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Duct System
Energy Efficiency, Comfort, and Health | Function and Optimal Operation | Components | Supply and Return Plenums | Hard-type Ducts | Flexible Ducts | Duct Insulation | Duct Sealant and Mastic | Tape | Dampers and Regulators | Boots | RegistersQuick Facts
- Part of larger HVAC system which delivers conditioned air to living space
- Can be used in conjunction with a traditional furnace or with a hydronic air handler
- Sealing leaky ducts or replacing uninsulated or damaged ducts can improve HVAC efficiency on average by up to 20% by improving the distribution efficiency
- Correctly sized, well sealed ducts can reduce a home’s heating and cooling needs, saving money
Energy Efficiency, Comfort, and Health
The duct system is an integral part of your HVAC system and directly affects your comfort, indoor air quality, and utility bills. Having the home’s duct system professionally sealed or replaced with home performance in mind will eliminate wasteful duct leakage, and the infiltration of attic and crawlspace pollutants that can occur as a result. Properly sealing the duct system can significantly reduce heating and cooling costs (reducing CO2 emissions), improve building durability, and create a healthier more comfortable indoor environment.
Function and Optimal Operation
The duct system is a network of components which work together to supply warm or cool air to the living space and to return air to the furnace or hydronic air handler to be filtered. Proper techniques, materials, and standards are often not followed when installing a duct system and installation tends to be very error prone. Attention to detail and building science concepts must be applied to the design and repair of an effective long-lasting system. Such design and repair is among the most cost effective of all home performance retrofits.
If possible, ducts should be designed to be located inside the conditioned area of the home or building envelope.
Components
Supply and Return Plenums | Hard-type Ducts | Flexible Ducts | Duct Insulation | Duct Sealant and Mastic | Tape | Dampers and Regulators | Boots | Registers
There are four main characteristics which contribute to a properly functioning duct system:
- Proper sizing of ducts to deliver the correct amount of air to each room; sizing should be matched to the furnace and A/C size
- Proper routing of ductwork to reduce the resistance to airflow (static pressure)
- Proper installation with mastic and zip ties to ensure a tight system with minimal leakage
- Proper insulation to reduce heat loss
Each of the following system components contribute to these characteristics.
Supply and Return Plenums
These are the primary connections between the furnace or air handler and the duct system. They are typically sheet metal boxes that should be insulated and attached with sheet metal screws, mastic, and heat resistant tape to ensure a tight connection with minimal heat loss or air leakage.
Hard-type Ducts
Main duct supply and return lines are generally designed using hard rectangular or cylindrical sheet metal ducts when run in in accessible areas, garages, or areas with reduced clearances or rodents. Hard-type ducts must be appropriately sealed and insulated for proper performance.
Flexible Ducts
Due to their flexibility of installation, flexible ducts are commonly used in the system to bring conditioned air to and from supply and return registers in the living space. They must be installed with care to minimize any restriction of air flow.
Duct Insulation
Since ducts are generally run through unconditioned spaces, such as attics and crawlspaces, which are subject to extreme temperatures, ducts must be appropriately insulated to minimize heat loss or gain of conditioned air as it travels from the furnace/air handler to the living space. R-6 to R-8 rated insulation is typically recommended.
Duct Sealant and Mastic
Even if ducts have proper insulation, they will severely under perform if they air not properly sealed with appropriate mastic. Leaky ducts lead to sub-optimal airflow, loss of heat energy, and exposure to harmful particles and gases from attics and crawlspaces.
Tape
In most cases mastic is a more effective sealant than tape. However, some applications need more heat resistance or tidy installation than mastic can provide. NEVER use duct tape; there are a variety of tapes appropriate for a variety of tasks, but duct tape, despite its name should never used anywhere in a duct system as it is ineffective and will soon fall off.
Dampers
Dampers can be used to control the amount of air supplied to certain rooms and regions of a home. Their settings should be engineered and calibrated by a home performance expert using concepts of building science. If the layout of the home makes their usage appropriate, dampers can be used in conjunction with a zoned thermostat to create the most effective heating system, since conditioned air only goes where it is needed, instead of equally throughout the whole house.
Boots
These integral components of the duct system are the connection between the ducts and the registers which supply conditioned air to the living space. If installed incorrectly, they can be the source of many leaks in the duct system, and when left unsealed (against the subfloor or sheetrock) they provide a direct connection for air to move from the crawlspace or attic into the home, reducing indoor air quality.
Registers
Registers are the most visible component of the duct system. They are the delivery mechanism of conditioned air to the living space. Properly designed and installed registers can have a tremendous effect on the efficiency of the whole HVAC system. Properly designed registers will direct air strategically through the room to maximize mixing of different temperature air, creating even temperatures throughout the room without blowing air directly on the occupants.
High quality linear blade and curved blade diffusers are most commonly utilized in a high performance system.