16
SUPPLY AIR
TEMPERATURE
The temperature rise of the air through
a furnace will vary with each furnace.
For proper temperature rise of the
furnace in question, check the rating
plate on the furnace or Table 7 in this
manual.
A. Temperature rise is the
temperature difference between
the air entering and leaving the
furnace.
B. The proper way to measure the
temperature rise follows: First
open all registers and dampers to
allow proper air flow. Then allow
the furnace to operate 10 to 15
minutes before taking the
readings.
C. Use two thermometers; insert one
in the supply air duct and one in
the return air duct. Do not place a
thermometer in the supply duct
directly above the heat exchanger
as the thermometer will pick up
radiant heat from the heat ex-
changer. (See Figure 19.)
D. Calculate by subtracting the return
air temperature from the supply air
temperature.
Example
Supply air
temperature 155 degrees
Return air
temperature
75 degrees
Temperature
rise = 80 degrees
E. Compare the calculated tempera-
ture rise with the specified
temperature rise on the rating
plate.
TEMPERATURE RISE TOO
HIGH
• Check for proper fan speed
• Check static pressure on duct
system for proper air flow.
• Check proper cooling coil size.
• Check duct static pressures for
proper air flow.
• Check for proper oil pressure.
• Check nozzle size for overfiring.
• Check for a clean air filter.
• Check duct sizes to assure
adequate size for desired air flow.
The blower speed can be increased to
deliver more air over the heat
exchanger if the duct system is large
enough to allow the increase. If the
duct system is not of adequate size in-
creasing fan speed may result in less
air flow than originally found.
TEMPERATURE RISE TOO
LOW
• Check for proper fan speed.
• Check nozzle size to make sure it
is of proper size.
• Check for excessive soot in the
heat exchanger. Low system
efficiency may indicate a sooted
heat exchanger.
The motor speed can be changed to a
lower speed to decrease the air flow
over the heat exchanger.
TRUE-SPOT
True-Spot smoke tester (see figure 19)
by Bacharach is used to check smoke
levels in flue products.
Figure 19
Determining Temperature Rise
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