§150.1(c)9
A typical residential construction practice in California is to place ducts and associated air handling equipment in the attic. When meeting the prescriptive requirements, there are two options for where this equipment can be located:
1. If meeting the prescriptive requirements for a high-performance attic (HPA) as explained above, the duct system and air handlers of HVAC systems are allowed to be located in the attic.
2. If meeting the prescriptive requirements for ducts in conditioned space (DCS) as explained above, the duct system and air handlers of HVAC systems must be located in conditioned space, which includes a joist cavity between conditioned floors, or in a sealed cavity below attic insulation.
If the DCS requirements are to be met, additional requirements apply:
1. Air handlers containing a combustion component should be direct-vent (sealed combustion chambers) and shall not use air from conditioned space as combustion air. Other types of combustion heating systems are possible given the system installer adheres to the combustion air requirements found in Chapter 7 of the California Mechanical Code.
2. Duct location needs to be verified through a visual inspection per RA 3.1.4.1.3.
3. Duct leakage to outside needs to be confirmed by field verification and diagnostic testing in accordance with RA3.1.4.3.8.
4. Ducts are insulated to a level required in Table 150.1-A.
Figure 4-9: Checklist for Prescriptive Requirement − Option C DCS (§ 150.1(c)1)
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-Vented attic -R30 or R38 ceiling insulation (climate zone specific) -R6 ducts (climate zone specific) -Radiant Barrier -Verified ducts in conditioned space |
The checklist in Figure 4-9 lists all the requirements for complying prescriptively using DCS strategy. It is not enough to locate ducts in conditioned space, the insulation must also meet prescriptive values. If a building is not able to meet all of the requirements in this checklist, it must use the performance approach or Option B from Section150.1(c).1. Refer to Section 3.5 of the Residential Compliance Manual for more information on these options.
There are several methods of achieving the goal of DCS. The basic information of the strategies, related benefits, challenges, and potential solutions to those challenges are outlined below.
A. Vented Attic, Dropped Ceiling
This strategy places ducts within the thermal envelope without affecting the standard construction of the attic space. This strategy works well in linear plans where rooms branch out from a central hallway with the dropped ceiling.
Figure 4-10: Ducts in Conditioned Space Using a Dropped Ceiling
Figure 4-11: Ducts Routed Through a Dropped Ceiling
Benefits of selecting this strategy include the following:
Attic ventilation remains the same as standard practice.
This strategy does not affect attic assembly or insulation; there are no changes to truss design.
The strategy works with simple and linear designs with rooms off the main hallway but can work with more complex plans.
The strategy can be integrated into architectural accents.
There are challenges associated with this strategy as outlined below, but they can be overcome with good design and installation practices.
Need to address air handler location – there may not be sufficient space (height, width) in the dropped ceiling to accommodate the air handler. In this case, the air handler would need to be installed in a separate closet within the thermal boundary of the home.
Coordination needed between trades – moving the ducts and air handlers and the need to isolate and seal the dropped ceiling would necessitate coordination between different trades (HVAC installer, drywall, framing, and electrical contractors) to ensure thermal integrity of the dropped ceiling.
B. Vented Attic, Conditioned Plenum Space
A conditioned plenum is created when a space within the attic is sealed off and insulated from the rest of the attic. To use this design option, a builder can specify two types of modified trusses: either scissor trusses or a truss configuration that creates a plenum box. Another way to create a conditioned plenum does not involve modified trusses, but rather to create the space by framing, sealing and insulating the plenum space above the ceiling plane.
Figure 4-12: Plenum Truss Design Example
Similar to a dropped ceiling, this design is easier with a linear plan that allows the conditioned space in the attic to cover a central “spine” throughout the floor plan that can reach all spaces in need of supply registers. This design option allows for ducts in the attic space and does not affect aesthetics of the home.
Benefits for selecting the strategy:
Vented attic space, same as standard construction
Aesthetically less disruptive than dropped ceiling
Works with simple and linear designs with rooms off main hallway
There are challenges associated with this strategy as outlined below, but they can be overcome with good design and installation practices.
Need to seal the plenum from attic – as with most of the DCS strategies, it is important that care and attention are provided to air-sealing the plenum space from the attic space.
May require modified trusses, in which case manufacturers need to be provided with specifications that can be met.
C. Vented Attic, Open Web Floor Truss
Figure 4-13: Open Web Floor Truss Example
This option can work for two-story construction and makes use of the space between floors to house ducts. Open-web floor trusses are uncommon in residential construction but are available from several floor joist manufacturers. The depth of floor joists may need to be increased to create a large enough space for supply ducts. The increased joist depth may affect interior details and wall heights. Because of the size constraints from using the floor truss, there is a need to preserve construction quality and prevent undesirable construction practices such as forcing 14-inch ducts into 12-inch joist spaces. Another option is to use alternatives to wire helix plastic flexible ducts that take up less space. Coordination between the architect and the HVAC engineer and/or contractor is needed to ensure that ducts are correctly sized and truss depths are appropriately selected. Using the area between floors to house ducts prescribes that supply registers be at the floor or lower wall in the second story and the ceiling or upper wall in the first story.
D. Mechanical Closet and Placement of Sealed Combustion Furnace
Figure 4-14: Mechanical Closet Placement Example
As part of the requirement for moving the duct system and air handler into a conditioned space, construction of a mechanical closet is necessary with some DCS strategies. For example, if ducts are placed in dropped ceiling space but there is not enough room to accommodate the air handler in that space, the mechanical closet could be placed inside the thermal boundary of the building. A conditioned plenum could provide enough space for ducts and equipment; therefore, a mechanical closet may not be needed.
One potential location for a mechanical closet is within the garage or other spaces normally not conditioned. In such instances, the air handler must be located within a specially built closet that is insulated to the same level as the exterior of the house so that the closet is not a part of the unconditioned space. Combustion air for the air handler must be taken directly from the outside through a direct vent to the outside.