The deleterious effects of poor indoor-air quality (IAQ) on health, comfort, and performance are fairly well-known. Perhaps in no other buildings are they more difficult to combat than schools, for several reasons:
Schools require high-density occupancy. Students in elementary and secondary schools typically occupy 500-sq-ft rooms at densities of 20 to 25 sq ft per student. Complicating matters is that school schedules allow little or no opportunity for students to relieve their surroundings if they sense they are having an allergic reaction. Additionally, teachers and other members of school staffs may not understand the nature of students' complaints that they are tired or “don't feel good.” The U.S. Environmental Protection Agency (EPA) reports that asthma, often triggered by indoor exposure to mold, causes 10 million missed school days a year.1
Schools are easy targets when it comes to cutting taxpayer expenditures. During the energy crunch of the 1970s, some of the greatest travesties in HVAC history were committed in schools. “Strategies” for conserving energy included the elimination of outside air, the removal of filters, and the shutting down of HVAC systems during occupancy. Today, systems are shut down for long unoccupied periods (e.g., winter and summer vacations), including when outdoor humidity is high. In the long run, such decisions result in toxic reactions among students. According to the EPA, in 1999, IAQ was reported unsatisfactory in about one in five public schools in the United States, while ventilation was reported unsatisfactory in about one in four.1
Schools tend to have a high ratio of foundation footprint to floor area. Built with access and security foremost in mind, most schools are one- or two-story structures. As a result, a significant amount of interior space is exposed to surfaces at ground (cool) temperatures. Moisture is drawn to these spaces and difficult to evaporate into the building air.
Children tend to be more susceptible to allergens. Unlike adults, children tend not to know what makes them sick or how to avoid it. Additionally, fewer drugs are available to help reduce their sensitivities. That exposure to an allergen may not affect an adult staff member the way it does a student can make a student's claims of feeling sick all that more difficult to understand.
This article will discuss methods of controlling moisture — both that introduced by outside sources, such as ventilation air, and that introduced by inside ones, such as the human body, clothing, and cleaning — in schools through central HVAC systems.
Traditionally, the control of humidity admitted with ventilation air has been addressed during design, with systems that cool ventilation air for summer air conditioning, causing moisture to condense on cooling coils. As the theory goes, the cool air will be heated by building occupants, interior lighting, and solar loads, resulting in 40- to 60-percent relative humidity in interior spaces. While this process can be traced on a psychrometric chart and, given the right coil leaving-air temperature, shown to be feasible, based on mold lawsuits alone, engineers in humid climates know it is only marginally effective (at best).
For outside ventilation air to be used effectively in the control of building humidity, it must be subcooled and reheated or chemically dehumidified with desiccants. No other method takes enough moisture from air streams to make up for internal gains from people, cleaning, etc. This is the case especially in schools and other spaces with high occupant loading.
When outside conditions are warm and air is dehumidified as part of mechanical cooling, reheating and the resultant decrease in relative humidity are assumed to occur uniformly throughout an occupied space. Nothing, however, could be further from the truth. The air that is introduced into a space mixes (somewhat) with overhumidified air and condenses on cool surfaces (such as floor slabs and lower wall corners with low local air circulation). The air then is drawn up into the return-air plenum, where it absorbs the heat of the fluorescent-light ballasts and returns to the air-handling unit to be recombined with humid outside air.
Note that saturated supply air does not have to condense water into puddles to cause mold growth. It can be absorbed into carpet and condense between the carpet and foundation slab, creating, in combination with nutrients tracked in from the outside, an ideal environment for fungal growth. When the carpet is cleaned, what are the chances of it being “dried” by saturated air coming, more or less, directly off the cooling coil? The moisture is much more likely to travel downward and condense on the cool slab than evaporate into humid air above.
The same process takes place between gypsum wallboard and its paper covering. In cool lower corners, wallboard is cooled by the slab, and moisture is absorbed through the paper covering, condensing in microscopic droplets. Fungal spores find nutrients in the adhesive that bonds the paper, and mold proliferates in the absence of light. Does “vapor-barrier” paint stop this moisture migration? No, it only slows it. Unfortunately for occupants, mold has all of the time in the world.
