The goal of an effective ductwork system is to provide constant temperature and humidity control throughout the house without noticeable air stratification or noise. The best way to accomplish this is with a simple, direct duct design, which ensures both good performance and low cost.
Types of ductwork systems
Three types of ductwork systems are common in the U.S.:
- flexible (the most common type in Texas)
- rigid
- sheet metal
Flexible duct systems consist of insulated flexible duct connected with metal transitions and joint connections. This system is used for attic installations where flexible duct can be suspended over the joist and whenever crawl-space installations are possible. This design works best with centrally located air conditioning systems.
Sizing
A properly sized and installed air distribution system is as important as properly sized and installed heating, ventilation and air conditioning (HVAC) equipment.
When designing an air distribution system, you should consider:
- capacity of the HVAC unit
- sound level
- quantity of air supplied to each individual room
Design and insulation
A ductwork system that is not properly designed, installed, insulated and sealed can increase total heating and cooling costs up to 40% and may decrease the comfort, health and safety of your home.
Different rooms need different volumes of air to maintain even temperatures throughout your home. Duct sizes that are not properly matched with room sizes cause hot and cold spots in a house. Long duct runs – the length the duct travels from the air handler (typically located in the attic) to the room – can raise the temperature of cooled air and lower the temperature of heated air before it reaches the room. This same temperature loss occurs when ducts run through unconditioned spaces, such as attics.
Placement of ducts can’t always be controlled in the design of most houses; however, any ill effects can be minimized with good duct insulation. Ducts are typically located in the attic. If they’re poorly insulated, the oven-like temperatures during the summer tend to transfer into the ducts, which heats up the cool air from the air conditioner before it gets into the living space. In winter, the opposite occurs. The warm air from the furnace loses heat to the surrounding cooler air in the attic, which makes the equipment work harder. Energy costs increase and the comfort level of your home decreases.
Recommended duct design
Good duct design includes thermal efficiency achieved with adequate insulation and airtight seals. We recommend ducts insulated to an R-Value of R-6. Ductwork should be kept as straight as possible. Any turns, bends or ‘S’ loops will cause additional pressure loss and reduced airflow.
Duct insulation should have a vapor barrier on the outer surface to prevent moisture in ambient air from passing through the insulation and condensing on the duct surfaces. Condensation will cause the insulation to be saturated with water, destroying its insulating ability and deteriorating the duct material.
Properly sealing an air distribution system
- All air duct transitions, joints and connections (including where duct attaches to supply boot) should be sealed airtight using a UL 181 approved mastic sealant or a UL 181-approved mastic tape.
- All seams on air handlers, furnaces and plenums should be sealed with mastic and reinforced with fiberglass mesh tape or UL 181 tape, or sealed with UL 181-approved mastic tape. Do not block combustion air passages on fossil fuel furnaces. Mastic alone is acceptable on unit access panels.
- If using duct board, all seams should be sealed with mastic on the foil side reinforced with fiberglass mesh tape or UL 181 tape or sealed with UL 181-approved mastic tape. Duct board should be used within a return chase, turned foil side in.
- Return air chases should be sealed with mastic at the sole plate and top plate and at all corner joints and seams. The preferred method of return air design is a ducted return. Avoid using building cavities for the return duct chase. If building cavities are used as return air duct, duct chase should be extended into the attic or between floors so that the section penetrating the floor joist or ceiling is continuous to prevent leakage.
- Mastic sealant utilized should be water-based, non-toxic and consist of at least 50% solids and be UL 181 approved.
- Mastic or mastic tape should be applied according to manufacturer’s specifications.
- All plastic or nylon tie straps used for securing flex duct should be installed with a duct tie tool.