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RETURN AIR
In order for the furnace to work properly, a closet or alcove must have a
certain total free area opening for return air.
For Heating Only Furnace
2
2
Minimum 200 in
(1290 cm
) free area opening.
Use Return Grille 7900-287P/B, or any Return Grille with minimum 200
2
in
free area opening.
For A/C and HP Applications (Standard Blower)
2
2
Minimum 250 in
(1613 cm
) free area opening. Use accessary blower
package 3500-7901, or use Return Grille 7900-287P/B, 1FG0620BK
(hinged), or Louvered Door 3500-1581, 3500-5851 (bulk pack), or any
Return Grille with minimum 250 in
For A/C and HP Applications (Accessory Blower)
2
2
Minimum 330 in
(2129 cm
) free area opening. Use 5 ton blower
accessory 3500-7901 use Return Grille 1RF1025BK, 1FG0125
(hinged), or Louvered Door 3500-1591, 3500-5861 (bulk pack), or any
Return Grille with minimum 330 in
The return air opening can be located in a closet front door or a sidewall
above the furnace casing, or in a louvered door on the furnace. If open-
ing for the return air is located in the floor, side walls or closet door any-
where below furnace casing height, 6" (15.2 cm) minimum clearance
must be provided on the furnace side where return is located to provide
for proper air flow. See Figure 2. The 6" (15.2 cm) minimum clearance
is not required if there is a return grille installed above the furnace
height. This return grille cannot start more than three feet above the fur-
nace height.
For Upflow installations, a closet 32" (81.3 cm) wide by 30" (76.2 cm)
deep with a 30" (76.2 cm) wide door is necessary. See Figure 3.
When installing furnace in a separate closet or room which is accessible
only through an outside door, a minimum of 200 in
opening for return air must be provided. The supply and return air must
be ducted, securely attached and be sealed to the furnace casing if
there are grilles in the outside door to the closet. Openings where ducts
pass through walls, the floor or the ceiling must be sealed to prevent air
leakage into or from closet and the living area.
1-5/8" Min., 2-3/8 Max.
Return Air
30"
Pass Through
Frame Width
FIGURE 2: Upflow Closet Clearances
Provisions shall be made to permit the return of circulating air from all
rooms and living spaces, except the bathroom(s) to the circulating air
supply inlet of the furnace. Failure to comply may cause improper heat-
ing and may cause the furnace to cycle on the limit.
DUCT SYSTEM DESIGN
Electric furnace is designed to operate at a given static pressure.
In order to assure proper air flow through the furnace, the distribution
system must be designed so that the static pressure external to the fur-
nace does not exceed the static pressure rating shown on the furnace
rating plate.
The number, size and placement of registers should be such that even
distribution of heat is provided throughout the home.
Johnson Controls Unitary Products
2
2
(1613 cm
) free area opening.
2
2
(2129 cm
) free area opening.
2
(1290 cm
32"
25-1/2""
30"
Closet
Wide Door
SECTION IV: DOWNFLOW FURNACE
INSTALLATION
7900 SERIES DUCT CONNECTOR
We have designed our duct connector to eliminate a sub-base require-
ment. Table 1 will help you in deciding the part number of the duct con-
nector you need.
Provide adequate clearance for servicing.
1.
Locate furnace conveniently away from wall facing or partitions to
permit easy removal of components.
2.
A six (6) inch space minimum should be maintained between the
furnace and closet door when door is used for return air.
3.
Two (2) feet of space must be available in front of furnace for
future servicing (blower, element or furnace removal, etc.).
TABLE 1: Duct Connector for Electric Furnaces
FLOOR TO
DUCT DIMENSIONS
1" (2.54 cm)
2" (5.1 cm)
3" (7.6 cm)
4" (10.2 cm)
5" (12.7 cm)
6" (15.2 cm)
7" (17.8 cm)
8" (20.3 cm)
9" (22.8 cm)
10" (25.4 cm)
11" (282 cm)
2
) free
12" (30.5 cm)
13" (33 cm)
- Indicates connector above or below could be used depending on
tolerance in floor to duct dimension.
- Indicates connector above could be used depending on toler-
ance in floor to duct dimension.
- Indicates connector below could be used depending on tolerance
in floor to duct dimension.
DUCT CONNECTORS (7990 SERIES)
FIGURE 3: Duct Connector Depth (7990 Series)
These duct connectors are for connecting the furnace to an under the
floor supply duct system. The furnace may be installed on combustible
flooring without a separate sub-base.
266180-BIM-B-0209
FINGERED
SCREW
STYLE
TAB STYLE
7990-6211
7990-6011
7990-6221
7990-6021
7990-6241
7990-6041
7990-6261
7990-6061
7990-6271
7990-6071
7990-6281
7990-6081
7990-6301
7990-6101
7990-6321
7990-6121
Floor
Duct Connector
Depth
SUPPLY DUCT
Floor
Joist
3
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