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Page 24 Installer’s Information Manual
SECTION 18. ADJUSTING
BLOWER SPEED
Determine initial heating and cooling speeds in system
design stage. See Specification Sheet for airflow data.
Depending on tests performed in following sections, you
may need to change blower motor speed.
CAUTION: Heating speed tap should not be reduced below
factory setting. Doing so may result in inadequate air
circulation, and could cause excessive air temperature rise
through furnace. This could cause a high-temperature limit
switch to cycle burners on and off, reducing furnace
efficiency and shortening heat exchanger life.
All models have these four motor speed designations:
High Speed (HI) Black wire
Medium High Speed (MH) Blue wire
Medium Low Speed (ML) Yellow wire
Low Speed (LO) Red wire
WARNING: To prevent electric shock, turn off
electrical power to furnace before changing blower motor
speed.
1. To change cooling speed, move desired motor lead to
terminal marked 'COOL' on Fan Timer. See Figure 22.
Initial factory setting for cooling is high speed (black
wire).
2. To change heating speed, move desired motor lead to
terminal marked 'HEAT' on Fan Timer. See Figure 22.
Factory setting for heating is listed in Table 9. Heating
speed should not be reduced below factory setting.
Figure 22. Main Blower Speed Selection.
3. Connect two unused motor leads to terminals marked
"UNUSED MOTOR LEADS" on Fan Timer. See Figure
22.
WARNING: Failure to properly terminate unused
motor leads could cause electrical shock.
Table 9. Factory Heating Speed and Temperature Rise.
GAS*
INPUT
(Btu/hr)
MOTOR
SIZE
(HP)
AIR
TEMPERATURE
RISE RANGE
(°F)
HEATING
OPERATION
**SPEED TAP /
WIRE COLOR
54,000 1/3 35-65 MH / BLUE
72,000 1/2 35-65 ML / YEL
72,000 3/4 30-60 ML / YEL
90,000 1/2 45-75 MH / BLUE
90,000 3/4 35-65 MH / BLUE
108,000 3/4 35-65 MH / BLUE
126,000 3/4 45-75 MH / BLUE
* Gas Input and Motor HP can be found on rating plate.
** These are factory settings.
Table 10. Cooling Speed Selection.
GAS* MOTOR AIR CONDITIONING TONNAGE
INPUT SIZE
AT 0.5 INCHES WC
(Btu/hr) (HP) 1.5 2 2.5
3 3.5 4 5
54,000
1/3 LO ML MH
HI -- -- --
72,000
1/2 -- LO ML HI -- -- --
72,000
3/4 -- -- LO ML MH HI --
90,000
1/2 -- LO ML HI -- -- --
90,000
3/4 -- -- LO ML MH HI --
108,000
3/4 -- -- -- -- LO ML HI
126,000
3/4 -- -- -- -- LO ML HI
* Gas Input and Motor HP can be found on rating plate.
This table gives speed tap settings for installations with duct
static pressure of 0.5” W.C. Duct work with a static pressure
less than 0.5” W.C. will have increased airflow. To determine
correct speed tap settings at ductwork static pressures other
than 0.5” W.C., see specification sheet.
SECTION 19. MEASURING DUCT
SYSTEM STATIC PRESSURE
You will need a 0 to 1 inch W.C. slope gauge with 0.01-inch
resolution and two pressure measurement taps to measure
duct system static pressure.
System airflow can be determined from Specification Sheet
when duct system static pressure is known. Improper
airflow in heating mode may result in poor heating
performance and reduced heat exchanger life. Improper
airflow in cooling mode may cause poor cooling
performance or air-conditioning coil freeze-up.
High duct system static pressure is an indication of an
overly restrictive duct system. Static pressure in excess of
0.5 inches W.C. indicates a need for duct system redesign
to ensure proper volume of airflow.
Follow this procedure:
1. Open supply-air registers and return-air grilles. Make
sure registers and grilles are free of obstruction from
rugs, carpets, drapes or furniture.
2. Set balancing dampers in supply duct system.
3. Check ductwork for obstructions or leaks.
4. Make sure filters are clean and in place.