89
Rev. 2
SERVICING
S-21
CHECKING DUCT STATIC
The maximum and minimum allowable external static pres-
sures 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 insuffi-
cient air that can cause excessive temperature rise, result-
ing 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 (in-
clined manometer) to measure the static pressure of the
return duct at the inlet of the furnace. (Negative Pres-
sure)
2. Measure the static pressure of the supply duct. (Posi-
tive 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 condition-
ing coil or Electronic Air Cleaner is used in conjunction with
the furnace, the readings must also include these compo-
nents, 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, reg-
isters, undersized and/or oversized poorly laid out duct work.
AIR COMMAND
HI EFFICIENCY 80 GAS FURNACE
Ama na
Elect ronic Air Cl eaner
Caution
High Vo lta ge
To avoid personal injury, wait 15
seconds after de- energizing unit
before touching unit interior .
CUTAWAY OF DUCTWORK
TO EXPOSE COIL
SUPPLY
AIR
INCLINED
MANOMETER
RETURN
AIR
Checking Static Pressure
S-22
CHECKING TEMPERATURE RISE
Temperature rise is related to the BTUH output of the fur-
nace and the amount of air (CFM) circulated over the heat
exchanger.
All furnaces are designed for a given range of temperature
increase; that is the temperature of the air leaving the fur-
nace minus the temperature of the air entering the furnace.
The more air (CFM) being delivered through a given furnace,
the less the rise will be; so the less air (CFM) being deliv-
ered, the greater the rise. The temperature rise should be
adjusted in accordance to a given furnace specifications and
its external static pressure.
1. Check BTU input to furnace - do not exceed input rating
stamped on rating plate.
2. Take entering and leaving air temperatures.
CAREFUL: Be sure the location selected for the leaving air
temperature will not allow radiation from the heater exchanger
to the thermometer.
3. Adjust belt drive furnaces motor pulley accordingly and/
or select the proper speed tap for direct drive furnaces.
4. Take motor amperage draw to determine that the motor
is not overloaded during adjustments.
AIR COMMAND
HI EFFICIENCY 80 GAS FURNACE
RISE =
T
SUPPLY -
T
RETURN
T
SUPPLY
SUPPLY
AIR
HEAT EXCHANGER
RADIATION "LINE OF SIGHT"
T
RETURN
RETURN
AIR
Checking Temperature Rise
