When building managers are considering hiring HVAC engineers to make improvements for their HVAC systems, energy efficiency tends to receive more attention that indoor air quality (IAQ). This can be explained by the fact than saving energy impacts the bottom line directly, while the benefits are not so evident for indoor air quality. However, IAQ can be just as important as energy efficiency, if not more, and has a significant impact on productivity, comfort and most importantly human health. A key example of what poor air quality can do is Sick Building Syndrome, when the health of occupants deteriorates in proportion to how much time they spend indoors and improves when they are away.
To improve indoor air quality, two complementary approaches are necessary, and HVAC systems play a very important role in both cases:
- Maintaining the physical properties of indoor air at values that are optimal for human occupancy. The three main variables to control are temperature, humidity and air speed. In the case of temperature, it is also important to ensure stability – if the average temperature value is as required, but fluctuates between warm and cool, it can cause both discomfort and health issues.
- Keeping indoor air free from hazardous pollutants such as carbon monoxide, radon, mold spores, dust, pollen and volatile organic compounds (VOCs). These compounds must be monitored and eliminated. Unpleasant odors should also be controlled; they don’t have the same health impact as direct exposure to pollutants, but can cause great discomfort.
Indoor air quality benefits from a properly designed HVAC system and using sensors that detect pollutants. Of course, all of this should be complemented with habits that enhance IAQ, and there are various plant species that improve IAQ by removing common pollutants.
Controlling the Physical Properties of Indoor Air
The functions of an HVAC system go far beyond heating indoor air in the winter and cooling it in the summer; it is also necessary to control humidity and air speed to provide comfortable and healthy conditions for humans.
Issues with air speed and relative humidity often arise when HVAC systems are designed by HVAC engineers with only temperature in mind. Oversized cooling equipment tends to cause excessive humidity, while oversized heating equipment tends to dry the air more than necessary. In both cases, indoor air draftiness is also common because fan horsepower is too high for the application.
Temperature issues are common when heating or cooling equipment is oversized, since the unit tends to run on a short cycle and causes the temperature to fluctuate above and below the desired value. Properly-sized HVAC equipment provides a more constant air temperature and lasts longer, since it is not subject to the wear caused by frequent cycling.
ASHRAE recommends that indoor air relative humidity be kept at values within the range of 30 to 60 percent. When relative humidity falls outside this range there are several consequences that are detrimental for indoor air quality:
- Low relative humidity tends to irritate the respiratory tract, eyes and skin.
- On the other hand, high relative humidity can cause air conditioned spaces to feel cold and humid like a refrigerator, which causes discomfort and potentially health issues. Humidity also stimulates the growth of fungi and bacteria.
- Viruses are notorious for their ability to proliferate at both low and high humidity values. The optimal relative humidity range recommended by ASHRAE (30 to 60%) minimizes the spread of viral infections.
As stated before, these issues can be prevented in great part with proper HVAC design. If the existing HVAC installation is only capable of controlling temperature properly, it can be complemented with an air dryer or humidifier.
Using “rules of thumb” in HVAC design almost always causes issues with oversized equipment, inadequate humidity values and drafty indoor environments. Systems end up being more expensive than necessary while performing poorly.
When indoor air moves at excessive speed, it generally causes two types of problems:
- Air ducts can become noisy, which is detrimental for concentration.
- Fast-moving air enhances thermal sensation. For example, if the air in a room is being cooled more than necessary, fast-moving air will only make it feel colder.
Air speed can be controlled by sizing HVAC equipment properly, especially fans and air-handling units. For greater effect, speed control measures can be deployed, ensuring the airflow provided matches demand. Variable frequency drives are effective for controlling fans above 1 HP, while electronically-commutated motors (ECM) provide the best speed control solution for fractional horsepower motors.
HVAC System Integration
Ideally, an HVAC system should keep temperature and relative humidity at values that enhance comfort and prevent the proliferation of fungi, bacteria and viruses. At the same time, speed control for ventilation equipment minimizes noise and prevents uncomfortable air drafts. Once this point is reached, the next step is eliminating air pollutants.
