Technical Updates

Posts Tagged ‘HVAC systems’

Shortly after construction was completed, a seven-story, four-star hotel in Charleston, South Carolina, developed severe moisture and mold problems. The investigators attributed the problems to rainwater intrusion through the hotel’s exterior brick veneer. Following that diagnosis, the hotel owner spent more than $10 million on renovations, including a completely redesigned and reconstructed building envelope.

The summer after the renovations were completed, the moisture and mold problems returned. While focusing on the envelope leaks, the investigators had overlooked the significant secondary source of moisture: outside air infiltration.

In areas like South Carolina, where hot, humid conditions persist, IAQ problems are largely due to a combination of high ambient moisture, improper interaction between the building envelope and the HVAC system, and misapplication of design and operation principles.

1) High ambient moisture – Given the high ambient moisture levels in humid climates during the summer months and the dehumidification limitations of many AC systems, excessive moisture accumulation within buildings and the resulting microbial growth are understandably major problems. Microbial-related IAQ problems in buildings can also occur in temperate climates, although more serious errors in the design, construction, or operation of a building normally must occur for such problems to develop in these areas. Cold climates are just as susceptible to moisture problems as hot, humid climates, and building envelopes must be designed accordingly. Many microbial problems in temperate climates are more commonly a result of water intrusion (rainwater and subsurface water) through breaches in the building envelope system, including subsurface envelope systems.

In all climates, anything that elevates the indoor RH or results in damp materials (leaky pipes, for example) for an extended period can cause microbial IAQ problems. Landscape irrigation systems, indoor swimming pools, and building humidification systems can provide enough moisture to create microclimates and microbial growth problems, even in dry climates. Buildings in Boise, Idaho; Denver, Colorado; and Kona, Hawaii have all been hit with severe IAQ problems from microbial growth as a result of introduced moisture, despite the fact that they are considered arid climates.

To be continued…

According to ASHRAE, a humid climate can be defined as one in which one or both of the following conditions occur:

1) A 67 degrees Fahrenheit [20 degrees Celsius] or higher wet bulb temperature for 3,000 hours or more during the warmest six consecutive months of the year.

2) A 73 degrees Fahrenheit [23 degrees Celsius] or higher wet bulb temperature for 1,500 hours or more during the warmest six consecutive months of the year.

This definition is somewhat problematic. First, it is difficult to interpret and apply to problem solving. Second, high dew-point conditions can also indicate areas where moisture problems occur. Atlanta, Georgia, for example, does not qualify as a humid climate under the ASHRAE definition, but high dew points are experienced in this area and problem buildings are often found there.

Industry experience with building failures suggests the need for a new definition of humid climates that more clearly identifies the geography where problem buildings are more likely to be found, and better explains why these problems occur at all. This new definition is based on observations about latent and sensible load: A humid climate is defined as one where the average monthly latent load of outside air meets or exceeds the average monthly sensible load for any month during the cooling season. (Latent load is the moisture in outside air that is brought into the building and requires removal via dehumidification. Sensible load is the air temperature that is sensed and addressed by the HVAC system, either by heating or cooling the air, to reach the established set point.)

Infiltration of air with a high latent load will cause moisture to accumulate in building materials such as gypsum wallboard, with subsequent material degradation and mold growth. This infiltration may also exceed the ability of the HVAC system to remove moisture from the supply air. On any given day in many temperate areas, the latent load may be greater than the sensible load without causing problems; however, when these conditions persist for a longer period (a month, for example), the resulting moisture accumulation is sufficient to cause building failure.

The occurrence of a high latent load during the cooling season is a critical factor in building failure. Thus, defining hot, humid climates in terms of the relationship of sensible to latent load in ambient air expands the ASHRAE humid climate zone to include other parts of the  United States that are highly susceptible to moisture-related building failures.

To be continued…

In the summer of 1988, construction of a large luxury resort was coming to a close. Because the vinyl wall covering on the interior side of the exterior walls had an impermeable finish, it functioned as a vapor retarder (also referred to as a vapor barrier). The HVAC system consisted of a continuous toilet exhaust and packaged terminal air-conditioner (PTAC) units. The outside air exchange rate in each guest room averaged six times an hour, all from infiltration. In this case, problems developed inside the building and inside the wall.

