25
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 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
(80% Furnace Shown)
Figure 16
Safety Circuit Description
General
T
O
AVOID
PERSONAL
INJURY
OR
DEATH
,
DO
NOT
REMOVE
ANY
INTERNAL
COMPARTMENT
COVERS
OR
ATTEMPT
ANY
ADJUSTMENT
.
E
LECTRICAL
COMPONENTS
ARE
CONTAINED
IN
BOTH
COMPARTMENTS
.
C
ONTACT
A
QUALIFIED
SERVICE
AGENT
AT
ONCE
IF
AN
ABNORMAL
OPERATION
SHOULD
DEVELOP
.
WARNING
A number of safety circuits are employed to ensure safe and
proper furnace operation. These circuits serve to control any
potential safety hazards and serve as inputs in the monitoring
and diagnosis of abnormal function. These circuits are contin-
uously monitored during furnace operation by the integrated
control module.
Integrated Control Module
The integrated control module is an electronic device which
controls all furnace operations. Responding to the thermostat,
the module initiates and controls normal furnace operation,
and monitors and addresses all safety circuits. If a potential
safety concern is detected, the module will take the neces-
sary precautions and provide diagnostic information through
an LED.
Primary Limit
The primary limit control is located on the partition panel and
monitors heat exchanger compartment temperatures. It is a
normally closed (electrically), automatic reset, temperature
activated sensor. The limit guards against the overheating
resulting from insufficient air passing over the heat exchanger.
Operational Checks
T
O
AVOID
PERSONAL
INJURY
OR
DEATH
,
DO
NOT
REMOVE
ANY
INTERNAL
COMPARTMENT
COVERS
OR
ATTEMPT
ANY
ADJUSTMENT
.
E
LECTRICAL
COMPONENTS
ARE
CONTAINED
IN
BOTH
COMPARTMENTS
.
C
ONTACT
A
QUALIFIED
SERVICE
AGENT
AT
ONCE
IF
AN
ABNORMAL
OPERATION
SHOULD
DEVELOP
.
WARNING
Checking Duct Static
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, 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.
Example:
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.
