SERVICING
35
To determine proper air movement, proceed as follows:
1.
With clean filters in the furnace, use a 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. (Pos-
itive Pressure)
3. Add the two (2) readings together for total external
static pressure.
NOTE:
Both readings may be taken simultaneously and
read directly on the manometer if so desired. If an air con-
ditioner 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.
4. Consult proper tables for the quantity of air.
If the total external static pressure exceeds the minimum
or maximum allowable statics, check for closed dampers,
registers, undersized and/or oversized poorly laid out duct
work.
.
Checking Static Pressure
CHECKING TEMPERATURE RISE
The more air (CFM) being delivered through a given fur-
nace, 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 ex-
ceed 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 sta-
bilize.
4. Adjust temperature rise by adjusting the circulator
blower speed. Increase blower speed to reduce tem-
perature 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 relat-
ed to the BTUH output of the furnace and the amount
of air (CFM) circulated over the heat exchanger. Mea-
sure motor current draw to determine that the motor is
not overloaded during adjustments.
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