The combined effect of excessive outside air infiltration and an improperly located vapor retarder caused $5.5 million in moisture and mold damage, even before the facility was opened. If these same design combinations had occurred in a more temperate climate, the problems would have been limited to increased energy consumption and possibly to complaints about guest comfort.

This is one example of how hot, humid climates present unique challenges that are often overlooked by the design and construction community. However, challenges also occur for buildings located in other climates. Meeting these challenges depends on understanding a building’s local climate conditions and how those contribute to IAQ problems.

Cold climates offer challenges for moisture flow through the building envelope that are similar to those in hot, humid climes. Cold climates are defined by the American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE) as those that experience at last 4,000 heating degree days (HDD at 65 degrees Fahrenheit [18 degrees Celsius] base) per year. Most problems occur during the winter, when the warm and relatively moist interior air is forced (due to high differential vapor pressures between indoors and outdoors) to the dryer and colder outdoor conditions. Moisture flow can be trapped and condensed on an improperly located vapor retarder. In addition, if the building is air-conditioned during the summer, the wall systems designed to address the heating condition can experience moisture damage inside the walls during the air-conditioned months. Therefore, few locations in the United States are completely free of potential moisture problems.

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Mold is an important part of the earth’s ecosystem, breaking down dead organic matter. For fungal growth to occur, four elements must be present: fungal spores, nutrient sources, appropriate temperature, and water. Of these four elements, water is the easiest to control in occupied buildings.

It is estimated that more than 1.5 million species of fungi exist. Fungal spores are ubiquitous; they are found in indoor and outdoor air, and on, and imbedded in, the surfaces of building materials. Non-HEPA (high-efficiency particulate air) filters, which are found in many building HVAC systems, cannot remove fungal spores from the air because of the spores’ very small size. These spores tend to disperse through the air and settle on all building surfaces, where they can remain dormant for years. It is impossible, or at best impractical, to remove fungal spores from the indoor environment.

Nutrient sources are any organic materials. The paper surface of gypsum wallboard is a prime nutrient, because it easily absorbs moisture. Even on inorganic materials, such as the vinyl in furniture, nutrients exist in settled surface dust. Such nutrient sources cannot be easily eliminated from the indoor environment.

The temperatures that are best for humans are also ideal for fungi. Different species of fungi have different optimal temperature ranges. However, in general, fungi grow well between 40 degrees Fahrenheit (4 degrees Celsius) and 100 degrees Fahrenheit (38 degrees Celsius). (Some can survive at temperatures down to approximately -23 degrees Fahrenheit [-30 degrees Celsius] or up to approximately 140 degrees Fahrenheit [60 degrees Celsius].) Because the comfort range for people is well within the comfort range for fungi, modifying temperature is not an option for controlling mold growth. 

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The HVAC system is typically designed to control the temperature inside a building and, as a by-product, also may control relative humidity (RH). In addition to keeping most people comfortable, the HVAC system should also help control contaminants in three ways: by filtration (filtering contaminants out of the air before they reach the building occupants); by ventilation (diluting the contaminants in the air by adding fresh outside air); and by pressurization (maintaining the right pressure balances between building spaces to keep contaminants from moving into the wrong place). If the HVAC system fails to operate properly, IAQ problems usually occur.

Pathways involve both a route for contaminants to travel through a building and a mechanism like air pressure to push the contaminant along that route. Pathways are affected by the building design, the operation of the HVAC system, and the building use.

Building occupants who spend an extended period of time (an eight-hour work day, for example) in a building are likely to report symptoms when IAQ problems occur. They are a good barometer of the health of a building.

All four factors combine to create IAQ problems. A change in any one of them can cause a dramatic change in the types of problems and symptoms that occur.

A large office building in Los Angeles illustrates this interaction. Workers in one section of the building were exposed to chemicals, including paints and adhesives, from another section of the building that was being renovated. The fumes were migrating to the workers’ area through the HVAC system that served both areas. The workers sued the building owners and managers, as well as the contractors, product manufacturers, and installers, and won a large financial settlement. If the building owner or manager had been aware of the four IAQ factors and taken proactive measures, the problem could have been easily avoided. For example, the pathway or pressure that enabled the chemicals to reach the occupants could have been removed by setting up a temporary exhaust system in the renovation area and blocking the return vents to the building’s HVAC system. These simple steps would have prevented the chemical fumes from getting into the common HVAC system where they could travel to the occupied areas of the building.

To be continued….