18
TWA-SVX01A-EN
Supplementary Heat
The 1
st
stage of supplementary heat is cycled on when the
indoor temperature drops approximately 2 degrees below the
thermostat setting. TSH-1 establishes compressor operation
through the “Y” terminal. TSH-2 contacts close providing
power to the “W” terminal on the indoor thermostat, which
supplies power to the supplementary heater control circuit
when the auxiliary fan contacts (F-AUX) are closed.
Additional supplementary heat or stages of heat may have
been added with the use of outdoor thermostats, (not shown
on the schematic). If the outdoor temperature falls below the
setpoint on the outdoor thermostat, the additional stags of
heat will come on. When it rises above the set point, the
system will revert back to 2
nd
stage supplementary heating.
As the indoor temperature approaches indoor thermostat set
point, TSH-2 contacts will open, shutting off supplementary
heat and TSH-1 contacts will remain closed until indoor
temperature is satisfied, shutting off the complete system.
Emergency Heat
An emergency heat switch (EMERG) is provided within the
thermostat. When placed in the emergency heat position,
TSH-1 circuit opens to “Y” terminal and closes to “X2”
terminal. Supplementary heat now operates on 1
st
call for
heating. “G” terminal sequence remains unchanged at this
point. The outdoor thermostats (if used) are by-passed.
System Start Procedure
“T” (Thermistor) Circuit
The “T” circuit, used with both the manual and automatic
changeover thermostats in conjunction with heat pump
applications, is used for anticipation of outdoor condition
changes. This thermistor varies the resistance as temperature
varies in an inverse proportional relationship. It is located in the
outdoor unit on the defrost control timer (DFC) and is in series
with a fixed value resistor on the thermostat between “R” and “T”
terminals. Should this “T” circuit from the outdoor unit become
inoperative, the thermostat could be out of calibration by as
much as 6 to 10 degrees. Before attempting to calibrate the
thermostat, be sure to check this circuit for proper operation.
Indoor Fan
The evaporator fan is controlled by an ON/AUTO switch on the
room thermostat. With the switch positioned at AUTO and the
unit operating in the cooling mode, fan operation coincides with
the thermostat calling for cooling. If the unit is operating in the
heating mode while the fan switch is at AUTO, fan operation
coincides with the thermostat calling for heating. When the fan
switch is positioned at ON, fan operation is continuous.
Low Outdoor Ambient Cooling
The Evaporator Defrost Control is standard equipment on Air
Handlers and will permit low ambient cooling down to 35
degrees F. For cooling operation down to 0 degrees F, use an
accessory Head Pressure control on the outdoor unit.
