SERVICING
25
CHECKING DUCT STATIC
The maximum and minimum allowable external static
pressures are found in the specification section. These
tables also show the amount of air being delivered at a
given static by a given motor speed or pulley adjustment.
The furnace motor cannot deliver proper air quantities
(CFM) against statics other than those listed.
Too great of an external static pressure will result in in
-
sufficient air that can cause excessive temperature rise,
resulting in limit tripping, etc. Whereas not enough static
may result in motor overloading.
To determine proper air movement, 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. 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 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 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 related
to the BTUH output of the furnace 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.
