Everyone knows that an airtight home saves energy so finding and fixing leaks can save energy significantly. Weather stripping and caulking around windows, doors and floor molding can reduce air leakage considerably, but not completely, but the most important leaks are those not easy to find within your heating and cooling duct system.
Duct leakage affects homes in many ways. Obviously, air leakage both into and out of your house, called infiltration, reduces the efficiency of heating and cooling systems, reducing household comfort and wasting energy. Duct leakage also affects health and safety by drawing air pollutants from the garage, attic, soil, or combustion equipment into your home.
In a normal operating A/C system, room air is pulled into the return side of the system by the air handler fan and pushed into the supply side of the system. Without leaks, the duct system is an efficient closed loop allowing the same air that comes out of the supply registers to re-enter the return grills. Problems develop when the duct system is not airtight.
Even small holes can result in a large amount of leakage. When this occurs, the system develops pressure problems much like a balloon. Outdoor air is pulled into the duct system causing your home to have excess air, creating high pressure in your home. House air is then forced out through all holes in walls, floor and ceilings. In order to be at optimum equalized pressure, a cubic foot of air must go out for each cubic foot of air coming in, therefore, the amount of air leaving the house is the same as that being drawn in from a leak.
When you have supply leaks on the other hand, you lose twice. First, highly conditioned air is lost from your ducts to the outdoors. This in turn, causes your home to develop negative pressure much like a deflated balloon. In order to equalize, your home sucks heated outside air to replace the lost air. You lose a second time because this excessive hot or cold air must then be heated or cooled causing your heater or cooler to work twice as hard.
HVAC systems move air across a cold evaporator coil to cool and dehumidify the air or through a heat pump. With duct leaks, it is harder for the HVAC to work well. With heated summer air drawn into the duct system, the air temperature entering the coil is raised so the temperature coming out of the coil area is not as cold. As a result, the cooling capacity of your air conditioning system is often reduced by 25%. The problem is more serious when the air entering the return leak comes from the attic, which often reaches 120º F, reducing your system’s cooling capacity by more than 50%! Similar problems occur in the winter, with cold air instead of hot.
The energy penalty of duct leakage depends on the temperature difference between the air leaving the house and the air entering. Supply leaks waste even more energy than return leaks because the air lost from the supplied duct is colder than house air in the summer and hotter than house air in the winter. Supply leaks depressurize the home, causing air to be drawn in through all the leaks in the cracks of the house structure. In many cases, the big leaks are between the house and the attic, so the air is hotter as mentioned previously.
Some of the symptoms of a duct leak problem include high humidity, your HVAC system runs excessively, you home is too hot or cold, and high utility bills. One of the big clues for an HVAC specialist is a high infiltration rate. Natural infiltration of air flowing into the home as a result of wind and temperature changes about 25% of the house air each hour. When the HVAC system turns on, however, air infiltration increases to double or triple this amount, indicating that duct leaks are pulling too much air into your home.
The effects of duct leak infiltration are worse than you might imagine for two reasons. First, duct leak infiltration only occurs when the air handler is operating which is mostly when outdoor conditions are at their worst—on hot summer afternoons and cold winter mornings. Therefore, the temperature of the air brought into your home is hotter or colder than the average outdoor air. Second, duct leaks often draw hot attic air into the house so the impact on air conditioning is even greater.
Since some leaks only occur when the air handler fan is blowing, duct leak infiltration is greatest when outdoor conditions are at their worst. In the typical home, duct leakage increases air conditioning electricity used to about a 33% increase. TRS-10 Fortunately, duct repair can substantially reduce this energy waste. Since the costs of such repairs are in the range of $100 – $400, a repair can pay for itself in about 2 years or less. Duct repair can lower HVAC electricity use during the utilities peak demand period, reducing the number of power plants needed. Therefore, many utilities are beginning to create duct repair programs. It is estimated that duct repair in 3 million homes at a cost of about $600 million could eliminate the need for 5 large power plants costing about $2 billion.
So the pressure is really on to find and fix leaks in your home. HVAC duct leaks occur at seams in joints because tape adhesive dries out or was not properly installed in the first place. TRS-11 In some cases, ducts separate or collapse and the result is a catastrophic leakage that overwhelms the HVAC.
