Checking Duct Static
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 and 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 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
To determine total external duct static pressure, proceed as
1. With clean filters in the furnace, use a draft gauge (inclined
manometer) to measure the static pressure of the return air
duct at the inlet of the furnace (negative pressure).
2. Measure the static pressure of the supply air duct (positive
3. The difference between the 2 numbers is 0.4" W.C.
Static reading from the return air duct = -0.1" W.C.
Static reading from the supply air 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 illustration.
4. Consult the 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 ductwork.
Checking Static Pressure—80% Furnace Shown, 90%
A. Supply air
B. Cutaway of ductwork to expose coil
C. Inclined manometer
D. Return air
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.
1. To remove, simply press out the lanced sections by hand to
expose the metal strips retaining the sheet metal over the
2. Using tin snips, cut the metal strips and remove the sheet
metal covering the duct opening.
3. In the corners of the opening, cut the sheet metal along the
scribe lines to free the duct flanges.
4. 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.
Edges of sheet metal holes may be sharp. Use gloves as
a precaution when removing hole plugs.
Duct Flange Cutouts
A. Cut using tin snips.
C. Scribe lines outlining duct flanges.
B. Press out by hand.
D. Cut 4 corners after removing sheet metal.
Filters—Read This Section Before Installing the
Return Air Ductwork
Filters must be used with this furnace. Discuss filter
maintenance with the building owner.
Filters do not ship with this furnace, but must be provided,
sized and installed externally by the installer.
Filters must comply with UL900 or CAN/ULCS111 standards.
If the furnace is installed without filters, the warranty will be
On upflow units, guide dimples locate the side return cutout
locations. Use a straight edge to scribe lines connecting the
dimples. Cut out the opening on these lines.
NOTE: An undersized opening will cause reduced airflow.
Refer to Minimum Filter Requirement charts to determine filter