The behavior of moist, condensing air moving across a cooling coil is complex. The air exits the coil as a combination of air streams of differing temperatures containing entrained, microscopic water droplets. Its relative humidity is 90 to 100 percent, meaning it will condense on surfaces cooler than the discharge temperature. These include duct surfaces cooled by adjacent chilled- or domestic-water piping, the foundation slab, or an air leak to the outside. As for the entrained, microscopic water droplets, they will coalesce on steel or aluminum ductwork at the first sharp turn leaving the coil. A portion of the condensate will flow into the drain pan and, if the drain was installed properly (few are), flow safely to waste.
Saturated supply air is dangerous in that if anything goes wrong, moisture has nowhere to condense but in ductwork. Wet duct liner never dries. Mold growing between it and cool metal duct can go undetected, spreading microorganisms throughout a building, for years.
The lesson to be learned is that effective dehumidification of outside air is active mechanical or desiccant dehumidification of outside air. If a desiccant wheel is used, outside air should be dried to the point that when it is mixed with return air and cooled to the discharge set point, there is no condensation on the cooling coil. Energy savings will accrue from the elimination of latent heat load on the coil and refrigeration equipment. Will these savings offset the energy used by the desiccant wheel? No; the basic laws of entropy still apply. The best one can do is to minimize the additional energy through careful design and make sure mold growth is prevented.
Alternately, dehumidification can be accomplished by subcooling supply air with a cooling coil and reheating it to the discharge set point. The reheat coil can be a steam/hydronic coil, a heat-recovery coil, or a “hot-gas” coil connected to the condenser side of the refrigerant circuit. Heat-recovery and hot-gas systems add complexity and maintenance, but decrease the energy required for dehumidification.
But what happens during unoccupied shutdowns? As part of a commissioning project, my office recently reviewed plans for a high school utilizing water-source heat pumps. The design team specified a heat pump employing a hot-gas reheat coil that allows supply air to be subcooled and reheated to well below saturation. Both the subcooling and reheating were to take place in the same self-contained cabinet, and the use of hot-gas reheat was to minimize energy consumption. Unfortunately, the sequence of operation was written to shut down the entire system during off hours, allowing moisture to migrate to cool spaces, condense, and foster mold growth. The solution was to use both humidistats and thermostats to control the heat pumps. During occupied hours, the thermostats have primary control, while the humidistats activate dehumidification via the hot-gas reheat coils when needed. During unoccupied hours, the heat pumps are controlled primarily by the humidistats, cooling is locked off, and heating is in a setback mode. In this manner, dehumidification is continuous on an as-needed basis. Does this require more energy than simply shutting down the entire system when the school is unoccupied? Yes. Would the school rather go to court to fight a mold lawsuit? Probably not.
Returning to the issue of water-source-heat-pump control, some promising advances have been made in dehumidification via refrigerant reheat. Notably, combinations of liquid and gas reheat that greatly enhance control and effectiveness are now available. In the past, the application of heat-pump systems suffered because the control and temperature differential required for positive dehumidification was lacking. The short-cycling of oversized heat pumps causes condensed moisture to be re-entrained into air streams during extended off cycles and reduces dehumidification effectiveness. Improved hot-gas/liquid reheat allows heat pumps to operate in a steady-state manner and continuously wring moisture from air, eliminating re-entrainment during off cycles. This promises to make energy-smart heat-pump systems good IAQ performers.
In humid climates, one of the fastest ways to turn a building into a mold incubator is to shut off the outside-air ventilation system and leave restroom and other exhaust fans on during unoccupied hours. The resulting negative pressure will pull humid air into the building walls and interstitial spaces. Moisture in the air will condense on cool interior surfaces either after the cooling cycle that same day or immediately after the cooling system is started the next day.
The best approach to building-shutdown periods is to install positive dehumidification for outside air and leave that system on at low capacity during unoccupied hours, with heating/cooling locked off unless required by outdoor conditions. This fits with variable-air-volume systems and can be tailored to the needs of a building over time.
As the public becomes more aware of IAQ issues, the positive, functional control of building humidity needs to be a specific, commissioned goal of projects. It needs to be stated clearly in the design-intent section of control specifications, followed through on in the basis-of-design portion, and field-tested. Assuming it will happen on its own is a mistake.
EPA. (2002, October). Indoor air quality tools for schools program: Benefits of improving air quality in the school environment (EPA-402-K-02-005). Washington, DC: U.S. Environmental Protection Agency. Available at http://www.epa.gov/iaq/schools
A member of HPAC Engineering's Editorial Advisory Board, Ronald J. Wilkinson, PE, is vice president of operations, commissioning services, for Dome-Tech Engineering, a full-service field- and design-engineering firm specializing in HVAC and site utility systems.