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Residential Ventilation
Heating, Ventilating and Air-Conditioning Systems in Residences

Heating, Ventilating and Air-Conditioning Systems in Residences What ventilation is Infiltration Natural ventilation Mechanical ventilation Reducing indoor air problems in the home Local exhaust ventilation General ventilation Problems that can be reduced by using mechanical ventilation Combustion pollutants Radon Problems associated with ventilation systems Air furnace filters Cleaning ducts Purchasing a mobile home What Ventilation Is Infiltration Natural ventilation Mechanical ventilation Ventilation is the process of bringing clean, fresh (outdoor) air into the house and removing stale indoor air from the house. This is done by clean infiltration, natural ventilation, and mechanical ventilation. Insufficient ventilation is one of the main causes of poor indoor air quality, because the lack of clean, outdoor air can result in the build up of pollutants, irritants, and odors that may adversely affect the indoor environment. Without ventilation, the pollutants and odors cannot be flushed out of the home. Infiltration: This is when outdoor air gets into the house through openings, cracks in the walls and floors, and around windows and doors. This type of airflow may not provide enough ventilation, can make a house feel too hot or too cold, and can increase energy bills. If air is being exhausted out of the home or being burned up by combustion appliances (such as gas burning stoves and fireplaces) and the windows are closed, then infiltration is the only way that makeup air can get into the house. Often it gets into the house by being sucked in because the house is under negative pressure. There are two major problems with a home under negative pressure: Makeup air is from soil gas: In this case, the makeup air is sucked into the house from the ground. If the ground contains radon or is contaminated because of chemicals from an unknown hazardous waste site (like Love Canal) or is near a leaking underground storage tank, these toxic contaminants can be drawn into the air inside. Additional moisture can also be brought into the house this way, which can lead to the growth of mold and dust mites. Backdrafting: Since air will enter through the path of least resistance, it is possible for air to enter a home by coming in through the chimney. This can cause combustion products such as deadly carbon monoxide to build up inside the home. Natural ventilation: This is when air moves into the house through open doors or windows. Though natural ventilation can improve indoor air quality, it has several limitations. These are:¹ It is an inefficient means to dilute contaminants in a home because the air is not uniformly distributed. The flow cannot be controlled. Increased ventilation from outside can increase pollen and mold spore levels, if outdoor counts are elevated, and moisture. If there is not much wind outside and not much temperature difference between indoors and outdoors, there will not be enough ventilation in the home when windows are opened. If there are high winds outside or big temperature differences between indoors and outdoors, there will be too much ventilation that can make humidity levels too low inside. This kind of ventilation in the winter also wastes energy by heating outdoor air. If the outside air is humid, it can make the indoor air humid, which can create a favorable environment for dust mites and mold. Mechanical ventilation: Heating and air-conditioning systems with fans are used to bring fresh outdoor air inside, circulate the air through the house, and exhaust stale indoor air outside. Individual fans can also help to add or remove air. For example, properly installed and operated exhaust fans can remove stale, moist air from bathrooms and kitchens. However, there needs to be a way that fresh air can be brought in to make up for the air being vented out. Many homes actively exhaust air but rely on natural ventilation to bring air into the home. A man who had severe asthma installed a powerful exhaust fan in his home to try to get rid of any moisture. However, he did not include any makeup air for the air being exhausted and found that the moisture problem did not improve and his asthma symptoms got worse. To find out why the exhaust fan was not getting rid of the moisture, he brought in somebody to figure out what was wrong. They found that since there was no makeup air being provided, humid air from outside was being drawn in through the walls of the home. The fan had been so powerful that this air went through the vinyl siding on the outside of his house, through polystyrene insulation under the siding, through gaps in the plywood under the insulation that was contaminated with mold from the moisture, through gaps in the sprayed-in cellulose wall insulation, and finally into the living space through the electrical outlets!² When there is little infiltration, natural ventilation, or mechanical ventilation, the air exchange rate (fresh air coming in and stale air going out) is low and pollutant levels can increase. The American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) recommends a minimum ventilation rate of 0.35 ACH (air changes per hour) for new homes. This air exchange rate measures how many times all of the air inside the house is replaced with fresh outside air. If there are not enough air exchanges in the home, then pollutants can build up inside. This can make the indoor air uncomfortable and possibly even unhealthy. Some people may choose not to "tighten" up homes that are "leaky" or that rely upon infiltration to bring in the needed outside air. There are several problems with doing that: Ventilation through infiltration cannot be controlled so there may not be enough ventilation in areas of the home that need it most. As a result, pollutants can build up in even "leaky" homes. Infiltration and natural ventilation