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Home Duct System Improvement
Let's face it, considering improvements to your duct system is not as exciting as choosing new tile or kitchen cabinets, but the benefits can be huge. The October 1998 Appraisal Journal reports that the value of a home increases $20 for every $1 decrease in annual utility costs. Translation: If your yearly heating and cooling bills total $800, and you make a change that saves 25 percent ($200), you will realize $4,000 in added value when you sell the house. And that doesn't include the $200 savings you pocket every year until then! Of course you have already added insulation to your attic, replaced your old single-glazed windows, caulked and weather-stripped every crack and joint, and installed a clock thermostat, so what could you possibly do that would save another 25 percent? You can optimize your central air-distribution system (ADS)! In a study concluded in 1990, researchers at the Florida Solar Energy Center found that the amount of energy lost from heating and cooling ducts was even greater than that due to infiltration through the home's walls, ceilings, windows and doors! Even though the holes in the ducts were physically smaller than those in the building shell, they accounted for 70 percent of the total energy loss while the blower was running. This is because the air pressure in duct systems is much greater than the pressure differences across building surfaces, and thus it results in greater airflow. This rather surprising information caused government- and utility-sponsored weatherization programs nationwide to add sealing heating and air-conditioning ducts to their hit lists. Problems With ADS An ADS consists not only of ducts, but every part of the heating and cooling system involved in moving conditioned air. Its purpose is to collect house air; heat, cool, humidify or dehumidify it to comfortable levels; then transport it back to various zones of the house. It should do so efficiently and quietly while providing circulation. The problem is that many, if not most, home systems are just getting by. They work well enough to make the indoor environment tolerable, but at much greater cost than a well-designed and -maintained system. Duct leakage, unbalanced airflow and poor system design affect not only energy efficiency, but health, safety, comfort and building durability. Four ADS problems we will consider here are:
Increased heating and cooling costs,
Poor air-quality control,
Poor comfort control, and
Increased potential for mold and moisture problems. Duct Leakage Much of what we expect from air conditioners cannot be realized if the ducts leak. Leakage negates efforts toward both energy efficiency and good air quality. Imagine paying for the best air-filtration system money can buy, only to have duct leakage downstream of the filter pull polluted air back into the system and distribute it throughout your home. You can't control air quality if you don't have control over where the air flows. Although you may have a brand-new home that is built to be more efficient than older homes, you are still likely to have significant duct leakage according to the most recent research. A 2002 FSEC study of homes no more than one year old concluded that ADSs are still very leaky. In fact, they are approximately 35 times leakier than the Sheet Metal & Air Conditioning Contractors' National Association standards for class-6-duct air tightness. Research in other regions of the country confirms that duct leakage is a nationwide problem. Locating Duct Leakage Building cavities, such as wall-stud spaces and floor-joist spaces, are sometimes used for ducting air. For example, the space between two floor joists can be used as a duct by placing sheet metal over the bottom of the joists. This is called a panned floor joist and may extend all the way to the exterior wall or foundation. It is poor practice because such a space is difficult to insulate and seal effectively. More often, the leakage is due to myriad small seams and joints in a fabricated system. This makes it difficult to pinpoint leakage visually. Even small holes and cracks can result in a lot of leakage if located where high pressures occur. Leaks in the supply ducts (the ducts delivering conditioned air from the furnace or air conditioner to the home) blow air out of the duct and can be detected on your skin, but leaks in the return ducts (those bringing air back to the furnace or air conditioner) suck air into the duct, which is difficult to feel by hand. (Blow air into your hand, then suck air away from it to feel the difference.) Professionals use a special smoke-like vapor to be able to see small leaks. It is well worth having your duct system tested by a certified energy rater or other trained professional who has the equipment and the know-how to map the leaks in your system. The FSEC study also found that, on average, 5 percent of the total airflow through the air handler comes from leaks in the air handler itself - and these were brand new! Many of the leaks were in locations near the fan and after the filter. Not even the best filtration system can capture whatever happens to be in the air around a leaky air handler. Pollution Goes Where Air Flows Return leaks pull air into the system like a pollution highway. A pollutant such as dust or radon in the area of the leak will be sucked in and distributed throughout the house. If any part of an ADS is in an attached garage, don't warm up your car engine in the garage, and store volatile chemicals in an outside shed. Sealing Leaks To be airtight, ducts must form a continuous air barrier from section to section. Duct materials are airtight, so the key is to connect the sections with sealants that will last the life of the building. The old standby cloth duct tape is not a durable sealant. Metal-foil tapes are better, but should be UL181-approved and applied only to clean surfaces. Even these will fail if not applied correctly. Non-hardening, code-approved mastic may last the life of the building if applied correctly. Mastic coatings should be thick, about 4 inches wide, and centered over the duct seams. Again, have a qualified professional seal your system, and then insist on a final test to verify that the total system