Bryant DUAL FUEL THERMOSTAT TSTAT - Manual (Page 2)

If selection of balance point temperature is such that it is below
thermal balance point, all operation between these 2 temperatures
will be part HP and part furnace. A heating cycle will begin with
HP operation. Because HP has insufficient capacity, structure
temperature will slowly fall. After a time, thermostat will issue
next stage call, turning off HP and turning on furnace. Furnace will
operate until thermostat is completely satisfied at which time
furnace will turn off. The next heating cycle will begin with HP
and cycle will repeat.
This operating mode is acceptable as long as selected balance point
temperature is not too far below thermal balance point. This
thermostat has no "droop" between stages, and structure will not
cool significantly below setpoint before furnace comes on. How-
ever, there is a 15-minute timer which requires HP to operate for
15 minutes before furnace comes on. If balance point temperature
is set too far below thermal balance point, the temperature loss
during this 15 minute period could be uncomfortable.
For most installations balance point temperature setting should be
in the range of 25˚F to 45˚F. If set too high, HP will not be used
enough and its advantages not realized. If set too low, structure
temperature loss during HP operation will become uncomfortable.
In case of 2-speed heat pumps or 2-stage furnaces, thermostat will
start equipment at low capacity and if this is not sufficient, will
advance to higher capacity. If load demand is between low and
high capacities, it will cycle between these 2 stages, i.e. between
low and high heat pump speeds or between furnace low and high
fire. However, once operation proceeds from HP to furnace, it will
NOT cycle directly back to HP. Furnace will remain on until all
stages of thermostat are satisfied.
DESCRIPTION OF OPERATION
COOLING OPERATION—SINGLE SPEED
HP cooling:
O/W2 energizes reversing valve to select cooling operation.
G energizes furnace blower.
Y/Y2 energizes compressor and selects high blower speed at
furnace.
When cooling demand is satisfied, G and Y/Y2 are de-energized.
O/W2 will remain on to minimize cycling of reversing valve. It
will turn off only when a call for heat occurs.
COOLING OPERATION—TWO SPEED
Low-speed HP cooling:
O/W2 energizes reversing valve to select cooling operation.
G energizes furnace blower.
Y1/W2 energizes compressor at low speed and may determine
furnace blower speed.
High-speed HP cooling:
Y/Y2 is added to low-speed cooling call to operate compressor at
high speed and increase blower speed.
HEATING OPERATION—SINGLE-SPEED HP
HP heating:
O/W2 remains off to select heating operation.
G turns on furnace blower.
Y/Y2 energizes compressor and increases furnace blower speed.
HEATING OPERATION—TWO-SPEED HP
Low-Speed HP heating:
O/W2 remains off to select heating operation.
G turns on furnace blower.
Y1/W2 turns on compressor at low speed and may also adjust
blower speed for low-speed HP operation.
High-Speed HP heating:
Y/Y2 is added to low-speed HP heating call to operate compressor
at high speed and adjust blower for high-speed HP operation.
HEATING OPERATION—SINGLE-STAGE FURNACE
W/W1 causes furnace to operate. Furnace controls its own blower
at heating speed.
HEATING OPERATION—TWO-STAGE FURNACE—
THERMOSTAT CONTROL
Low fire:
W/W1 causes furnace to operate at low fire. Furnace controls its
own blower at low heating speed.
High fire: Y1/W2 is added to low-fire heating call to operate
furnace in high-fire mode. Furnace controls its own blower at high
heating speed.
HEATING OPERATION—TWO-STAGE FURNACE—
ALGORITHM CONTROL
W/W1 causes furnace to operate. Based on amount of time it is
required to be on, it determines when to operate at low fire and
when to operate at high fire. It controls its own blower at proper
speed.
DEFROST OPERATION
A unique feature of this thermostat is that a defrost cycle, once
started, will always be completed. This is true even if thermostat is
satisfied during defrost. The result is that a heating cycle never
begins with an uncompleted defrost and its associated "cold blow".
During operation of HP in heating mode, HP itself determines
when a defrost is necessary. It initiates defrost by energizing its O
and W wires. The signal on the O wire switches HP from heating
to cooling mode and W signal starts furnace. Thermostat monitors
this action by sensing the signal (which it did not create) on the O
line. It responds by turning on its W outputs (both if 2-stage
furnace) to hold furnace on high fire. At completion of defrost,
indicated by removal of signal from HP on O wire, thermostat does
1 of 2 things. If it is satisfied, it turns off all Y, G, and W outputs
which results in all equipment off. If not satisfied, it will turn off
Y and G, leaving W on until it becomes satisfied. In this way it
assures that furnace will be used to satisfy a heat demand existing
after a defrost cycle is completed.
EMERGENCY HEAT
If EHEAT mode is selected at thermostat, all heating will be done
by furnace.
CONTINUOUS FAN
If FAN ON mode in selected, thermostat energizes G terminal,
which causes fan to operate when both heating and cooling are off.
STAGING SEQUENCE—HEATING
Depending on whether HP is single or 2 speed and whether furnace
is single or 2 stage, the staging of thermostat outputs with
increasing demand varies. Table 1 shows the heating staging
sequence for all 4 possible combinations of single- and multi-stage
equipment. Note that thermostat itself has a maximum of 3 heating
stages. Note also that at outdoor temperatures below selected
balance point temperature no HP operation is allowed. As dis-
cussed earlier, once furnace is turned on, it remains on to satisfy all
demand.
A "freeze protect" feature is built into the dual fuel thermostat. If
indoor temperature sensor fails, and thermostat is in HEAT or
AUTO mode, heat will be cycled when outdoor temperature is
below 50˚F. The amount of heat supplied will increase as outdoor
temperature decreases, keeping structure from freezing.
INSTALLATION
NOTE: All TSTAT models do not have batteries and are not
"power stealing". They do require 24VAC (R and C terminals) to
be connected for proper operation. Thermostat will not operate
without these 2 connections.
—2—