Checking Duct Static; Bottom Return Air Opening [Upflow Models ] - Goodman GMH95 Installation Instructions Manual

A closed return duct system must be used, with the return duct
connected to the furnace. NOTE: Ductwork must never be attached
to the back of the furnace. Flexible joints may be used for supply
and return connections to reduce noise transmission. To prevent
the blower from interfering with combustion air or draft when a
central return is used, a connecting duct must be installed be-
tween the unit and the utility room wall. Never us a room, closet, or
alcove as a return air chamber.
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 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 en-
tering the furnace must be between 55°F and 100°F.
C D S
HECKING UCT TATIC
Refer to your furnace rating plate for the maximum ESP (external
duct static) rating.
Total external static refers to everything external to the furnace cabi-
net. Cooling coils, filters, ducts, grilles, registers must all be con-
sidered when reading your total external static pressure. The sup-
ply duct pressure must be read between the furnace and the cool-
ing 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 fol-
lows;
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 Pres-
sure)
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 di-
rectly 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.
Checking Static Pressure
(80% Furnace Shown, 90% Similar)
B R A O
OTTOM ETURN IR
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 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 is-
sues and noise issues.
E
DGES OF SHEET METAL HOLES MAY BE SHARP
PRECAUTION WHEN REMOVING HOLE PLUGS
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 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
32
[U M
PENING PFLOW ODELS
WARNING
. U
SE GLOVES AS A
.
PRESS OUT BY HAND
SCRIBE LINES OUTLINING
DUCT FLANGES
]