HVAC AND THE ENVIRONMENT
- By Michael Fickes
- 02/01/04
As students returned to school last August, more than a dozen school principals in Guilford County, N.C., phoned the superintendent’s office to report the discovery of mold in their facilities. At the same time, a mold infestation swept through St. Petersburg High School in Florida. Up north in Massachusetts, 50 schools faced off against slimy black outbreaks of mold.
The immediate cause — one of the rainiest summers in memory — raised humidity levels in schools up and down the east coast and created conditions that allow mold to grow. In many cases, heating, ventilating and air conditioning (HVAC) systems fostered the growth of mold by dispersing mold spores throughout the air inside the buildings.
It worked this way. Outside moisture and humidity led to condensation inside school air conditioning systems that had been turned off for the summer. Wet coils, damp ventilation shafts and other soggy components bred the mold. When classes convened and the air conditioning clicked on, blowers pushed mold spores into the ventilation system and spread them through the indoor air. Had the air conditioning been running throughout the summer, moisture would have condensed on the coils, dripped into drain pans and flowed safely away, reducing if not eliminating mold growth within the HVAC system. In addition, the flow of conditioned air would have helped prevent moisture condensation on surfaces inside the school and further reduced mold growth.
To be sure, HVAC systems can’t take all the blame for mold. Architects note that improperly designed or built moisture barriers in walls, leaky windows and open doors can also contribute to moisture build-ups in school buildings. Moreover, carpet and wallboard, which have replaced moisture-resistant tile and plaster in modern schools, absorb moisture and promote mold growth.
Still, expertly designed HVAC systems might have restrained and perhaps prevented the growth of mold in many east-coast schools late last summer.
In fact, HVAC can help control a host of indoor air quality (IAQ) problems in schools, from mold-producing moisture to contaminants given off by cleaning fluids to chemical gases emitted during experiments in chemistry labs. HVAC can also deal with relatively harmless but distracting problems like odors from the cafeteria kitchen.
Controlling Moisture With HVAC
While it’s not often done, an HVAC system can be designed to go beyond basic heating and cooling. Such designs include auxiliary devices that reduce the amount of moisture in the air to acceptable levels.
The Environmental Protection Agency (EPA) recommends considering both temperature and moisture control in HVAC designs.“Air that lacks sufficient moisture causes respiratory problems,” says Bob Thompson, an environmental engineer with the EPA and the acting chief of the EPA’s Research and Development Office for Indoor Environments.“On the other hand, if the air is too moist, mold can grow.”
To control moisture in the air, conventional HVAC designs need the help of auxiliary energy recovery ventilation (ERV). Long available, but little used, ERV devices are available from all major air conditioning vendors.
ERV systems fall into two classifications. The first recovers “sensible” or heat energy from exhaust air and returns it to the system. The second, a total-energy recovery system, transfers both sensible heat energy and “latent” or moisture energy from exhaust air back into the system.
“When properly designed, ERV systems take energy — sensible, latent or both — out of the exhaust air and put it into incoming fresh air,” Thompson says. “So, they save energy and add moisture or eliminate moisture as appropriate.
Since ERV designs depend upon the local climate, the EPA has developed a software tool to help school officials, architects and engineers determine the best ERV design for particular locations and climatic conditions. Called School Advanced Ventilation Engineering Software (SAVES), the free program can be downloaded at: .
SAVES can compare the performance of a conventional HVAC system with ERV methods, Thompson says. The comparison looks at first costs and operating costs for conventional and ERV designs. SAVES also calculates
a building’s susceptibility to mold given a particular climate.
Finally, SAVES estimates a payback schedule for installing an ERV system. According to a map on the SAVES Website, facilities in most regions of the country can recover costs relatively fast. In the Northeast and Midwest, for example, an ERV system will pay for itself in two years or less. Schools installing ERV systems in locations near the Rocky Mountains will earn back the cost of their systems within two to seven years. Payback will take longer in the moderate climates along the Pacific coast. Systems installed there could take seven or more years to pay for themselves. But after the payback period, the efficiencies provided by ERV systems will cut utility costs.
Thompson notes that ERV systems can also reduce the first costs of HVAC equipment by allowing for smaller, less expensive air conditioning and heating components, including ductwork and fans.
