My Library

walls and between joists in floors. While supply air ducts are always formed from sheet metal, return air ducts can take advantage of stud and floor joist spaces to transfer air back to the furnace. When an enclosed joist or stud space is used as a return air duct, non-combustible material such as sheet metal should be used to line the joist space that is within 600 mm (24 in.) of the furnace, under floor registers and at the bottom of vertical ducts. It should be noted that framed spaces tend to be very leaky and therefore may not return air from the different rooms of the house as expected. Sealed sheet metal return ducts provide more predictable and better performance but are less commonly provided due to increased material and installation costs. Where ducts cannot be concealed within the floor and wall framing, bulkhead chases—typically aligned along wall-ceiling intersections—can be constructed to conceal the ducts where they pass through finished rooms. Such framing work should be considered at the design stage to avoid large and unsightly bulkheads in locations that intrude too much on room space and head room. Provisions such as access hatches must be made in the framing and room finishes for access to any dampers or other adjustable fittings installed in the ductwork system. The spread of fire is a major concern for ducts and associated fittings. Heating ducts, duct connectors, associated fittings and plenums must be constructed of steel, aluminum alloy, copper or appropriate non-combustible material. An exception is made for systems where the air temperature will not exceed 120°C (250°F), which is typical for most residential space heating systems. This means that polyvinylchloride pipe-type ducts are permitted for high velocity heating systems. Return air grilles are usually located on inside walls at floor level. The bottom plate of the wall and subfloor below are cut to make a
passage between the wall grille and the return air duct located in the joist below. When a partition that houses a supply heating or return air duct is supported on doubled floor joists, the joists should be separated by blocking to provide room for the duct and to avoid cutting framing members. A lintel is required when a stud has to be cut to accommodate a large return air grille. It is framed in the same way as the door opening shown in Figure 69. Even if no studs are cut, blocking must be tightly secured in place within the stud space above the return air grille opening to ensure that air is returned from the room and not the wall space above. Return air grilles may also be located at high-wall locations near the ceiling. In this case, the wall framing must accommodate the cross section of the return air duct branch from ceiling to floor. Blocking above the return air grille is not required due to the presence of the wall top plate. A return air grille should be provided in each bedroom, living room, dining room, family room and any other space where people may spend time—apart from kitchens and bathrooms. As a minimum, one or two return air grilles can be located in central locations on each floor, but the airflow route between the supply air diffusers and return air grilles can become restricted when interior doors are closed. Restricted airflow routes can reduce the amount of heat delivered to a room, which, in turn, can cause comfort and condensation problems within the room. It can also cause unfavourable air pressures between rooms and across the building envelope. Transom grilles installed above doorways, or air transfer grilles installed in interior doors, can help prevent these problems but sound transmission and privacy problems may result. The supply air ducts running between the main supply air trunk duct and each room are typically 100 mm (4 in.), 125 mm (5 in.) or 150 mm (6 in.) in diameter, depending on