24
Total external static refers to everything external to the fur
-
nace cabinet. Cooling coils, filters, ducts, grilles, registers
must all be considered when reading your total external static
pressure. The supply duct pressure must be read between
the furnace and the cooling coil. This reading is usually taken
by removing the “A” shaped block off plate from the end on
the coil; drilling a test hole in it and reinstalling the block off
plate. Take a duct static reading at the test hole. Tape up the
test hole after your test is complete. The negative pressure
must be read between the filter and the furnace blower.
Too much external static pressure will result in insufficient air
that can cause excessive temperature rise. This can cause
limit switch tripping and heat exchanger failure.
To determine total external duct static pressure, proceed as
follows;
1. With clean filters in the furnace, use a draft gauge (inclined
manometer) to measure the static pressure of the return
duct at the inlet of the furnace. (Negative Pressure)
2. Measure the static pressure of the supply duct. (Positive
Pressure)
3. The difference between the two numbers is .4” w.c.
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 tem
-
perature rise range listed on the furnace rating plate.
Bottom Return Air Opening [Upflow Models]
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 cor
-
ners of the opening, 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 approximately 18% if duct
flanges are not unfolded. This could cause performance
issues and noise issues.
CUT FOUR CORNERS
AFTER REMOVING SHEET
METAL
CUT USING TIN SNIPS
PRESS OUT BY HAND
SCRIBE LINES OUTLINING
DUCT FLANGES
Duct Flange Cut Out
Figure 32
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 condensa tion in the heating element.
With a parallel flow arrange ment, 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 rec
-
ommended 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 ex
-
changer 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.
NOTE: In a horizontal installation the air conditioning coil
must be adequately supported by proper brackets and sup
-
ports. Inadequate coil support can result in furnace cabinet
distortion and air leakage.
When the furnace is heating, the temperature of the return
air entering the furnace must be between 55°F and 100°F.
Filters - Read This Section Before Installing The
Return Air Duct work
Filters must be used with this furnace. Discuss filter maintenance
with the building owner. Filters do not ship with this furnace, but
must be provided, sized and installed externally by the installer.
Filters must comply with UL900 or CAN/ULCS111 standards.
Damage or repairs due to the installation of the furnace
without filters is not covered under the warranty.
On upflow units, guide dimples locate the side return cutout loca
-
tions. Use a straight edge to scribe lines connecting the dimples.
Cut out the opening on these lines.
NOTE:
An undersized
opening will cause reduced airflow.
Refer to the Filter Sizing Chart to determine filter area re
-
quirements.
