31
A closed return duct system must be used, with the return duct
connected to the furnace.
NOTE:
Ductwork must never be at-
tached to the back of the furnace.
For upflow installations
requiring 1800 CFM or more, use either two side returns or
bottom return or a combination of side /bottom. Flexible joints
may be used for supply and return connections to reduce noise
transmission. To prevent the blower from interfering with com-
bustion 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 cham-
ber.
C
HECKING
D
UCT
S
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 cabinet. Cooling coils, filters, ducts, grilles, regis-
ters must all be considered when reading your total exter-
nal 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 rein-
stalling the block off plate. Take a duct static reading at
the test hole. Tape up the test hole after your test is com-
plete. 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 (in-
clined 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. (Posi-
tive 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
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 damp-
ers, registers, undersized and/or oversized poorly laid out
duct work.
Checking Static Pressure
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 sec-
tions 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 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 perfor-
mance issues and noise issues.
E
DGES
OF
SHEET
METAL
HOLES
MAY
BE
SHARP
. U
SE
GLOVES
AS
A
PRECAUTION
WHEN
REMOVING
SHEET
METAL
FROM
RETURN
AIR
OPENINGS
.
WARNING
OPERATION
