30
Checking Static Pressure
(80% Furnace Shown, 90% Similar)
Figure 36
CUT FOUR CORNERS
AFTER REMOVING SHEET
METAL
CUT USING TIN SNIPS
PRESS OUT BY HAND
SCRIBE LINES OUTLINING
DUCT FLANGES
Duct Flange Cut Outs
Figure 37
E
DGES
OF
SHEET
METAL
HOLES
MAY
BE
SHARP
. U
SE
GLOVES
AS
A
PRECAUTION
WHEN
REMOVING
SHEET
METAL
FROM
RETURN
AIR
OPENINGS
.
WARNING
Example:
static reading from return duct = -.1" w.c.
static reading from supply duct = .3" w.c.
total external static pressure on this system = .4" w.c.
NOTE:
Both readings may be taken simultaneously and read directly on
the manometer if so desired. If an air conditioner coil or Electronic Air
Cleaner is used in conjunction with the furnace, the readings must also
include theses components, as shown in the following drawing.
4. Consult proper tables for the quantity of air.
If the total external static pressure exceeds the maximum listed on the
furnace rating plate, check for closed dampers, registers, undersized and/
or oversized poorly laid out duct work.
The temperature rise of the furnace must be within the temperature rise
range listed on the furnace rating plate.
B
OTTOM
R
ETURN
A
IR
O
PENING
[U
PFLOW
M
ODELS
]
The bottom return air opening on upflow models utilizes a “lance
and cut” method to remove sheet metal from the duct opening in
the base pan. To remove, simply press out the lanced sections by
hand to expose the metal strips retaining the sheet metal over the
duct opening. Using tin snips, cut the metal strips and remove the
sheet metal covering the duct opening. In the corners of the open-
ing, cut the sheet metal along the scribe lines to free the duct
flanges. Using the scribe line along the duct flange as a guide, unfold the
duct flanges around the perimeter of the opening using a pair of seamer
pliers or seamer tongs.
NOTE:
Airflow area will be reduced by approxi-
mately 18% if duct flanges are not unfolded. This could cause perfor-
mance issues and noise issues.
When the furnace is used in connection with a cooling unit, the furnace
should be installed in parallel with or on the upstream side of the cooling
unit to avoid condensation in the heating element. With a parallel flow
arrangement, the dampers or other means used to control the flow of air
must be adequate to prevent chilled air from entering the furnace and, if
manually operated, must be equipped with means to prevent operation
of either unit unless the damper is in the full heat or cool position.
When the furnace is installed without a cooling coil, it is recommended
that a removable access panel be provided in the outlet air duct. This opening shall be accessible when the furnace is installed and
shall be of such a size that the heat exchanger can be viewed for visual light inspection or such that a sampling probe can be inserted
into the airstream. The access panel must be made to prevent air leaks when the furnace is in operation.
When the furnace is heating, the temperature of the return air entering the furnace must be between 55°F and 100°F.
