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T7200/T7300 RELAY LOGIC
The T7300 contains three switching relays. When the
mini-jumper is set for conventional applications, the relays
control first stage cooling, first stage heating, and fan.
When the mini-jumper is set for heat pump applications,
the relays control the heat pump compressor, auxiliary heat,
and fan. Because of this change in switching logic, it is
important to use Q7300C,D, or F subbases when the T7300
is set for heat pump applications and use Q7300A,B,E,G
subbases when the T7300 is set for conventional applica-
tions.
The T7200A Relay Logic is for conventional applica-
tions. The T7300B Relay Logic is for heat pump applica-
tions.
T7200/T7300 AND CONVENTIONAL
THERMOSTATS
P+I Control
The T7200/T7300 microprocessor based control requires
that the user understands temperature control and thermo-
stat performance. A conventional electromechanical or elec-
tronic thermostat does not control temperature precisely at
setpoint. Typically, there is an offset (droop) in the control
point as the system load changes. This is a phenomenon
that most people in the industry know and accept. Many
factors contribute to offset including switch differential,
thermal lag, overshoot, cycle rates and system load.
The thermostat microprocessor simultaneously gathers,
compares and computes data. Using this data, it controls a
wide variety of functions. The special proprietary algo-
rithm (program) in the thermostat eliminates the factors
causing offset. This makes temperature control more accu-
rate than the conventional electromechanical or electronic
thermostats. The temperature control algorithm is called
proportional plus integral (P+I) control.
The thermostat sensor, located on the thermostat or
remote, senses the current space temperature. The propor-
tional error is calculated by comparing the sensed tempera-
ture to the programmed setpoint. The deviation from the
setpoint is the proportional error.
The thermostat also determines integral error, which is a
deviation based on the length of error time. The sum of the
two errors is the (P+I) error. The cycle rate used to reach
and maintain the setpoint temperature is computed using
the P+I. The addition of the integral error is what differen-
tiates the thermostat from many other electronic and elec-
tromechanical thermostats. See Fig. 21.
Fig. 21—Proportional temperature control
versus P+I temperature control.
PROPORTIONAL COOL
CONTROL POINT
P+I CONTROL
POINT
PROPORTIONAL HEAT
CONTROL POINT
THERMOSTAT
HEAT SETPOINT
HEATING
100
50
While the thermostat is designed to eliminate droop,
allowing droop can be desirable in applications where
building visitors are dressed for the weather and their stays
are brief. Examples of these applications include fast food
restaurants and convenience stores. The droopless feature
of the thermostat can be overridden by placing the droop/
droopless select switch located on the subbase in the OFF
position. This removes the integral calculation from the
thermostat control scheme, allowing only the proportional
control to occur.
EQUIPMENT PROTECTION
As part of the operational sequence, the T7200/T7300
microprocessor also incorporates minimum On and Off
times for all heating and cooling stages. Minimum On and
Off times assure that rapid cycling of equipment will not
occur, extending equipment life. Minimum On/Off times
for compressor stages are two minutes On and four minutes
Off. Minimum On/Off times for heat (electric resistive or
gas) are two minutes.
T7200/T7300 THERMOSTAT OPERATION
Startup
When power to the thermostat is turned on, a startup and
initialization program begins. The startup takes place after
total loss of power (24 Vac and battery backup). At this
time, the system default values are put into memory (re-
placing the user program that was lost). Default values are
heating 68°F (20°C) and cooling 78°F (26°C) for the occu-
pied modes. Unoccupied default values are heating 55°F
(13°C) and cooling 90°F (32°C).
15
T7200/T7300/Q7300
Operation
THERMOSTAT
COOL SETPOINT
P+I CONTROL POINT
COOLING
0
50
100
PERCENT LOAD
M4414
63-4038—7
OPERATION
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