30
Total external static refers to everything external to the
furnace 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 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
(inclined 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.
(Positive Pressure)
3. The difference between the two numbers is .4” w.c.
For example:
-.1
0
.1
.2
.3
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: B
otH
reaDIngs
MaY
Be
taken
sIMultaneouslY
anD
reaD
DIrectlY
on
tHe
ManoMeter
If
so
DesIreD
. I
f
an
aIr
conDItIoner
coIl
or
e
lectronIc
a
Ir
c
leaner
Is
useD
In
conjunctIon
wItH
tHe
furnace
,
tHe
reaDIngs
Must
also
IncluDe
tHeses
coMPonents
,
as
sHown
In
tHe
followIng
DrawIng
.
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 duct work.
note: t
He
teMPerature
rIse
of
tHe
furnace
Must
Be
wItHIn
tHe
teMPerature
rIse
range
lIsteD
on
tHe
furnace
ratIng
Plate
.
Complete information regarding tank sizing for vaporiza-
tion, recommended regulator settings, and pipe sizing is
available from most regulator manufacturers and propane
gas suppliers.
Always use a pipe thread sealant approved for all gases.
DUCT WORK
Duct systems and register sizes must be properly designed
for the CFM and external static pressure rat ing of the
furnace. Design the ductwork in accor dance with the
recommended methods of “Air Conditioning Contractors of
America” Manual D.
Install the duct system in accordance with Standards of
the National Board of Fire Underwriters for the Installation
of Air Conditioning, 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
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
con nections 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.
When furnace duct(s) supply air outside the space
containing the furnace, a return air duct must terminate in
the same space as the supply duct and be sealed to the
furnace casing.
c
HeckIng
D
uct
s
tatIc
WARNING
n
eVer
allow
tHe
ProDucts
of
coMBustIon
,
IncluDIng
carBon
MonoxIDe
,
to
enter
tHe
return
Duct
work
or
cIrculatIon
aIr
suPPlY
.
Refer to your furnace rating plate for the maximum ESP
(external duct static) rating.