leakage is less than 5 percent of the maximum system-rated airflow when the duct is pressurized to 25 pascals. Begin your search for a qualified energy rater at www.fsec.ucf.edu, with a click on the Home Energy Ratings section in the lower right corner of the page. Dangers of Unbalanced Leakage Sealing more of the supply leakage than of the return leakage (and vice versa) may result in pressure differentials inside the home. In addition, sealing leaks in ducts that are outside the conditioned space (in an attic, garage or crawlspace, for example) may alter the pressure inside the conditioned space. Consider this example: A home has large supply and return leaks in the unconditioned attic that leak equal amounts. Since the leakage is balanced, no pressure imbalances occur in the home. Then a well-intentioned person locates and seals several of the large return leaks, unaware of the large supply leaks. The supply leaks then pump more air out of the house than is returned through the now-diminished return leaks, and the house becomes depressurized. Anyone sealing duct leakage should be certain not to create significant pressure imbalances in the process. Measuring such pressure differences requires a differential pressure manometer - another reason to hire a professional. Zone Pressure and Air Quality Probably due to its complexity, not all building-trade professionals understand the connection between airflow, pressure and air quality. One reason zone pressure is ignored may be that it is typically very small. However, very small pressures can sometimes be responsible for very large air movements and consequences. Pressure differences can pull hot and humid attic air into your house, or push conditioned air out of the house. Some of these pressure differences are created by natural conditions, such as wind or temperature differences between indoor and outdoor air. An ADS with unbalanced airflow also creates zonal pressures (pressure differences). The pressures associated with zones in a building are extremely small compared with other pressures you are familiar with. For instance, a tire pressure of 35 pounds per square inch is equal to 241,000 pascals. A pressure difference of one pascal would feel like a piece of paper in your hand. A pressure difference of a few pascals is not much, but it is enough to create big problems under certain circumstances. Depressurization Depressurization creates great potential for building and occupant problems, including improper combustion-appliance venting, moisture-related damage to building materials, and increased pollutant entry. Depressurization of a zone can result from:
Large return leaks that pull air out of a zone such as utility room, closet or garage,
Large supply leaks outside the conditioned space,
Unbalanced return and supply air within the same zone, or
Unbalanced exhaust-fan operation. Unbalanced exhaust-fan operation is not part of the ADS, but can significantly depressurize homes. The continuous operation of bathroom, kitchen and even attic exhaust fans can play a large role in building problems. Combustion Appliances Atmospheric-vented combustion appliances are devices like gas water heaters and fireplaces that are not vented by exhaust fans. When located in a depressurized zone, they may not be able to vent combustion gases. This means carbon monoxide could collect in unhealthy concentrations. In Florida, a new home with an air handler and gas water heater in the garage was found to have return leaks large enough to depressurize the garage to 5 pascals below outdoor pressure. This is enough pressure to cause combustion gas to be pulled back into the garage instead of venting outside. Zones with combustion appliances should not have a pressure difference more than 3 pascals lower than outside. As an added measure of safety, homes with combustion appliances should have a carbon-monoxide detector as well as smoke detectors. Moisture Damage In hot and humid climates, depressurization results in a lot of moisture problems. Humid air is pulled from outdoors into conditioned spaces. Moisture in the air condenses on the first surface it encounters that is below the outdoor dewpoint temperature. Often this occurs behind vinyl wallpaper, where mold and mildew stains may appear. If the air-conditioning system cannot keep up with the moisture load, wood floors may warp and other materials become damaged, as they remain too wet. Such problems are often symptoms of unbalanced airflow or of poor air-conditioner performance. Pollutants Any air pathway, regardless of size, is an entry point for air into a depressurized zone. Cracks in floor slabs are Grand Canyons for soil gas, and the rate of entry increases as the zone becomes more negative. The soil gas of most concern is radon. Balancing Airflow When more air is delivered to a zone than is transferred back out, pressure imbalances occur. Consider an ADS with a single centrally located return and supply registers in every room. Closing a bedroom door restricts the flow of air back to the central return register. The bedroom thus becomes pressurized, pushing conditioned air out around the door and through electrical switches, receptacles and fixtures in the walls and ceiling to outdoors. At the same time, the central core of the house becomes depressurized. The more closed doors, the greater the depressurization. One method of fixing unbalanced return airflow is to provide a duct from each room to the return plenum. The duct must be sized correctly so that it transfers the same amount of air as the supply. Another method is to provide a short section of insulated flexible duct with one end connected to a grille in the bedroom and the other end connected to a grille in central area. Each zone of a zonal heating and cooling control, where the amount of air into the zone can be varied manually or automatically, should be tested for balance. Supply air can be decreased either by adjusting a damper inside the duct or by closing off its outlet register. Ideally, the pressure differences between zones should be zero pascals, but 2.5 pascals is considered a reasonable limit. Duct Location Restricting ducts to the conditioned space provides a lot of forgiveness:
Air lost or gained from duct leaks has minimal energy penalties.
