Operation - Honeywell T7350 Product Data

T7350 COMMERCIAL PROGRAMMABLE THERMOSTAT

OPERATION

Startup Operation
Upon initial thermostat powerup, a startup and initialization
program begins. This startup occurs only on initial powerup.
After total loss of power for an extended period, the current
time and day can be lost (requiring reset). However, the
thermostat retains the user program.
NOTE: With no program set, the thermostat controls to the
Occupied default setpoints of 70°F (21°C) for heat
and 75°F (24°C) for cool.
T7350 Relay Logic
All T7350 model thermostats contain four switching relays. In
conventional applications, the relays control first stage cooling,
first stage heating, fan, and auxiliary. In heat pump
applications, the relays control the heat pump compressor,
changeover, fan, and emergency heat.
NOTE: Models T7350B,D, T7350H1009 have subbases with
more relays to accommodate extra stages.
Logic with Modulating Outputs (T7350H1017, M, only)
The specific relay logic depends on whether or not staging is
employed. See Table 14.
NOTE: The modulating outputs always provide a signal.
Table 14. T7350 Modulating Relay Logic Exceptions.
Modulating
a
Output Stages
On 0 On
(1 to 100
percent)
1 or 2 Staged
Cycling
Off 0,1, or 2 Off
(0 percent)
a
See Table 15 for modulating output signal details.
To display the status of the modulating outputs:
1. Cycle through information displays using the info key.
2. The device displays a percentage value for each output:
a. MODHEAT on one screen.
b. MODCOOL on another screen.
Table 15. Modulating Output Signal.
Action 0 percent
Direct 4 mA
Reverse 20 mA
O/B Terminal for Heat or Cool Changeover
The O/B terminal controls heat pump changeover. The default
operation is for the terminal to be powered when calling for
heat (or while the most recent call was for heat). This
corresponds to a typical O terminal.
NOTE: The O/B terminal can be configured to operate as a
typical B terminal (powered on call for cool).
63-2605—5
Relay Action
Heat/
Cool Fan
Output reaches 10%: On
Output falls below 5%: Off
On/Off matches equipment
stage operation
Off
100 percent
20 mA
4 mA
Emergency Heat
(select T7350 Thermostats only)
With the system set for Em Heat, auxiliary heat serves as
stage one; compressor stages are locked off. The fan cycles
with the auxiliary heat.
Equipment Protection
As part of the operational sequence, the T7350
microprocessor incorporates cycle rate, and minimum on and
off times for all heating and cooling stages. This extends
equipment life as it prevents rapid cycling of equipment.
NOTE: Minimum on and off times are fixed.
Cycle Rates
The thermostat control algorithm maintains the temperature by
cycling stages of heating or cooling to meet setpoint. Cycle
rates, in cycles per hour (cph) are set using a PDA.
NOTE: Defaults are: heat: 6cph, cool: 3cph.
P+I+D Control
The T7350 microprocessor-based control requires that the
user understands temperature control and thermostat
performance. A conventional electromechanical or electronic
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.
IMPORTANT
• P+I+D (Loop Tuning) parameters are optimized for
proper operation of a vast majority of HVAC systems.
Only when completely certain of necessary and
proper changes should you alter these values.
• Improper changes result in poor system performance
and equipment problems such as compressor short
cycling. Other problems include wide swings in space
temperature and excessive overdriving of modulating
outputs.
All adjustments to Loop Tuning parameters should be gradual.
After each change, allow the system to stabilize to accurately
observe the effects of the change. Then, as needed, make
further refinements until the system operates as desired.
If adjustment of PID parameters is required, use the following:
NOTE: In the items that follow, the term "error" refers to the
difference between the measured space temperature
and the current actual space temperature setpoint:
— The Throttling Range (TR), also called Proportional Gain,
determines the impact of the error on the output signal.
Decreasing TR amplifies the error effect; that is, for a given
error, smaller TR causes higher output signal.
— The Integral Time (IT), also called Integral Gain,
determines the impact of the error-over-time on the output
signal. Error-over-time has two components making up its
value: amount of time the error exists; and size of the error.
The higher the IT, the slower the control response. In other
words, a decrease in IT causes a more rapid change to the
output signal.
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