MG3P Installation Manual

32
Verifying Operation of the Supply Air Limit Switch
A properly functioning limit switch should turn off the gas valve
when the return is blocked (time depends on how well the return
air is blocked). The circulating air and combustion blowers should
continue to run when the limit switch opens.
1. Verify the blower door is securely mounted in place and that
there is power to the furnace.
2. Block the return airflow to the furnace by installing a close-off
plate in place of or upstream of the filter(s).
3. Set the thermostat above room temperature and observe the
Operating Sequence.
4. Remove the close-off immediately after the limit switch opens.
If the furnace continues to operate with no return air, set the
thermostat below room temperature, shut off the power to
the furnace, and replace the limit switch.
MG3P OPERATING SEQUENCE
The operating sequences for the heating, cooling, and fan modes
are described below. Refer to the field and furnace wiring and
ladder diagrams: Figure 45 (page 42), Figure 46 (page 43),
Figure 47 (page 44), & Figure 48 (page 45).
Heating Cycle
1. The thermostat calls for heat by energizing the W terminal
with 24VAC.
2. The control verifies the pressure switch is open.
3. If the pressure switch is open, the control energizes the inducer
and waits for the pressure switch to close. The pressure switch
must close within 10 seconds.
4. The inducer runs for a 30 second pre-purge.
5. The control energizes the Hot Surface Igniter (HSI) output
for the appropriate warm-up time limit.
6. The main gas valve energizes for 3 seconds.
7. If the flame proved and ignites the gas, the control de-energizes
the HSI. The gas valve and inducer remains energized. The
control goes to blower on delay.
8. If flame is present, the control energizes the blower on the
selected HEAT speed 30 seconds after the gas valve opened.
The gas valve and inducer remain energized.
9. When the thermostat demand for heat is satisfied, the control
de-energizes the gas valve. The Inducer output remains on
for a 30 second post-purge period.
10. Blower off timing begins when the thermostat is satisfied.
The control will operate at the selected HEAT speed of 60,
90, 120, or 180 seconds. If the blower off delay jumper is
not present, the fan should still operate for 120 seconds
at the selected HEAT speed. The Indoor blower motor is
de-energized after a blower off delay as selected by the
movable jumper.
Cooling Cycle
1. The thermostat calls for cooling by energizing the G & Y
terminals with 24VAC.
2. The control energizes the blower in cooling speed and sends
24VAC to the contactor in the condensing unit.
3. When the thermostat removes the call for cooling, the contactor
in the outdoor condensing unit is de-energized and the control
continues to run the fan for a period of 60 seconds.
Fan Mode
• When the thermostat energizes the G terminal for continuous
fan (without calling for heat or cooling), the indoor fan is
energized on the selected FAN speed.
• If a call for cooling occurs during continuous fan, the blower
will switch over to the selected COOL speed.
• If the W terminal receives a call for heat during continuous
fan, the blower will de energize.
• A call for fan is ignored while in lockout.
EXAMPLE:
• Time for 1 revolution of a gas meter with a 1 cubic ft dial
= 40 seconds.
• From Table 14 read 90 cubic ft gas per hr.
• Local heating value of the gas (obtained from gas supplier)
= 1,040 Btu per cubic ft.
• Input rate = 1,040 x 90 = 93,600 Btuh.
5. Multiply the gas flow rate in cubic ft per hr by the heating
value of the gas in Btu per cubic ft to obtain the input rate in
Btuh. See example.
6. The manifold pressure must be set to the appropriate value
for each installation by a qualified installer, service agency
or the gas supplier. See page 27.
Verifying & Adjusting Temperature Rise
Confirm the temperature rise through the furnace is within the
limits specified on the furnace rating plate. Any temperature rise
outside the specified limits could result in premature failure of
the heat exchanger.
1. Place thermometers in the return and supply air stream as
close to the furnace as possible. The thermometer on the
supply air side must be shielded from direct radiation from
the heat exchanger to avoid false readings.
2. Adjust all registers and duct dampers to the desired position
and run the furnace for 10 to 15 minutes before taking any
temperature readings. The temperature rise is the difference
between the supply and return air temperatures.
For typical duct systems, the temperature rise will fall within the
limits specified on the rating plate with the blower speed at the
factory recommended setting. If the measured temperature rise
is outside the specified limits, it may be necessary to change
the speed of the blower.
NOTE: Lowering the blower speed will increase the temperature
rise and a higher blower speed will decrease the temperature rise.
The furnace is equipped with a 5-speed ECM motor. Heating
and cooling speed selection is made by moving the switch on the
integrated control inside of the furnace. The furnace is shipped
from the factory pre-wired for 4 blower speeds. There is a fifth
alternate speed available on the motor. To utilize the alternate
speed, remove the wire at the motor plug of the desired speed to
be changed (H, MH, ML, L). Reconnect this wire to the alternate
motor plug speed tap.
Verifying Burner Operation
CAUTION:
The door over the burners may only be opened
for inspection purposes only. The door must be
installed during unattended operation.
1. Remove the burner compartment door.
2. Set the thermostat 5 degrees above room temperature and
observe the ignition sequence.
NOTE: The flame should carry over immediately between
all burners without lifting off, curling, or floating. The flames
should be blue, without yellow tips.
3. After validating the flame, change thermostat setting to below
room temperature.
4. Verify the burner flame is completely extinguished.
5. Replace the burner compartment door.