If your HVAC equipment is the heart of your home, then the ducts make up its circulatory system. Ductwork channels conditioned air to vents, takes return air back into the system and transports exhaust air and waste heat to the exterior of your home. Ducts serve a triple purpose: They carry warmth in winter, hold cool air in summer and provide ventilation for better indoor air quality throughout the year. Because they're in use almost every day, your ducts need occasional maintenance to improve the longevity of your heating and air conditioning equipment.
How Your AC Ducts Work
Unless your home has a ductless system or relies solely on window units for cooling, it has ducts. You probably have a good idea how ducts carry air from the central inside unit of your AC or your heater, but those ducts represent only part of the system. To understand how air moves throughout your home, it's helpful to know about air sources and what your HVAC system does to keep you comfortable.
Cooling Your Home's Air
To an air conditioning specialist, your AC doesn't add cold air to your home; it subtracts the heat from the air already inside it. Technicians refer to this process as thermal transfer. Air conditioners and heat pumps use coolant as the vehicle for thermal transfer. Although the coolant never comes in contact with the air, it still absorbs and loses heat readily. Evaporator and condenser coils maximize the surface area over which heat transfer happens.
Air conditioners rely on a special property of gases and liquids. When the coolant in your AC changes form from a liquid to a gas in the evaporator coils, it cools rapidly and soaks up excess heat from the air moving over the coil array. The heated substance then travels to the exterior half of your air conditioner where the compressor turns it back into a liquid in the condenser coils. A fan removes the excess heat from the coolant as it passes through the coils to prepare it for another trip through the system.
When your air conditioner performs efficiently, it can cool air by as much as 20 degrees. On a 90-degree day, that brings your home's temperature to a cool 70 degrees, but how does your AC cope with sweltering triple-digit heat? Your AC is designed to recirculate some of the already conditioned air in your home to bring temperatures down to comfortable levels even on the hottest days. To do that, it must have an adequate supply of conditioned air via your ducts.
Your AC processes two air sources: return air and supply air. Depending on your building's construction, you may also have exhaust air ducts in your home, especially if you live in a multi-story building, but most single-family homes use only supply and return ducts. Supply air comes from outside the building via your ventilation system. Return air goes through your filter and into ducts that mix it with supply air in a space called the plenum. The air in the plenum cools slightly as it combines external air and recirculated air, but it isn't until it moves over the evaporator coils that it loses most of its heat.
The ducts that carry air to vents are separate from the return air ducts that bring air to the plenum and through the main blower. The filters that remove smoke, dust and other particulate matter from return air serve a vital purpose and help keep your AC system's coils clean. Clean coils are efficient coils; because the evaporator coils carry coolant ready for its freight of excess heat, the coils' fins must be free of soil that could hamper thermal transfer.
Types of Air Ducts
More than a few movies have featured people moving through metal air ducts, but most household duct systems are not large enough to accommodate anything much larger than diagnostic camera equipment. Ducts can be made of rigid materials or from flexible components; many buildings use a combination of types to suit different purposes.
Rigid ducts are typically more expensive to install, but they are durable and quiet if your home has the space for them. Rigid materials are also easy to seal and clean. For forced-air systems, rigid materials maintain air pressure better than flexible options, which is why many homes use them for main trunk ductwork and use flexible materials for offshoots. The most common material for rigid ducting is galvanized steel, but polyurethane panels with additional insulation are becoming increasingly popular for new construction. Fiberglass panels are another possibility for rigid duct systems; they are well insulated and lightweight.
Flexible ducting, or flex, allows your AC installation specialist to fit ducts into smaller spaces and around existing architectural elements. Flex ducts are economical and compact, making them a good choice for reaching vents near corners or close to the house's eaves. Today, most flexible ducts are made from metal coils wrapped with insulating materials and polyethylene plastic, but older ducts may incorporate other plastics and metal films. When installing flexible ducts, ventilation technicians should avoid bending or kinking the wire within the tubing. Over short distances of under 15 feet and with no sharp bends, flexible ducts can approach the efficiency of rigid duct materials.
Care and Maintenance of HVAC Ducts
Under normal circumstances, your air conditioner's ducts need little special care. During regular maintenance inspections, your AC and ventilation technician should examine ducts for breaks or tears, loose connections or insulation loss, but problems are rare.
Cleaning Air Ducts
One of the most common concerns about your ducts is their cleanliness. You can easily change filters and even clean your system's condenser and evaporator coils, but cleaning air ducts is beyond the scope of most do-it-yourself projects. Professional duct cleaning uses specialized equipment, including vacuums, forced air, antimicrobial solutions and other tools to clean your system. As the Environmental Protection Agency points out, accumulated dust and soil in ducts has not been definitively linked to increased allergic reactions or other health concerns. However, dust build-up can limit the efficiency of your system. The EPA also recommends cleaning if you see the following signs of trouble:
- Substantial and obvious mold growth on duct interiors
- Signs of animal or insect activity such as droppings or insect parts
- Heavy accumulation of soil and dust around air vents
The EPA's website also notes that cleaning ducts has not shown any negative effects. If you're considering a thorough cleaning of your ducts and other parts of your HVAC system, talk to a cleaning and maintenance specialist such as one of the certified technicians at AC Southeast to decide if your system needs cleaning. A specialist can assess your ducts from the inside out with diagnostic equipment and help you make an informed decision about the project.
Duct sealing can have a dramatic effect on your utility bills. By properly insulating and sealing ducts, you reduce the amount of cool air lost to your attic or basement. According to data on the EPA's EnergyStar site, leaking ducts can cost your heating and air conditioning system as much as 20 percent of its efficiency. Sealing also protects the system from animal or insect invasions that could necessitate a complete duct cleaning. Sealed ducts improve indoor air quality by creating a barrier between your ventilation network and potential pollutants that could find their way into the system.
Troubleshooting Duct Problems
While your ducts will probably remain trouble-free as long as you change your return air filters regularly, problems can occur. Duct problems are usually silent; unlike problems that affect your compressor, blower or other mechanical parts of your AC system, trouble with ducts can go undiagnosed for years.
- Insufficient air flow: When you hold your hand to an intake vent or register, you should feel air movement. If you detect little or no air, you may have a breach or kink in the duct. Check the damper on the vent, too. Dampers are meant to limit air flow, and if someone in your household inadvertently closed it, the solution may be as simple as sliding a lever.
- Dust collecting on vents: If you see soil near vents, check your filter first. A dirty filter lets dust into the return air system and sends it throughout the house via the vents. After cleaning the dust away from the vents and changing the filter, monitor the amount of dust the AC produces. If the system still leaves visible dust after a week or two of observation, you may have a breach that needs professional attention.
- Hot and cold zones in your home instead of uniform temperatures: Improperly installed ducts can lead to unwanted temperature differentials. A duct that's too small for the room it serves or that becomes constricted can't deliver enough cool air for comfort. Before contacting a professional, check the dampers on vents to ensure that they haven't been closed.
- Unpleasant or musty smells: Ducts normally deliver clean, scentless air, but a marked odor could indicate that an animal has made its home in your ducts. Musty aromas can come from mold growth, especially if your AC system is inefficient at removing humidity from the air.
Your HVAC system's ductwork is vital to its efficiency and to your comfort. If you suspect a problem with your ducts, contact a professional duct cleaning and maintenance service such as the team at AC Southeast for an assessment.
Written by Robert Koch