SERVICING
47
limit tripping, etc. Whereas not enough static may result in
motor overloading.
To determine proper air movement, proceed as follows:
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.
Both readings may be taken simultaneously and read
directly on the manometer if so desired. If an air conditioner
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.
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.
temperature rise as follows:
minutes. Check BTU input to furnace - do not exceed
input rating stamped on rating plate. Ensure all registers
partially open) position.
2. Place thermometers in the return and supply ducts as
close to the furnace as possible. Thermometers must not
heat exchanger.
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
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.
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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