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Filters
Filter installation
CAUTION
All installations must have a filter installed. All filters
and mounting provision must be field-supplied.
Unauthorized modifications to Johnson Controls
products are prohibited as they may create hazards
which could result in equipment damage.
All applications require the use of a field-installed filter. All
filters and mounting provision must be field-supplied.
Install filters external to the furnace cabinet. Do not
attempt to install filters inside the furnace.
Table 3: Required minimum filter sizes (high velocity
600 FPM)
CFM (m³/min) Cabinet size Side (in.)
1200 (34.0)
A
1200 (34.0)
B
1600 (45.3)
C
2000 (56.6)
C
2000 (56.6)
D
1.
Air velocity through disposable-type filters may not
exceed 300 ft/min (91.4 m/min). All velocities over this
require the use of high velocity filters.
2.
Do not exceed 1800 CFM using one side return and a
16 x 25 filter. For CFM greater than 1800, you may use
two side returns, one side and the bottom, or one side
return with a transition to allow use of a 20x25 filter.
NOTICE
Single side return above 1800 CFM is approved
as long as the filter velocity does not exceed filter
manufacturer's recommendation and a transition is
used to allow use on a 20x25 filter.
Side return
Locate the L shaped corner locators. These indicate the
size of the cutout to make in the furnace side panel as
shown in Figure 13.
16
Installation Manual: RL18 Single-Stage Standard ECM Residential Gas Furnaces (Non-condensing Multi-position Standard
Low NOx)
Bottom (in.)
16 x 25
14 x 25
16 x 25
16 x 25
16 x 25
20 x 25
(2) 16 x 25 20 x 25
(2) 16 x 25 22 x 25
Johnson Controls Ducted Systems
Figure 13: Side return cutout markings
Install the side filter rack following the instructions
provided with that accessory. If one or more filters are
provided at another location in the return air system,
attach the ductwork directly to the furnace side panel.
Important: Some accessories such as electronic air
cleaners and pleated media may require a larger
side opening. Follow the instructions supplied with
that accessory for side opening requirements. Do
not cut the opening larger than the dimensions for
the optional return air cutout shown in Figure 12.
Horizontal filters
CAUTION
In horizontal furnace arrangement, the filter must be
located a minimum of 18 in. from the return air inlet of
furnace.
Any branch duct (rectangular or round duct) attached
to the plenum must attach to the vertical plenum before
the filter. It is important to use straps and supports as
required to support the weight of the external filter box.
Downflow filters
CAUTION
In downflow furnace arrangement, the filter must be
located a minimum of 12 in. from the return air inlet of
furnace.
Downflow furnaces are typically installed with the filters
located above the furnace, extending into the return air
plenum or duct. Any branch duct (rectangular or round
duct) attached to the plenum must attach to the vertical
plenum above the filter height. Filters may be located in
the duct system external to the furnace using an external
duct filter box attached to the furnace plenum or at the
end of the duct in a return filter grille.
Use straps and supports as required to support the
weight of the external filter box.
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