HVAC Engineers Must be Mindful of Controlling Indoor Air Pollutants
ASHRAE has determined that pollutant concentrations are 2 to 5 times higher indoors than outdoors, and this applies even for urban areas with heavy traffic. Modern humans spend most of their time indoors, and therefore it is important to ensure that pollutants are kept under check.
One of the most important steps to prevent the accumulation of indoor air pollutants is meeting the ventilation requirements as set forth in building codes. The NYC Mechanical Code establishes the requirements depending on the type of occupancy, generally in terms of cubic feet per minute (CFM) or air changes per hour (ACH). These values are often specified in terms of the number of occupants.
Silent Killers: Carbon Monoxide and Radon
Although many compounds are detrimental for indoor air quality, there are two that require special attention due to how damaging they can be for humans: carbon monoxide and radon.
- Carbon monoxide is an odorless and colorless combustion product, making it impossible to detect with human senses alone. However, it can cause nausea and dizziness within minutes, or even coma and death if the concentration is high enough. To prevent carbon monoxide leaks, it is very important to ensure that the exhaust systems of combustion-based appliances are properly designed. Sensors should also be deployed to detect leaks, since the gas is impossible to see and smell.
- Radon is a radioactive gas that leaks out of the ground and is also colorless and odorless. Radon does not have drastic immediate effects on humans like carbon monoxide but can be as damaging as cigarette smoke in the long run – the only difference is that radon is invisible and lacks the unpleasant smell, making it more difficult to avoid. Measuring radon concentrations in indoor air is the only surefire way to detect the gas. It is also important to note that radon concentrations change throughout the year, so one-time measurements are not enough to keep it under watch. Radon concentrations are site-specific.
Other notorious air pollutants to monitor include allergens such as dust, mold and pollen; as well as volatile organic compounds (VOCs), which are often found in artificial fragrances, aerosol sprays and other cleaning products.
Mold tends to grow very fast whenever there is accumulated moisture, especially in poorly-maintained air ducts. The best way to control mold is to control humidity, since it is required by spores.
General Measures to Control Indoor Air Pollutants
Indoor air pollutants can be controlled in great part with a properly designed ventilation system. It also helps to implement habits that reduce the presence of pollutants, while using plants that can absorb them.
The department of buildings provides a series of guidelines to enhance a ventilation system’s ability to remove pollutants and odors from indoor spaces. The following are some general recommendations:
- Some areas are notorious for their release of odors and compounds, and the extraction system serving them should be designed to exhaust the air directly outdoors, without passing through other areas. Some examples of areas that require dedicated extraction systems include generator rooms, kitchens, bathrooms and laundries.
- Designing the ventilation system to keep a slightly positive pressure also prevents the accumulation of pollutants that may leak in from outdoors or from other building areas. Likewise, a poorly-designed ventilation system that does not account for negative pressures can have the opposite effect, drawing in odors and pollutants from the surroundings. Positive pressure is very effective for preventing the ingress of dust and pollen.
In addition to properly designed HVAC systems, volatile organic compounds (VOCs) can be controlled in great part by reducing the use of products that release them. For example, aromatic plants can be used instead of synthetic spray fragrances. Dry cleaning processes normally impregnate clothes with VOCs, so it is recommended that dry-cleaned clothes be kept outside for a while, to let the VOCs dissipate.
Many plant species are known for their ability to filter harmful compounds from the air. NASA has studied this subject to determine how plants can improve air quality in space stations, and has concluded that the following ten are solid choices:
- Aloe Vera
- Peace lily
- Spider plant
- English ivy
- Boston fern
- Heart leaf philodendron
- African violet
- Chinese evergreen
All these plants are effective pollutant filters, and can greatly improve indoor air quality if present in residential and commercial buildings.
Indoor air quality starts with a proper HVAC design, capable of controlling humidity to prevent the proliferation of fungi, viruses and bacteria. It is very important to meet airflow requirements as specified in the applicable building codes, and positive pressure can help keep airborne particles out of occupied spaces. Experienced HVAC engineers know the ins and outs of these matters. Some air pollutants can be lethal, and their presence must be monitored on an ongoing basis; carbon monoxide and radon are the two main examples.
It is also possible to improve IAQ through passive measures, such as the use of plants that are capable of filtering toxins and pollutants from the air. The use of cleaning products that emit VOCs should be minimized as well.