external static pressure. The supply duct pressure must be
read between the furnace and the cooling coil. This read-
ing 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
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
2. Measure the static pressure of the supply duct. (Positive
3. The difference between the two numbers is .4" w.c.
Difference is 4
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 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 damp-
ers, registers, undersized and/or oversized poorly laid out
The temperature rise of the furnace must be within the
temperature rise range listed on the furnace rating plate.
Checking Static Pressure
(80% Furnace Shown, 90% Similar)
DGES OF SHEET METAL HOLES MAY BE SHARP
PRECAUTION WHEN REMOVING SHEET METAL FROM 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. 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 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 approximate-
ly 18% if duct flanges are not unfolded. This could cause
performance issues and noise issues.
CUT USING TIN SNIPS
CUT FOUR CORNERS
AFTER REMOVING SHEET
Duct Flange Cut Outs
SE GLOVES AS A
PRESS OUT BY HAND
SCRIBE LINES OUTLINING