Central Air Conditioning and Heating: the Importance of Proper Duct System Design

Andre Olagues, Attaway, Denise

Central Heating, Ventilation and Air Conditioning (HVAC) equipment distributes air through out a house by a system called “duct work.” When this system is properly designed, air distribution is even and a uniform temperature is maintained. The efficiency of duct work systems has been found to be as much as 60 percent less than it could be, because of poor design and leaks in the ducts. Many of these flaws can be corrected for little or no extra cost. Proper duct work design can allow for the correct sizing of systems, thus, increasing savings.

Leaking ducts can cause energy bills to rise and system capacities to be reduced. Leaking ducts are also are a source of unbalanced temperatures throughout a house. Return-air duct leakage can cause dirty coils, thus reducing capacity. Return-duct leakage is also blamed for increased humidity levels in a house. Return duct leakage can also allow contaminants to enter the air stream of a living space.

Duct work can be oversized or undersized. Undersized duct work leads to low air flow, which also can occur from needless bends, pinches and turns. It also increases air noises and impacts the efficiency of the system. Oversized ducts cause low air flow and uneven temperature throughout the house. Oversized ducts also impact equipment efficiency.

The objectives of good duct work design are occupant comfort, proper air distribution, economical cooling and heating, and an economical duct installation. These are established by the duct design process. The final layout is called a duct system. Such as system includes supply and return duct plenums, ducts, fittings, insulation, sealants, boots, grilles, and registers.

The objectives of the duct system are to:

  • Supply conditioned air to each room.
  • Be sized to get the proper pressure drop across the air handler’s coil.
  • Be sealed to provide proper air flow and stop leakage.
  • Have a balanced supply and return system.
  • Reduce the losses by temperature gain for surrounding conditions (high attic temperatures).

Types of supply systems typical for Southern Louisiana:

  • Trunk and Branch System.
  • Spider System.

Trunk and Branch System:

The trunk and branch system is made up a large main supply trunk that is connected to the air handler. The smaller branch runs are connected to the main trunk supply. There are two variations of the trunk and branch system. One is, the main trunk is sized the same for the complete length of the run. This is limited to 24-foot runs. The other is similar, but the length can be run out, to up to 50 feet. The trunk is, then, reduced to accommodate the loss of air volume, air velocity and pressure. The branches are sized to the calculate air volume requirements of the particular room it serves.

Spider System:

A spider system is a variation of the trunk and branch system. A large supply trunk or box plenum connects directly to the air handler. This supply trunk is large enough to connect the large supply ducts that run to remote mixing boxes. Smaller branch ducts are connected to the remote mixing boxes. These smaller ducts are connected to each room in the residence. The larger supply ducts are many times large and may be flexible ducts. This system leads to possible duct restrictions when flexible ducts are used for the small-duct runs.

Return air systems:

This part of the system is almost always ignored. The return duct removes air from the roomand delivers it back to the air handler for filtering and reconditioning.

Two return air systems generally used in residences are:

  • Central Return Air
  • Multiple Room Return Air

Central Return Air is the most common system used in houses built in south Louisiana, but is not necessarily the best. It is a system that consists of one or more large grilles located in central areas of the house and often close to the air handler. In multi-story houses a return air system should be located on each floor. Undercutting of doors on rooms that can be closed off has been the norm. Some use the 1-inch rule to provide clearance. This usually is not a sufficient amount of clearance. Using transfer grills or jump ducts can avoid this problem. This system provides more privacy and is quieter than the under the door method.

Multiple Room Return Air is the ultimate return duct system. This system ensures air flow is returned from all rooms even when the door is closed. This system is more expensive and may have higher friction loss requiring increased blower requirements. It is the quietest and allows for more privacy.

Duct materials:

Duct systems are usually made of one of the following:

  • Sheet metal,
  • Rigid fiberglass duct board, or
  • Flexible non metallic round duct.

Sheet metal is the most common duct material and should be use for plenums, trunks, branches and runners. Sheet metal is smooth on the inside and offers little resistance to air flow. When located in unconditioned spaces, sheet metal should be insulated and all joints mastic sealed before the insulation is installed. Each joint should be mechanically fastened with screws.

Rigid fiberglass duct board (RFDB) is usually designed into a supply rectangular duct and sealed. Mastic sealing is preferred. RFDB can be used for return and supply ducts, run-outs are connected to the supply ducts. These run-outs can be sheet metal or flex duct. Sheet metal is the best and can not be damaged like flex duct. Flex duct is faster to install, but the drawbacks are great, including it is easily torn, crushed, pinched and /or damaged during installation. Connections still need to be mastic sealed. This also is true of all sheet metal connection. These connections too must be mastic sealed. Rigid fiberglass duct board has a long life if installed to the manufacture specifications.

Flexible non metallic duct is made of a duct inner liner supported by helix wire coil and covered with a blanket of insulation with a flexible vapor barrier jacket on the outside. Flexible duct negatives are pointed out in the previous section. They are used in hard to get to places and mastic sealing is required.

Location of ducts and registers:

The LSU AgCenter recommends ducts be located within conditioned spaces. This is because any duct losses will return to inside the house envelope. This reduction of duct loss allows for consideration of smaller air conditioner units and lower operating costs.

This can be accomplished by placing the ducts in a furred down chase or located the ducts between the floors in a two story house.

Supply registers in houses in the South should be located so the ceiling diffusers discharge the air parallel to the ceiling. In today’s well insulated homes, it not necessary to locate the registers more than a few feet into a room, on the ceiling. The preferred types of register to use are curved blade adjustable types. These types avoid the air flow stream from blowing direct on occupants. The curved blade type is mandatory for heat pump applications.

The location of the return air register has only a secondary effect on air motion in the room. The location can improve stratification and mixing of room air. This is especially true in two story houses. The upper level return air register should be placed high and lower level placed low.

Duct design:

The duct layout should be considered in the drawing of plans for a house, not as an afterthought. The Air Conditioning Contractors of America (ACCA) Manual D is the suggested method to use when designing duct work. Many air conditioning manufacturers have their own methods.

The following steps should be followed when designing duct work:

  • Select the type of HVAC equipment.
  • Confirm fuel availability.
  • Select the air distribution system best for the climate.
  • Confirm all envelope insulation and sealing measures.
  • Calculate the heat loss /heat gain of the structure using a Manual J method
  • Select the models of HVAC equipment and SEER ratings to be used.
  • Address any additional items such as a electronic filter or humidifier
  • Develop a scale drawing of the air distribution system, showing all sizes and ducts run to locations.
  • Select size and quality of registers and return air grill(s).
  • Select insulation level for duct system, the 2006 IRC Energy Conservation section defines this minimum.
  • Clarify that all joints and connectors are sealed with mastic for minimum duct loss.

Finally: “Best Job” recommendations are:

  • Design the air distribution system in the conditioned space (a insulated sealed attic is not conditioned space).
  • The total system should be hard ducted, including the return-air run.
  • Consider two units for two-story houses.
  • Consider installing supply ducts outlets near interior walls to reduce duct loss with shorter runs.
  • Consider installing volume dampers on all of the take-offs on the plenums or trunks, rather than using the register to balance the system.
  • Avoid the 1-inch of clearance method for return air from closed rooms
  • Specify higher duct insulation (R –value) than the Code minimum and specify loosely wrapped rather tightly wrapped
  • Mastic tape all joints and air handler cabinet connections.
  • Consider testing the duct work with a duct blaster when job is complete.

Audio clipHow can I know that my contractor is doing a good job installing my air and heat?

2/13/2009 3:28:50 AM
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