Goodman GMVM Service Instructions Manual page 45

OPERATION
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 (in-
clined manometer) to measure the static pressure of the
return duct at the inlet of the furnace. (Negative Pres-
sure)
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 = -0.1" w.c.
static reading from supply duct = 0.3" w.c.
total external static pressure on this system = 0.4"
w.c.
NOTE: Both readings may be taken simultaneously and
read directly on the manometer if so desired. If an air condi-
tioner coil or Electronic Air Cleaner is used in conjunction
with the furnace, the readings must also include theses com-
ponents, 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.
Checking Static Pressure
B R A O
OTTOM ETURN IR PENING
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 re-
move the sheet metal covering the duct opening. In the corners
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 is-
sues.
E
DGES OF SHEET METAL HOLES MAY BE SHARP
PRECAUTION WHEN REMOVING SHEET METAL FROM RETURN AIR
.
OPENINGS
CUT USING TIN SNIPS
CUT FOUR CORNERS
AFTER REMOVING SHEET
METAL
Duct Flange Cut Outs
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 ele-
ment. 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 recom-
mended 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.
[U M ]
PFLOW ODELS
WARNING
. U
SE GLOVES AS A
PRESS OUT BY HAND
SCRIBE LINES OUTLINING
DUCT FLANGES
45