SECTION II: DUCTWORK
DUCTWORK GENERAL INFORMATION
The duct system’s design and installation must:
1.Handle an air volume appropriate for the served space and within the operating parameters of the furnace specifications.
2.Be installed in accordance of National Fire Protection Association as outlined in NFPA standard 90B (latest editions) or applicable national, provincial, state, and local fire and safety codes.
3.Create a closed duct system. For residential and non-HUD Modular Home installations, when a furnace is installed so that the supply ducts carry air circulated by the furnace to areas outside the space containing the furnace, the return air shall also be handled by a duct(s) sealed to the furnace casing and terminating outside the space containing the furnace.
4.Complete a path for heated or cooled air to circulate through the air conditioning and heating equipment and to and from the condi- tioned space.
The cooling coil must be installed in the supply air duct, downstream of the furnace. Cooled air may not be passed over the heat exchanger.
When the furnace is used with a cooling coil, the coil must be installed parallel with, or in the supply air side of the furnace to avoid condensa- tion in the primary heat exchanger. When a parallel flow arrangement is used, dampers or other means used to control airflow must be ade- quate to prevent chilled air from entering the furnace. If manually oper- ated, the damper must be equipped with means to prevent the furnace or the air conditioner from operating unless the damper is in full heat or cool position.
When replacing an existing furnace, if the existing plenum is not the same size as the new furnace then the existing plenum must be removed and a new plenum installed that is the proper size for the new furnace. If the plenum is shorter than 12” (30.5 cm) the turbulent air flow may cause the limit controls not to operate as designed, or the limit con- trols may not operate at all.
The duct system is a very important part of the installation. If the duct system is improperly sized the furnace will not operate properly. The ducts attached to the furnace plenum, should be of sufficient size so that the furnace operates at the specified external static pressure and within the air temperature rise specified on the nameplate.
The minimum plenum height is 12” (30.5 cm). The furnace will not operate properly on a shorter plenum height. The minimum recom- mended rectangular duct height is 4” (10.2 cm) attached to the ple- num.
If a matching cooling coil is used, it may be placed directly on the fur- nace outlet and sealed to prevent leakage. If thermoplastic evaporator ‘A’ coil drain pans are to be installed in the upflow/horizontal configura- tion, then extra 2” (5.1 cm) minimum spacing may be needed to ensure against drain pan distortion.
On all installations without a coil, a removable access panel is recom- mended in the outlet duct such that smoke or reflected light would be observable inside the casing to indicate the presence of leaks in the heat exchanger. This access cover shall be attached in such a manner as to prevent leaks.
The duct system must be properly sized to obtain the correct airflow for the furnace size that is being installed.
Refer to Table 7 or the furnace rating plate for the correct rise range and static pressures.
If the ducts are undersized, the result will be high duct static pres- sures and/or high temperature rises which can result in a heat exchanger OVERHEATING CONDITION. This condition can result in premature heat exchanger failure, which can result in personal injury, property damage, or death.
DUCT FLANGES
Four flanges are provided to attach ductwork to the furnace. These flanges are rotated down for shipment. In order to use the flanges, remove the screw holding an individual flange, rotate the flange so it is in the upward position and reinstall the screw then repeat this for all 4 flanges.
If the flanges are not used, they must remain in the rotated down posi- tion as shipped.
For duct attachment, | Factory |
if needed. | installed |
FIGURE 1: Duct Attachment
DUCTWORK INSTALLATION AND SUPPLY PLENUM CONNECTION - UPFLOW/HORIZONTAL
Attach the supply plenum to the furnace outlet. The use of an approved flexible duct connector is recommended on all installations. This connection should be sealed to prevent air leakage. The sheet metal should be crosshatched to eliminate any popping of the sheet metal when the indoor fan is energized.
FLOOR BASE AND DUCTWORK INSTALLATION - DOWNFLOW
Installations on combustible material or directly on any floors must use a combustible floor base shown in Figure 8. Follow the instructions supplied with the combustible floor base accessory. This combustible floor base can be replaced with a matching cooling coil, properly sealed to prevent leaks. Follow the instructions supplied with the cooling coil cabinet for installing the cabinet to the duct con- nector. Plug intake and vent pipe holes in bottom panel and move grommet to desired vent side exit.
Downflow Air Conditioning Coil Cabinet
The furnace should be installed with coil cabinet part number specifi- cally intended for downflow application. If a matching cooling coil is used, it may be placed directly on the furnace outlet and sealed to pre- vent leakage. For details of the coil cabinet dimensions and installation requirements, refer to the installation instructions supplied with the coil cabinet.
Attach the air conditioning coil cabinet to the duct connector, and then position the furnace on top of the coil cabinet. The connection to the fur- nace, air conditioning coil cabinet, duct connector, and supply air duct must be sealed to prevent air leakage.
