MORTEX PRODUCTS INC. 501 TERMINAL RD FORT WORTH, TX 76106 Page 15
Furnace and Outdoor Unit With Separate Transformers
If the furnace and the outdoor unit have separate transformers, it
is important to use a thermostat with isolated heating and cool-
ing contacts “RC” and “RH” to prevent interconnection of Class II
24 VAC systems. Most modern thermostats have separate heating
and cooling contacts for use with homes that have an air handler
and outdoor unit that are completely separate and each have a
24 VAC transformer for system control. These thermostats have a
“RC” terminal for cooling and a “RH” terminal for heating. Connect
the RED wire from the “R” terminal on the outdoor unit to the “RC”
terminal on the thermostat and the “R” terminal from the furnace
low voltage terminal block (LVTB) to the “RH” terminal on the
thermostat. If the “R” circuits from the furnace and outdoor unit
with separate transformers are both connected to the thermostat
“R” terminal, a transformer burnout can occur or the outdoor
unit control system could go into lockout mode. If a furnace and
outdoor unit with separate transformers are being installed and
the thermostat does not have the “RC” and “RH” terminals, a new
thermostat with “RC” and “RH” terminals must be purchased and
installed.
IMPORTANT: Cycle the air handler and outdoor unit separately to
make sure both operate correctly.
Separate Thermostats For Heating and Cooling
If heating and cooling are controlled by separate thermostats, a
thermostat interlock switch (See Figure 21) is required to prevent
the furnace and the outdoor unit from operating at the same time.
When using separate thermostats for heating and cooling,
a thermostat interlock must be placed in the thermostat “R”
circuit. Simultaneous operation of heating and cooling can
result in equipment overheating, equipment damage, and
wasted energy.
Do not connect a wire to the thermostat Y terminal unless
an outdoor unit is installed.
Do not locate thermostat within 3 feet of any of the
following items:
1 Supply air registers
2 Lights or heat lamps
3 Aquariums
4 Televisions, stereo, amplifiers, surround sound systems
5 Stoves or any cooking appliance
6 Refrigerator
7 Washer and/or dryer
8 Hot water tank
9 Any hot water
10 Within 15 feet of an electric space heater
11 Within 2 feet of sunlight
Thermostat Heat Anticipator
Some thermostats have a heat anticipator setting that must be
set to the settings shown below in order to function correctly. If
the heat anticipator setting is too low, the system will short cycle.
If the heat anticipator setting is too high, the system will run
long heat cycles thus causing the temperature to overrun the
temperature setting. This will cause the homeowner or user to
feel too warm by the time the blower completes its cycle and too
cold by the time the system cycles on again. The heat anticipator
should be set to 0.4 for all heating kW’s.
The thermostat may be a “self-setting” type in which no heat
anticipator will be found on the thermostat, eliminating the need
for field adjustment.
Typical Heat/Cool Low Voltage Wiring Connections
1. Remove the front access panel.
2. Remove the control box cover.
3. Install a field supplied grommet in the 3/8” hole on the right
side of the furnace cabinet to protect the thermostat wire
cables.
4. Strip ½” of the insulation from the end of each wire in the low
voltage cables.
5. Insert the wire cables from the thermostat and outdoor
cooling unit through the grommet in the right side of the
furnace cabinet and through the 3/8” strain relief installed in
the control box.
6. Place the thermostat wire cable next to the furnace low
voltage terminal block (LVTB).
7. Connect the RED thermostat wire (24 VAC) to the “R” screw
terminal on the LVTB.
8. Single-Stage Heat Wiring Connection - Connect the WHITE
thermostat wire (heat) to the “W” screw terminal on the
furnace LVTB.
Two-Stage Heat Wiring Connection (12kW – 20kW Only)
- Connect the WHITE wire from the thermostat “W1” terminal
(1st stage heat) to the “W” screw terminal on the furnace
LVTB. Remove the BLACK/W2 wire (2nd stage heat)
connected to the spade terminal on the back of the furnace
LVTB “W” terminal, cut the terminal off, and strip ½” of the
insulation from the end of the wire. Connect this wire to the
BLACK wire from the thermostat W2 terminal with a wire nut.
9. Connect the GREEN thermostat wire (indoor fan) to the “G”
screw terminal on the LVTB.
10. Connect the YELLOW thermostat wire (cool) and the YELLOW
wire from the outdoor unit compressor contactor to the “Y”
screw terminal on the LVTB.
11. Connect BROWN thermostat wire (24 VAC common) and the
BROWN wire from the outdoor unit compressor contactor to
the “C” screw terminal on the LVTB.
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