4. Return Air Connections
a. For upflow applications:
Bottom return air usage is required as part of any upflow return air configuration. If additional return air is to enter 1 side of each furnace,
in addition to bottom return air, cut open 1 entire return-air opening in appropriate side of each furnace. (See Fig. 1.)
WARNING: UNIT DAMAGE AND FIRE HAZARD
Failure to follow this warning could result in unit damage, fire, personal injury or death.
DO NOT use the back of the furnace for return-air duct connections in upflow position, as limit switch cycling will occur.
b. For downflow applications:
Return air can only be connected to bottom opening of furnace. A common return air plenum is required for proper auxiliary limit switch
operation. (See Fig. 2.)
5. For upflow applications
a. Position furnaces against each other on return-air plenum. (See Fig. 21.)
b. Drill two 1/8-in. holes, approximately 1 in. below discharge air flange, from inside top of discharge opening and through both furnaces.
(See Fig. 25.)
c. Drive 1 factory-supplied No. 6 X 3/4-in. LG screw through each hole and tighten until furnaces are secure and foam strips have sealed
gap between furnaces.
6. For downflow applications
a. Position furnaces on downflow kits or plenum (no A/C coils) or on entering air-side of A/C coils. (See Fig. 21.)
b. Drill two 1/8-in. holes, approximately 1 in. below return air flange, from inside top of return air opening and through both furnaces. (See
Fig. 25.)
c. Drive 1 factory-supplied No. 6 X 3/4-in. LG screw through each hole and tighten until furnaces are secure and foam strips have sealed
gap between furnaces.
7. Determine which side of furnace will be used to route external extension harness. If condensate drain hole is available, use it, otherwise,
drill two 7/8-in holes in blower housing area, mid-point between blower deck and bottom of furnace.
NOTE:
The external extension harness cannot be used on the same side of the furnace that the return air ducts connect to. Locate harness on
opposite side of furnace where return air is used.
8. Connect return and supply-air ducts to furnaces. Seal duct connections to prevent air leakage.
9. Follow individual furnace installation instructions for upflow/downflow applications. This includes, but not limited to: condensate trap,
condensate/inducer housing tubing, pressure switch tubing venting and electrical connections.
C.
Horizontal, Back-to-Back Configuration
When twinning furnaces in the horizontal position, consideration must be made to the type of building construction. Attic floors should be
constructed to support normal live and dead loads of the furnaces and the person(s) servicing them.
Trusses, wood and metal are engineered for specific applications, and may not support the weight of two (2) furnaces suspended from the top
chords or the bottom chords of the trusses. Long horizontals spans may flex or sag, resulting in damage to the building. Contact the truss
manufacturer for additional design and engineering assistance.
Fig. 25—Attaching Furnaces Together-Downflow
A02219
—28—