Lennox MERIT Series Installation Instructions Manual page 9

Return Air -- Upflow Applications
Return air can be brought in through the bottom or either
side of the furnace installed in an upflow application. If the
furnace is installed on a platform with bottom return, make
an airtight seal between the bottom of the furnace and the
platform to ensure that the furnace operates properly and
safely. The furnace is equipped with a removable bottom
panel to facilitate installation.
Markings are provided on both sides of the furnace cab-
inet for installations that require side return air. Cut the
furnace cabinet at the maximum dimensions shown on
page 2.
NOTE - 60C and 60D units that require air volumes over
1800 cfm (850 L/s) must have one of the following:
1 - Single side return air with optional return air base
with transition that must accommodate required 20
x 25 x 1 in. (508 x 635 x 25 mm) air filter to maintain
proper air velocity.) See figure 8.
2 - Bottom return air.
3 - Return air from both sides.
4 - Bottom and one side return air.
Refer to Product Specifications for additional information.
AIR FLOW
14−1/2 (368) A Width (65W75)
17−1/2 (446) B Width (50W98)
21 (533) C Width (50W99)
24−1/2 (622) D Width (51W00)
NOTE- Optional Side Return Air Filter Kits are not for use with Optional Return Air Base.
1
Both the unit return air opening and the base return air opening must be covered by a single plenum or IAQ cabinet.
Minimum unit side return air opening dimensions for units requiring 1800 cfm or more of air (W x H): 23 x 11 in. (584 x 279 mm).
The opening can be cut as needed to accommodate plenum or IAQ cabinet while maintaining dimensions shown.
Side return air openings must be cut in the field. There are cutting guides stenciled on the cabinet for the side return air
opening. The size of the opening must not extend beyond the markings on the furnace cabinet.
2
To minimize pressure drop, the largest opening height possible (up to 14 inches 356 mm) is preferred.
Optional Return Air Base
(Upflow Applications Only -- For use with A, B, C and D cabinets)
FURNACE
FRONT
IF BASE
IS USED
WITHOUT
IAQ CABINET,
INDOOR AIR
A SINGLE
QUALITY
RETURN AIR
CABINET
PLENUM
(PCO, Filter MUST
COVER BOTH
Cabinet, etc.)
UNIT AND
RETURN
AIR BASE
OPENINGS
OPTIONAL
RETURN
AIR BASE
AIR FLOW
3−1/4
(83)
7−1/4
(184)
Figure 9
Page 9
Single Side Return Air
(with transition and filter)
20" X 25" X 1"
(508mm X 635mm X 25mm)
Cleanable Filter
Return Air
Plenum
Transition
Figure 8
1
23 (584)
Overall
1
(Maximum)
Minimum
11 (279)
Maximum
2
1
Unit side return air
14 (356)
Opening
SIDE RETURN
5−5/8
AIR OPENINGS
(143)
(Either Side)
23
(584)
26−7/8
(683)
SIDE VIEW
1
22−7/16
(570)
Overall
(Maximum)
3/4
(19)