rely heavily on outdoor weather so these conditions can drastically reduce the amount of outdoor air that enters a home. "Leaky" homes can cost more in the long run since they are not energy efficient and cost a lot to heat and cool. "Tight" homes are ones that have been weatherized by sealing cracks, having good insulation, and quality windows. If indoor air pollutants are controlled, the use of mechanical ventilation in these homes can be more comfortable since the temperature and humidity levels can be controlled. If homes are tight, pollutants are not controlled, there is an inadequate number of air exchanges, and levels of pollutants can build up inside the home. How to Reduce Indoor Air Problems in the Home Local exhaust ventilation General ventilation The best way to control indoor air quality is to control indoor pollutants at their source. This means doing things like: Using non-toxic, low-emitting, non-fragrant substitutes for indoor chemicals (cleaning, maintenance, hobby, and personal supplies) Not allowing smoking in the home Sealing cracks where moisture or radon could enter. Keeping humidity levels between 40 percent and 60 percent Not idling cars in attached garages Cleaning your home regularly and not leaving out any garbage However, not all indoor pollutants can be controlled at their source. In these cases, ventilation should be used. There are two types of mechanical ventilation: Local exhaust ventilation: This tries to capture pollutants at their source and exhaust them to the outside. One benefit to using local exhaust ventilation (exhaust fans) in kitchens and bathrooms is that they can reduce levels of organic pollutants that vaporize from hot water used in showers and dishwashers. Examples of this type of ventilation equipment include: Range hood over the stove Bathroom fans - There are models on the market that produce little noise. Outdoor vents for clothes dryers General ventilation: This is the ventilation supplied by the heating and air-conditioning system. Air is distributed through the ductwork of the house. The more general ventilation there is, the less concentrated the pollutants are in the home. It is important to have ventilation when pollutant processes occur such as the introduction of new furnishings, indoor remodeling, cooking, pesticide application, cleaning, and use of personal care products. Most modern commercial office buildings have a ventilation system that both brings in fresh air from outside and exhausts the stale air. In homes, this type of system is called an energy recovery unit or ventilator (sometimes called an air exchanger) and is one of the latest tools used in general ventilation. This system brings in fresh air and exhausts stale air without giving up the desired heat or cold in the air that is being exhausted. These quiet devices can be very effective and usually sit next to the heating/ventilation/air-conditioning (HVAC) system. Energy-recovery ventilators can reduce the buildup of moisture and pollutants. The downside to them is that they increase energy consumption. Problems That Can be Reduced b=By Using Mechanical Ventilation Two indoor air pollutants that need good ventilation are combustion pollutants and radon. Combustion pollutants These pollutants come from combustion appliances such as gas stoves, gas dryers and fireplaces. When these burn, they use oxygen. If fresh air is not brought in through mechanical ventilation or through open doors or windows, the house can come under negative ventilation that pulls in air from cracks and from the ground. To prevent a house from coming under negative pressure, combustion appliances should be provided with a source of air from outside the house; new appliances are designed this way. Exhaust fans should also have a source of air from outside the house. This can be done by using a fan with an outdoor air supply duct, or by installing specially designed vent openings in exterior walls. Radon Radon often enters a house by coming up through cracks in the foundation or basement floor. Once inside, radon can get into furnace ducts and be distributed throughout the house. Radon can also be drawn into the living space through openings around pipes in floors. One way that radon is controlled is by means of a plastic pipe that is installed through the concrete floor to the gravel under the slab. The pipe goes all the way to the attic of the house and through the roof. A special fan draws radon gas out of the soil from beneath the house, pulls it upward through the pipe, and exhausts it to the outside. By using this type of ventilation, radon is removed from the soil and sent outside before it has a chance to enter the house. Problems Associated with Ventilation Systems Occasionally there can be contaminants that are caused by the ventilation system. One of the most common of these is mold. The two most common sources of mold in home ventilation systems are: Central humidifiers that are not properly cleaned or installed. There was once a case where a woman hired a contractor to install a new humidifier to moisten the air from her gas furnace. However, the contractor mistakenly connected a water supply line from her water softener. This was a problem because her water softener, like most kinds, uses salt. The salt got into the humidifier and then into the air that was distributed through the air ducts. The heating ducts were made of steel coated with galvanized zinc. The salt condensed in the ducts, which allowed an unusual type of mold to grow that produced spores covered with zinc. The woman, her daughter, and even their pet dog got very sick. Doctors were not able to find out what was causing their illnesses for a long time. Finally, a special blood test found the elevated levels of zinc, but by then the woman had suffered many health problems including internal bleeding, severe anemia, and diabetes. She was declared disabled in 1990.