideal temperature for one person may be too warm or too cool for another.
Heat Loss from People
To manage thermal comfort, the HVAC system must allow for heat loss from people. The human body is a heat-producing system, and the food one eats is metabolically converted into heat energy. This process is quite inefficient, however; only about 20 percent of the energy produced by the human body is used for tissue-building, work, and movement. The remaining 80 percent of the metabolic energy is given up to the environment as waste heat.
Metabolic heat can be a major heat source in meeting rooms, auditoriums, and other assembly areas. The HVAC system helps maintain the body’s natural cooling processes, making people feel comfortable.
While there is no one definitive temperature for every building, there are guidelines. Energy conservation, building location, and operating budget all play a role in determining the setpoint of building systems.
Understanding the role human anatomy plays in regulating body heat also helps to inform decisions regarding the setpoint of building systems.
Factors Affecting Metabolic Heat Loss or Gain
The effectiveness of evaporation, radiation, or convection heat loss or gain is dependent upon various environmental conditions:
- Evaporation—factors impacting heat loss include:
- Skin surface temperature of the person, assuming that the dry-bulb (DB) air temperature is 80°F or below
- Relative humidity (RH), but only slightly, if the air temperature is 80°F or below; RH is much more important if the DB is above 80°F
- DB air temperature
- Velocity of the air; higher velocities increase the evaporation rate and, therefore, the heat loss
- Radiation—factors impacting heat loss include:
- Mean radiant temperature (MRT)
- Average temperature of exposed skin and clothing
- Surface emissivity, or how well the surface radiates and absorbs radiant energy
- Convection—factors impacting heat loss include:
- Dry-bulb air temperature
- Air velocity
- Average temperature of exposed skin and clothing
In typical indoor conditions, the human body continually gives up its metabolic heat through the combination of evaporation, radiation, and convection. This makes metabolic heat generated by the people in a building a significant source for the HVAC system to control.
Physiological Reactions to Thermal Extremes
HVAC systems are designed for specific purposes. When a system is designed, the total heat load for each space is carefully calculated and designed accordingly. The load is the resistance that a system must overcome to accomplish the job it was designed to meet.
If several people were engaged in activities ranging from performing light office tasks at a desk to participating in rigorous athletic activity in the same room at the same time, no heating/cooling system could keep all of them comfortable; their heat production would be too great. An average activity must be selected for the system to be properly designed for practical purposes.
Typically, designers of office building air-conditioning systems choose sedentary activity—an activity done while seated at rest or standing at ease—as an average condition for healthy people to be served by the system. Sedentary activity is also considered the most important case because it allows people to think about their comfort or discomfort. Thermal comfort exists when a person is in an environment where temperature and relative humidity permit the person to lose, without conscious effort, metabolic heat at the same rate he or she produces it.
For design purposes, a seated person doing light work is a good example. The average person generates about 400 Btu/hour (117.2 watts). By comparison, a 100-watt light bulb generates about 340 Btu/hour (100 watts).
The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) has also struggled with defining comfort. Recognizing that comfort has both physiological and psychological aspects, ASHRAE defined thermal comfort in its Thermal Environmental Conditions for Human Occupancy Standard 55-2004 as that condition of mind that expresses satisfaction with the thermal environment.
This positive feeling of comfort is based on the interior environment being neither too warm nor too cold.
Comfort and Energy Conservation
Energy savings are as much a factor in the initial design as they are in the ongoing operating costs. The same question is asked every time a building rises out of the ground: Can we establish balance between energy conservation, thermal comfort, and productivity?
For example, if an office building’s HVAC system were turned off, a tremendous amount of energy would be saved. But this tremendous energy savings could not compensate for the lost business. Most building owners and managers understand that if the amount of outside fresh air being brought into the building by the HVAC system is reduced, air quality will decline quickly, and worker production will drop dramatically.
The key is in improving energy performance while maintaining occupant comfort. Tenants have a choice of where to lease commercial space. In many cases, the difference between signing a new lease and moving from the building is determined by the experience the tenant has had in dealing with building staff on HVAC issues.
Designing for Comfort
When designing HVAC control systems, consider the following:
- RH does not adversely affect comfort as long as it is between 30 percent and 60 percent.
- Air temperature and MRT should be designed and programmed together.
- Tempered air is often introduced into the space near outside walls and windows to minimize the cold wall effect.
- Air velocity is not important if it is less than 50 feet per minute. Most systems are well below this figure.
- Short cycles of temperature and humidity changes are sources of discomfort and should be avoided.
To produce and maintain the desired environment, the designer must consider various factors, such as geographical location, weather conditions, building use, occupant activities, architectural details, and heating/cooling sources, to name a few. Most of these factors are addressed in HVAC literature and technical guides.
The architectural aspects of wall construction, insulation, window sizes and types, floor plan, and sun orientation all affect heating and cooling loads. These architectural details also affect how quickly a thermal exchange occurs within the structure and, to some extent, control the HVAC system’s ability to provide uniform conditions in response.
The heating/cooling source, the distribution means, and the HVAC system controls are within the system designer’s province. By carefully selecting these elements, the designer creates thermal comfort for building occupants. During office reconfigurations and in situations with high churn, the original design intent must be kept in mind.
Understanding the basic fundamentals of HVAC systems and the factors affecting human comfort is important to providing a healthy, productive environment in your building.
This article is adapted from BOMI International’s The Design, Operation, and Maintenance of Building Systems, Part I course, part of the RPA and FMA designation programs. More information regarding this course or the BOMI-HP™ credential is available by calling 1-800-235-2664. Visit BOMI International’s website, www.bomi.org.