At the end of the ducts where the supply air groups to meet the register, leaks are common. Most often there is no seal between the boot and ceiling sheet rock. This leakage could become worse when interior doors are closed, increasing pressure that backs up into the duct system. Leakage becomes dramatically worse if the supply registers are closed, blocking airflow. The volume of air flowing out of the leak may become 10 times greater, overcoming any possible savings achieved by closing a vent.
Leaks of the air handler occur at various penetrations, cracks, and around access panels, providing paths for air to enter or leave the air handler. Sometimes the filter access cover is left off, leaving a big opening. Even the smallest leaks near the air handler can be very serious because the system is under great pressure.
When wall sockets, chases, and spaces between floors are used, duct problems occur because these building cavities are not airtight. The biggest leaks happen when air handler support platforms are used as return plenums. A return plenum is basically a box from which the air handler pulls air. Return plenum leakage occurs primarily because adjacent walls are opened to the inside of the plenum. Air is pulled into the duct system from those walls and any spaces connected to them once wall cavities are depressurized. Air and pollutants can be drawn from anywhere, including the attic, crawl space, basement, garage, and outdoors. About 50% of all duct leaks are located in the support platforms.
Significant duct leakages are found in a large portion of homes. In fact, there is an 80% chance that you need to repair your home. Finding duct leaks, however, requires specialized tools and diagnostic methods. The blower door mounted in an exterior doorway is used to measure the air tightness of the house. When the registers are sealed off and the blower door test is repeated, the size of the holes in the duct system can be computed.
A smoke stick helps find duct leaks when the house is pressurized by the blower door. If the smoke races into the register, there are large duct leaks nearby. When smoke just sits in front of the grill, then there are no leaks. TRS-15 Measuring pressures is important to understand the indoor air quality because negative pressure can draw pollutants into the house from outside the home, such as dangerous and toxic materials stored in the garage. TRS-16 Even more importantly, negative pressure can cause flu gases from combustion equipment, fumes from gas water, heaters, furnaces and other devices, radon from soil, formaldehyde from building materials, pesticides, household cleaners, paints, solvents, volatile organic compounds, mold and mildew to flow into the house through the duct system. Because of this concern, duct tests include measuring draft pressure and carbon monoxide, as well as checking for proper venting of combustion gases.
Once duct leaks have been identified and health and safety concerns have been resolved, duct repair may begin. Duct tape is not suitable for duct repair, regardless of popular belief due to its name because the surfaces are too dirty for good adhesion. Repair should be done with mastic and embedded fiberglass fabric. TRS-17 Once dry, this combination forms a ridged, long lasting sealed return. Plenums should be sealed from the inside by entering through the return grill or by removing one side of the support platform. The inside should have a continuous air barrier separating it from air in adjacent walls, surrounding rooms and outdoors.
Various materials can be used to seal larger openings, but it is important to use non-toxic materials and be fire resistant. When leaks are difficult to reach, repairs are most easily achieved by cutting open the duct and repairing from the inside. Several steps are required to achieve an airtight seal, including removing portions of the insulation from the foil. The fiberglass side of duct board is not an air barrier so sealing must be done on the foil surface. The final closure is made with fiberglass mesh and mastic. This may be a difficult process best left to HVAC professionals such as TRS.
Mobile homes often have supply leaks, but fortunately, most of these leaks can be easily accessed from inside the structure. Duct repair in many mobile homes can be done easily and inexpensively and yet the energy savings can be very large indeed.
Duct leakage in homes has turned out to be a much larger and more complex issue than you ever imagined. An invisible problem without our knowledge may be robbing you of energy, comfortable living environment, and a safe and healthy home.
Nationwide, residential duct leaks may be draining $3 billion per year in wasted energy bills and account for 10% of electrical generation requirements. While the magnitude of the problem may be large, the opportunity for improvement is also great. At a typical cost of several $100 of repairs, you can significantly reduce your utility bills, may solve indoor air quality, moisture, and building degradation problems. Fortunately, many utilities are ready to assist you with the cost of duct repair since airtight duct systems help avoid the cost of building new power plants. Call your utility company to find out.
While the repair is a great opportunity, health and safety must be the first priority. It is not just a matter of finding duct leaks and fixing them. You must look at how repairs will affect your home’s air quality and be sure the combustion safety problems, if any, are properly dealt with. Therefore, specialized training or using a quality company such as TRS is essential to be sure that repairs are done right.
Information provided by FL Green Team.