Removing Pollutants
Conventional HVAC systems control and remove indoor air pollutants in three ways, according to William Kosik, president of OWP/P Engineers, a subsidiary of the Chicago-based OWP/P architectural firm.
First, HVAC systems exhaust pollutants at their source. Fume hoods in science labs, welding hoods in industrial arts departments, restroom vents and cooking hoods in kitchens vent polluted air directly out of the building.
Second, HVAC systems dilute contaminants that do get into the air by regularly replacing indoor air with outdoor air. The rule of thumb for this is 15 air changes per hour. Depending on the type of space, the number of air changes may rise or fall somewhat.
Third, HVAC creates pressure differentials to contain contaminants in certain areas. High-pressure air flow in a corridor next to a chemistry lab, for example, will force air into the lab and prevent the air in the lab from getting into general circulation.
Mixing Up the Air in Schools
HVAC systems use different techniques to move air into and through different kinds of spaces. In a hospital operating room, for example, designers aim to create a laminar flow of new air that pushes existing air down and out of the space. New air and old air do not mix.
In a typical classroom, designers strive to mix new air with old air. “We think of existing air in a room as water in a bucket,” Kosik says. “The goal is to mix new air thoroughly throughout the room like dye completely mixing into and coloring the water in a bucket.”
To achieve the right kind of mixing, designers select and place diffusers — the devices that dispense air into a room and vents which allow air to escape — in carefully planned configurations. In a gymnasium, for instance, architects select diffusers and configurations that push air to the floor, mix new air with existing air and create a floor to ceiling airflow. In a classroom, different diffusers create a different kind of airflow, one that moves more gently downward.
Ineffective diffusers and diffuser configurations have been blamed for causing sick building syndrome. In some sick buildings, air from diffusers fails to move down into the breathing zone, Kosik says. Left to breathe stale air in tightly sealed rooms, people can grow sleepy from accumulations of carbon dioxide. Worse, contaminants not diluted by fresh supplies of air may develop a variety of mild to severe health problems.
Kosik also notes that operable windows, employed in most school designs, create additional HVAC challenges. Opening windows while continuing to run air conditioners or heaters defeats the purpose of HVAC.
As a solution, Kosik suggests an intelligent HVAC system that senses when a window opens and shuts down the heat or air conditioning being delivered to the space. While automatic shutdowns save energy, Kosik emphasizes the need for overrides capable of switching the air system back on should carbon dioxide levels begin to rise.
Finally, Kosik urges architects and school officials not to sacrifice mechanical room space to solve other design problems. “I’ve seen mechanical rooms where you have to hang upside down to change the filters,” he says. “As a result, maintenance people will take the path of least resistance and fail to change the filters often enough.
“We tend to compartmentalize HVAC design, operations and maintenance. But, they are not independent of each other. A good design can’t overcome poor operating practices and maintenance or vice versa.”
Does IAQ Really Affect Health and Learning?
No one knows exactly how the quality of the air inside a school affects students, faculty and staff. Mold and other airborne contaminants seem to bother some people not at all. Others may become slightly or seriously ill. Many will feel some effects: watery, itchy eyes; joint stiffness; fatigue; nausea; and dizziness.
Asthma sufferers may find their conditions worsened. According to the Center for Disease Control, asthma causes U.S. students to miss 14 million school days per year. Some percentage of those missed days is likely attributable to poor IAQ in school buildings.
According to the EPA, a European study carried out in the late 1990s showed that airborne contaminants may adversely affect the ability of students to concentrate, calculate and memorize. But which contaminants cause such problems? Is it one thing or another? Is it a mixture? At what level do airborne contaminates begin to cause problems? No one knows.
Until credible research data arrives, Thompson says that school officials must take it on faith that IAQ affects absenteeism, student performance and teacher productivity. “We have come to believe that the indoor environment has a much greater impact on teaching and learning than many people think,” he says. “Educators say that improving student teacher ratios, revising curriculum and taking other steps directly related to the education process can improve student performance on tests by three percent. Some of the data we’ve looked at leads us to believe that those gains are small compared to the gains that can be achieved by fixing IAQ in schools. We honestly believe that IAQ can improve test scores in the range of 10 percent.”