Facilities search for answers in their efforts to prevent too little or too much humidity

— By James Piper

Controlling humidity in building spaces once was limited to specific areas, such as computer rooms where high levels of humidity could corrode electrical contacts and too-low levels could result in damaging static electricity. But growing concerns over building indoor air quality (IAQ) and an awareness of the role that relative humidity plays in health, comfort and productivity has made humidity control an important issue in most commercial and institutional facilities.

Many engineering and maintenance managers today are upgrading their facilities’ HVAC systems to include humidification and dehumidification equipment. As a result, humidity-control equipment is becoming common in a wide range of facilities, particularly in educational and health care facilities.

The most easily recognized problem with humidity control occurs when humidity levels are too high in facilities: Moisture condenses on cold surfaces. Molds and mildew form on damp surfaces, such as ceiling tiles, carpet, and upholstery. Wood and other materials warp.

But too little humidity also can be a problem. When humidity levels are too low, static electricity increases, high-speed printers can experience paper jams, and building occupants more often complain of eye dryness and respiratory irritation.

To prevent these and other problems, a building’s relative humidity must be kept within a limited range. During summer months, the ideal humidity range is 40-60 percent, and during winter months, between 20-30 percent. Without effective controls, building relative humidity typically is 70 percent or higher during the summer, and 10-15 percent in the winter.

The good news for managers is that it is relatively easy to regulate building relative humidity in the desired ranges. The technology to accomplish it has been available for years. Advances in controls technology have created significant improvements in the operation of these systems, resulting in systems that perform better, are easier to operate and require less maintenance.

But before investing in humidifier and dehumidifier systems, managers need to identify the sources of humidity problems in their facilities, and when possible, eliminate those problems at the source.

Causes of high humidity
A leading cause of high humidity levels is an undersized HVAC system. During the summer months, undersized systems cannot sufficiently cool the spaces and dehumidify supply air. While the systems might be able to maintain the desired space temperature, they will not be able to sufficiently dehumidify supply air. As a result, air supplied to the conditioned space is close to 100 percent relative humidity, and little or no dehumidification takes place.

Maintenance personnel often attempt to compensate for undersized systems by increasing the fan speed, which raises airflow to conditioned spaces. But increasing the airflow has exactly the opposite effect on a space's relative humidity. Higher airflow results in higher supply-air temperatures and even greater moisture content of the air, since the cooling coil could not sufficiently cool the air to condense the moisture from it.

Even worse, a higher airflow can lift moisture from the cooling coil and carry it downstream in the ductwork, creating damp areas that encourage the growth of mold and fungi. Careful analysis and redesign of the system often can correct problems linked to undersized systems.

Another cause for high humidity levels in buildings is poor maintenance. For example, cooling coils can become clogged with dirt, decreasing their surface area, cooling capacity and dehumidifying capacity. Condensate pans can become clogged, allowing condensate from coils to accumulate, and flood ductwork; further increasing the humidity level and creating additional IAQ problems.

Improperly set-up and balanced HVAC systems do not provide the airflow required to keep relative humidity levels within the desired range. Wind-driven rain can accumulate in outside air intakes that are not properly shielded.

If the humidity level suddenly becomes an issue in a particular area, the first step to take is to closely assess the HVAC system’s condition. Only when maintenance issues have been ruled out should other actions be taken.

High humidity levels also can be the result of high internal moisture loads. Buildings change. As they change, the loads on the building HVAC systems also change. Unless those systems are modified to meet those changed conditions, it is possible that the system will not be able to properly control the relative humidity.

Finally, leaks can be a source for moisture that contributes to high levels of humidity. Roof leaks, basement wall leaks, equipment leaks and leaks in piping all can lead to additional moisture being introduced into occupied spaces. Again, if excess humidity becomes a problem in a particular area, managers should make sure inspectors survey the building envelope and all mechanical systems to see if they are the sources of the additional moisture.

Correcting high humidity
The existence of properly designed and maintained systems does not guarantee facilities can avoid humidity levels that are too high. In those cases, managers need to add dehumidification equipment to the HVAC system. Facilities traditionally use one of two basic types of dehumidification systems — mechanical refrigeration or desiccant-based.

Mechanical refrigeration-based systems are the most widely used systems. They cool a portion of the building's supply air below its dew point, causing moisture to condense out of the air. Since the air is then at nearly 100 percent relative humidity, it must be mixed with warmer air or reheated before it is delivered to the conditioned space. Mechanical refrigeration systems offer the advantage of being able to closely regulate the relative humidity within a building. Their primary drawback is their energy cost.

Desiccant-based systems use a wheel or drum coated or filled with a material such as silica gel that absorbs moisture. Air supplied to the building passes across one portion of the wheel, where moisture is absorbed. As the wheel rotates, it passes through a second, heated airflow. As the wheel rotates through this airflow, it releases moisture absorbed by the desiccant, thus regenerating it for another cycle.

Desiccant-based systems are gaining in popularity due to their relatively low energy requirements, but their primary disadvantage is their size, as desiccant wheels and drums tend to be large.

Causes of low humidity
Low humidity is a natural byproduct of heating systems. The humidity in outside air, even if it is saturated, falls as a result of heating. The more its temperature rises, the more its relative humidity falls. The result is interior space conditions where relative humidity regularly falls as low as 10-15 percent. In buildings with high rates of infiltration, the additional air entering the conditioned spaces can lower relative humidity levels even more.

Compounding the problem is that most HVAC control systems are designed to control temperature, not humidity. As a result, humidity levels vary widely in conditioned spaces.

Correcting low humidity levels
Controlling the amount of outside air brought into a facility will improve relative humidity levels, particularly when outside-air temperatures are low. But limiting the volume of outside air can adversely impact IAQ. Also, some facilities, such as health care facilities, have specific requirements governing the rate at which outside air must be brought into the facility. So most facilities must install humidification systems to increase building relative humidity during heating seasons.

Facilities tend to use two basic types of systems to increase relative humidity — adiabatic and isothermal.

Adiabatic — also known as evaporative systems — evaporate water into the airstream. They require no additional energy to heat the water for evaporation. Rather, they inject water in micron-sized droplets small enough that they readily evaporate in the airstream. Various system types are available that use atomizing nozzles, rotating discs and ultrasonics to inject the moisture into the airstream. Adiabatic systems offer the advantage of low energy cost. Their biggest drawback is that they have a practical upper limit on the relative humidity they can produce — typically 30-50 percent.

Isothermal systems use an external heat source to generate steam, which is then injected into the airstream. Units are available as packaged electric or steam-driven systems. Since the steam injected into the airstream is generated within the unit itself, there is no problem with boiler feedwater chemicals typically found in most steam systems.

Isothermal humidification systems offer the advantages of low first costs and low maintenance costs, and they can increase relative humidity to almost any value. Their primary disadvantage is the energy costs associated with the steam generation.

Controls
No building humidification or dehumidification system can function properly without a good control system. Fortunately, humidity control systems have moved from a variety of organic and inorganic sensors used in the past to solid-state controls. Today, humidity controls are based on thin film capacitance or bulk polymer sensors that provide high sensing accuracy over a wide range of humidity conditions.

Humidity control systems can even use multiple sensors located throughout a facility to provide a more accurate picture of the conditions. As a result, today's systems can provide a healthier indoor environment for equipment and building occupants.

— James Piper, P.E., is a Bowie, Md.-based facilities management consultant with more than 20 years of experience.