Heat lost or gained by conduction has minimal energy penalties.
Air quality is improved, since there are fewer pollutants brought into the house. Homes on slabs and crawlspaces are most problematic, since they have no basement, and their ducts are usually located in the unconditioned crawlspace or attic. Energy-efficient homes with average duct systems and located in moderate heating and cooling climates could save 30 percent of their heating and cooling costs. Less efficient homes in severe climates could cut their heating and cooling costs by as much as half. Keeping the ADS within the conditioned space may also allow downsizing of heating and cooling equipment. Equipment capacity can also be reduced with more insulation and better windows. A simple solution for hiding ducts inside the conditioned space is to use a central hallway. The air handler is located in an airtight closet next to the hall, and the hallway ceiling is dropped to allow space for the ducts. The chase is constructed of drywall, and the seams are sealed with approved tape and sealant. Homes with 8-foot ceilings present a challenge, but a chase ceiling lowered to seven feet is possible with careful planning. For more information on designing interior duct systems, contact the FSEC and request Design and Construction of Interior Duct Systems FSEC-PF-365-01. ADS Layout To realize the greatest efficiency from the fewest materials, an ADS must be designed to the individual house. This requires expertise beyond the grasp of the average homeowner, but be sure to ask: Will the ADS be designed and built to Air Conditioning Contractors of America (ACCA) and Sheet Metal & Air Conditioning Contractors' National Association (SMACNA) standards and procedures? If the answer is not a definite "yes," find another company. After hearing "yes" to the first question, ask these questions: How will the ducts be made airtight? How will the surface area be minimized? How will optimum air velocities be maintained throughout the system? Planning for Efficiency In an ADS, there is one fan located at the air handler that must pull all air through the return ducts and heat exchangers, then push it back out through the supply ducts and branches. Turns, large surface areas and flow restrictions all increase friction and turbulence, reducing the amount of air the fan can move.
Locate the air handler and main ducts centrally along the longest axis of the layout. Planning the spaces for duct placement will prevent the need for lengthy detours around framing or plumbing obstructions.
Minimize the number of turns. Turning vanes inside the duct are beneficial where rectangular 90� turns are unavoidable. Use 45° turns if possible, since the frictional losses are about 3.5 times greater for 90° bends. Even two 45° turns with a short straight piece connecting them results in less than half the frictional losses of one 90° turn.
Minimize the lengths of branch ducts. If the house has energy-efficient windows and insulation that meets recommended R-values, then ducts should not have to extend to the exterior walls.
Size ducts for the amount of airflow required. The main supply duct should transition to smaller sizes as air is supplied to the branches. Undersized ducts restrict airflow, resulting in reduced comfort. The fan also operates at lower efficiency, since less airflow occurs for the same amount of fan power. Undersized ducts or other restrictions may also result in high air velocities and noise.
Insulated flexible ducts are common and convenient. They should not be compressed or turned in a way to cause a reduction in diameter, however. Take-offs from another part of the system should extend straight several inches before turning. The duct should be supported at least every five feet, and supports should not alter the round shape or significantly compress insulation. The duct should not sag more than 2.5 inches in five feet. Why Wait? With the help of qualified professionals who understand building science, your duct system can be optimized today. Optimizing will not only pay for itself in energy savings, but it will improve indoor air quality at the same time. Logic dictates that you first invest in your ADS, then pay for that kitchen or bath upgrade with the savings. Chuck Withers is a senior research analyst with the Florida Solar Energy Center. The views and opinions expressed in this article are those of the author, based on research and experience and are not intended to represent the views of the FSEC or the University of Central Florida.