³ Internal fiber glass liners on which mold can grow. Many homes built in the late 1970s and early 1980s have fiber glass-lined ductwork that is now deteriorating and providing a haven for fungal growth and other problems. Fungus can grow in this fiber glass lining since it can hold in moisture. In experiments, fiber glass has been known to hold moisture for up to 16 days.⁴ The reason fiber glass was used was to decrease the noise from the fan and to provide insulation. Not all ventilation systems have these liners. If you do have a fiber glass liner and it gets wet or moldy, you should have it removed. Air Furnace Filters According to a survey conducted by the American Lung Association, 41 percent of Americans fail to replace their furnace filters every two to three months as recommended and nine percent have never replaced the filter in their furnace.⁵ This is one of the easiest ways to keep your furnace in good working order. Common furnace filters are not designed to protect occupants in the residence. These filters are designed to remove large particles from the air to protect blower fans. These filters: Are installed in return air ducts of the furnace or air-conditioner Do not remove gases Are either disposable or washable (metal screen) Should be replaced every month or two, depending on conditions. The airflow will be blocked if the filters are not replaced or cleaned regularly. Are relatively cheap to replace There are other furnace filters that are called extended surface filters. They are denser, thicker, and more effective at removing particles. These filters: Are disposable so they must be replaced regularly Will block air flow if they are not replaced regularly Are more expensive than the common furnace filters Do not remove gases The most protective of furnace filters are high efficiency ones (HEPA). They not only protect the furnace, but also contain electrostatically charged fibers that capture particulates that could be harmful, such as pollen, pet dander and smoke. Only 13% of people use HEPA filters on their furnaces/central air-conditioners though almost 60% of people interviewed by the American Lung Association said they would be willing to spend a little more on an air filter every three months to improve air quality in the home.⁵ However, before installing a HEPA filter, make sure that your furnace/central air-conditioner is strong enough to blow air through this type of filter. Do not select a filtration system that gives off ozone. Ozone is a strong lung irritant and should not be added to the air. Cleaning Ducts With increased awareness about indoor air quality, many people are rushing to have their ducts cleaned as a way to solve their indoor air problems. However, the US Environmental Protection Agency (EPA) has said that there is no proof that duct cleaning actually prevents health problems. That is because there are bigger sources of pollutants in the home than just dirty air ducts. So, before you think about having your ducts cleaned, make sure that dirty ducts are the cause of your indoor air problems. There are times that you should have your ducts cleaned. These are when: You can see a lot of mold growing in the ducts or other parts of the ventilation system. There is vermin, such as rodents or insects, infesting your air ducts The ducts are clogged with so much dirt and debris that it is being released into the home from your supply registers As you can see, if any of these conditions exists, there are much bigger problems present than just dirt in your ducts. For example, what would be causing mold to grow there, or insects or mice to live there? The sources of these problems need to be taken care of first since this would indicate more than just a dirty air duct problem. With the duct cleaning industry expanding to meet demand and using extensive advertising to encourage people to use their services, you need to make sure of the following: That your ducts really need cleaning. That you also take care of any problems that would be causing the ducts to get dirty in the first place. For example, take care of any leaks in the ductwork that would allow dirt to get in. That you check the company's references to make sure they are reputable. That the air duct cleaning company takes proper steps to protect occupants in the residence from exposure to dislodged pollutants and chemicals used during the cleaning process. Such steps may range from using HEPA filtration on cleaning equipment, providing respirators for workers and having occupants vacate the premises during cleaning. Do not treat the ducts that carry air to your living spaces with biocides, paints, or chemicals The EPA just recommends having your ducts cleaned only when it is needed. Purchasing a Mobile Home Because of all the particleboard used in mobile homes that can release volatile organic compounds (VOCs) such as formaldehyde, be sure to ask about all available options for increased ventilation before buying one. Then choose one that will get enough outdoor air circulated throughout the home. Mobile home sellers are required under US Department of Housing and Urban Development (HUD) standards to give prospective buyers a ventilation improvement information sheet before a sales agreement is reached. This sheet must describe available ventilation options and the rated capacity of mechanical systems for bringing in outdoor air. References Fernández-Caldas E, Trudeau WL. Environmental control of indoor biologic agents. J Allergy Clin Immunol. 1994;94(issue 2, part 2):404-412. Turner WA, Bearg DW, Brennan T. Ventilation. Occup Med: State of the Art Reviews. 1995;10(1):41-57. Quinn J. "Homeowners grapple with indoor air pollution; impurities cause ailments from headaches to cancer." The Times-Picayune. November 13, 1999. Samimi BS. The environmental evaluation: Commercial and home. Occup Med: State of the Art Reviews. 1995;10(1):95-118. Air-Conditioning, Heating and Refrigeration News, June 21, 1999. "Survey: What homeowners don't know, may hurt them."