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C
A
& F
IRCULATING
IR
D
- A
F
UCT WORK
IR
LOW
Duct systems and register sizes must be properly designed for
the CFM and external static pressure rating of the furnace.
Design the ductwork in accordance with the recommended
methods of "Air Conditioning Contractors of America" Manual
D.
Install the duct system in accordance with Standards of the Na-
tional Board of Fire Underwriters for the Installation of Air Condi-
tioning, Warm Air Heating and Ventilating Systems. Pamphlets
No. 90A and 90B.
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. For upflow installa-
tions requiring 1800 CFM or more, use either two side returns
or bottom return or a combination of side and bottom. Flex-
ible joints may be used for supply and return connections to
reduce noise transmission. To prevent the blower from inter-
fering 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.
C
D
S
HECKING
UCT
TATIC
Refer to your furnace rating plate for the maximum ESP (ex-
ternal duct static) rating.
Total external static refers to everything external to the
furnace cabinet. Cooling coils, filters, ducts, grilles, reg-
isters 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 complete.
The negative pressure must be read between the filter and
the furnace blower.
Excessive external static pressure will result in insuffi-
cient air which can cause excessive temperature rise. This
can cause limit switch tripping and heat exchanger fail-
ure.
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. (Nega-
tive Pressure)
2. Measure the static pressure of the supply duct. (Positive
Pressure)
3. The difference between the two numbers is your total
external static pressure.
36
ILTERS
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 conditioner
coil or Electronic Air Cleaner is used in conjunction with the
furnace, the readings must also include these components,
as shown in the following drawing.
4. Consult proper tables for the quantity of air.
If the total external static pressure exceeds the maxi-
mum 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
temperature rise range listed on the furnace rating plate.
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
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, bend 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 folded open. This could cause performance is-
sues and noise issues.
Figure 41
[U
M
]
PFLOW
ODELS
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