MG3P Installation Manual

30
Grounding
WARNING:
To minimize personal injury, the furnace cabinet
must have an uninterrupted or unbroken electrical
ground. The controls used in this furnace require
an earth ground to operate properly. Acceptable
methods include electrical wire or conduit
approved for ground service. Do not use gas
piping as an electrical ground!
START-UP & ADJUSTMENTS
Pre-Start Check List
√ Verify the polarity of the connections are correct, the line
voltage power leads are securely connected and the furnace
is properly grounded.
√ Verify the thermostat wires (R, W, Y, & G) are securely
connected to the correct leads on the terminal strip of the
circuit board.
√ Verify the jumper setting (for fan speed) on the control board.
To select fan speed, see Figure 42 (page 31) and set the
jumper on the control board.
√ Verify the gas line service pressure does not exceed 10.0
inches of W.C., and is not less than 4.5 inches W.C. for natural
gas. For LP gas the line service pressure must not exceed
14 in. W.C., and must not be less than 11.0 in. W.C.
√ Verify the roll-out and manual reset switch is closed. If
necessary, press the red button to reset a switch. DO NOT
install a jumper wire across a switch to defeat its function.
If a switch reopens on startup, DO NOT reset the switch
without identifying and correcting the fault condition.
√ Verify the blower door is in place, closing the door switch in
the line voltage circuit.
√ Verify the gas line has been purged and all connections are
leak free.
Start-up Procedures
Do not perform these steps until all of the checks in the previous
steps have been completed:
1. Set the thermostat to the lowest setting.
2. Turn off all electrical power to the furnace.
3. Follow the Operating Instructions on the label attached to the
furnace.
4. Set the thermostat above room temperature and verify the
Operating Sequence (page 32).
5. After 5 minutes of operation, set the thermostat below room
temperature and verify steps 9 & 10 of the Operating Sequence.
Verifying & Adjusting Input Rate
IMPORTANT NOTE:
The input rate must not exceed the rate shown on the furnace
rating plate. At altitudes above 2,000 feet, it must not exceed
that on the rating plate less 4% for each 1,000 feet.
The input rate must be verified for each installation to prevent
over-firing of the furnace. To determine the exact input rate,
perform the following procedures:
1. Shut off all other gas fired appliances.
2. Start the furnace and run it for at least 3 minutes.
3. Measure the time (in seconds) required for the gas meter to
complete one revolution.
4. Convert the time per revolution to cubic feet of gas per hour
using Table 14 (page 47).
Heat Anticipator Setting
Please Set the heat anticipator according to the instructions
supplied by the thermostat manufacturer. To determine the
heat anticipator setting:
1. Add the current draw of the system components; or
2. Measure the current flow on the thermostat R-W circuit after
the circulating blower motor has started.
Twinning
WARNING:
When servicing either twinned furnace, power
must be turned off on both furnaces. Failure to
comply may result in improper operation leading
to damage to the furnaces or personal injury!
Single stage MG3 furnaces are not supplied with a built-in twinning
capability. For twinning of single stage furnaces, a twinning kit
is available for purchase. The two-stage MG3S furnace is not
approved for twinning. Refer to the technical sales literature
for part numbers. Please follow the instructions provided
with the kit.
Single stage furnaces are equipped with a fixed speed blower;
therefore, the twin terminal on the blower control boards Figure
43 (page 31) may be used to twin the single stage furnaces
only if the following criteria are met:
• Both furnaces and motors must be the same size.
• Both motors must be on the same speed for cooling and
heating.
• Both furnaces must have a common return duct and common
supply plenum.
• Both furnaces must be the same phase and on the same leg
of power.
• Furnaces equipped with variable speed iSEER ™ motors
may not be twinned under any circumstances.
For twinning of single stage furnaces equipped with fixed speed
blowers, refer to Figure 43 and the following instructions:
1. Turn off all power to both furnaces.
2. Attach a wire between the two twin terminals on the twinning
control boards. Use field supplied wire and two 3/16” wire
terminals. NOTE: One furnace can be used for one stage of
heating and the other furnace can be used for the second
stage of heating. The installer also has the choice of running
one furnace only or both furnaces. In both cases the blowers
will run at the same time and at the same speeds:
• Single stage heating: The W connection on each furnace
must be connected together and then connected to the W
connection of the thermostat. This will allow both furnaces
to ignite at the same time for one stage heating.
• Two stage heating: The W connection on one furnace must
be connected to the W1 connection of the thermostat. The W
connection of the second furnace must be connected to the
W2 connection of the thermostat. This will allow one furnace
to ignite for one stage of heating and the second furnace to
ignite for the second stage of heating.
3. Refer to Figure 43 (page 31) for the remaining thermostat
connections. The C connection must be made between the
twinned furnaces to ensure proper performance.