GMES96-U Installation Manual
25
sion. To prevent the blower from interfering with combustion air
or draft when a central return is used, a connecting duct must be
installed between the unit and the utility room wall. Never use a
room, closet, or alcove as a return air chamber.
CHECKING DUCT STATIC
Refer to your furnace rating plate for the maximum ESP (ex-
ternal duct static) rating.
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 pres-
sure 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 (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 = -.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 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.
The temperature rise of the furnace must be within the tem-
perature rise range listed on the furnace rating plate.
Checking Static Pressure
(80% Furnace Shown, 90% Similar)
Figure 30
E
DGES
OF
SHEET
METAL
HOLES
MAY
BE
SHARP
. U
SE
GLOVES
AS
A
PRECAUTION
WHEN
REMOVING
SHEET
METAL
FROM
RETURN
AIR
OPENINGS
.
WARNING
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 Outs
Figure 31