The importance of proper air flow over the heat exchanger cannot
be over emphasized .
One of the most common causes of heat exchanger failure is
overheating due to low air flow . An air flow table is located inside
the blower door and on the following pages .
TEMPERATURE RISE CHECK
To determine if the air flow is correct, make a temperature rise
check .
1 . Insert a thermometer in the supply air duct as close to the
furnace as possible yet ot of a direct line from the heat ex-
changer . See Figure 31 .
2 . Insert a thermometer in the return air duct as close to the fur-
nace as possible .
3 . Operate the furnace .
4 . When the thermometer in the supply air duct stops rising (ap-
proximately five minutes), subtract the return air temperature
from the supply air temperature . The difference is the tem-
perature rise .
5 . Compare the measured temperature rise to the approved
temperature rise range listed on the furnace name plate . See
Figure 32 .
If the measured temperature rise is below the nameplate range,
the gas input rate may need to be adjusted or the airflow may be
too high . If the gas input is correct, select the next lowest speed
tap for the heating function . If the measured temperature rise is
above the nameplate range, the gas input rate may need to be
adjusted or the duct system may be too restrictive due to under-
sized ducts . An undersized return duct is a common cause of an
overly restrictive duct system .
IMPORTANT: The measured temperature rise should be in the
middle of the range.
IMPORTANT: Some high-efficiency filters have a greater than
normal resistance to airflow. This can adversely affect furnace op-
eration. BE SURE TO CHECK AIRFLOW if using any filter other
than factory-provided filter.
45
Airflow
AIRFLOW
!
CAUTION
IT IS IMPORTANT THAT EACH DUCT SYSTEM BE SIZED
AND INSTALLED FOR THE SPECIFIC APPLICATION BY
PROPERLY APPLYING THE APPROPRIATE INDUSTRY
ACCEPTED STANDARD . IF LESS THAN MINIMUM STAN-
DARDS ARE APPLIED, THE EQUIPMENT USER COULD
EXPECT TO EXPERIENCE HIGHER UTILITY BILLS,
MAJOR COMPONENT FAILURE, VARYING DEGREES OF
AIR NOISE OR OTHER UNSATISFACTORY ISSUES, OVER
WHICH THE MANUFACTURER HAS NO CONTROL .
FIGURE 31
TEMPERATURE RISE MEASUREMENT
WARNING
THE MEASURED TEMPERATURE RISE MUST BE WITHIN
THE TEMPERATURE RISE RANGE SPECIFIED ON THE RAT-
ING PLATE . OPERATING THE UNIT WITH A RISE OUTSIDE
THE STATED RANGE COULD CAUSE DAMAGE TO THE
HEAT EXCHANGER OR INTERMITTENT OPERATION .
THIS COULD CAUSE INJURY OR DEATH AND WILL VOID
THE MANUFACTURER’S WARRANTY FOR THIS PRODUCT.
FIGURE 32
TYPICAL FURNACE NAME PLATE
!
Summary of Contents for 801C Series
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