SERVICING
47
Checking Static Pressure
S-201 CHECKING TEMPERATURE RISE
The more air (CFM) being delivered through a given furnace,
the less the rise will be; so the less air (CFM) being
delivered, the greater the rise. The temperature rise should
be adjusted in accordance to a given furnace specifications
and its external static pressure. An incorrect temperature
rise may result in condensing in or overheating of the heat
exchanger. An airflow and temperature rise table is provided
in the blower performance specification section. Determine
and adjust temperature rise as follows:
1. Operate furnace with burners firing for approximately ten
minutes. Check BTU input to furnace - do not exceed
input rating stamped on rating plate. Ensure all registers
are open and all duct dampers are in their final (fully or
partially open) position.
2. Place thermometers in the return and supply ducts as
close to the furnace as possible. Thermometers must not
be influenced by radiant heat by being able to “see” the
heat exchanger.
SUPPLY
AIR
RETURN
AIR
Checking Temperature Rise
3. Subtract the return air temperature from the supply air
temperature to determine the air temperature rise. Allow
adequate time for thermometer readings to stabilize.
4. Adjust temperature rise by adjusting the circulator blower
speed. Increase blower speed to reduce temperature rise.
Decrease blower speed to increase temperature rise.
Refer to
Circulator Blower Speed
section in the Product
Design section of this manual for speed changing details.
Temperature rise is related to the BTUH output of the fur-
nace and the amount of air (CFM) circulated over the heat
exchanger. Measure motor current draw to determine that
the motor is not overloaded during adjustments.
S-300 CHECKING PRIMARY LIMIT CONTROL
Primary limit controls are nonadjustable, automatic reset,
bi-metal type limit control. Refer to the following drawing for
the location of the primary limit.
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2
PRIMARY LIMIT
CONTROL
Primary Limit Control Location
The following drawing illustrates the style of limit switches
used on the 90% furnaces.
ENCLOSED DISK
FRONT VIEW
SIDE VIEW
Primary Limit Control Style
(90% Furnaces)
