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TB6575/TB8575 DIGITAL FAN COIL THERMOSTATS
17 62-0311—13
Table 7. Logic for 2 Pipes with Auxiliary Heat
If pipe temperature is between the two threshold values
after the 5 minute purge occurs, the thermostat will
activate a second 5 minute purge to double check the
water temperature. If after the second 5 minute purge, the
water temperature is still between the two thresholds, the
valve output will be disabled and only auxiliary heat will be
available for heating.
Auxiliary heat (Y/A) always de-energizes during purges.
Purge Cycles for 2 Pipe Seasonal Changeover
Applications
For 2 Pipe with Auto Changeover and 2 Pipe with
Auxiliary Heat applications, the thermostat will run purge
cycles to determine if there is hot or cold water in the
pipes.
A 5 minute purge will occur every 2 or 24 hours (IS 25) to
ensure that the pipe sensor is sensing the correct mode
during seasonal changeover months.
A 5 minute purge will also occur anytime the installer
setup or installer test menus are exited, whenever the
thermostat is switched from its “Off” position, and if the
power is reset.
NOTE: For the 2 Pipe with Auxiliary Heat application,
the thermostat must be in the “Off” position
for more than 30 minutes before a 5 minute
purge will occur when it is switched back into
“Heat” or “Cool.”
4 PIPES AUTO CHANGEOVER
Single Set Point Method
In 4 pipe auto changeover with a single setpoint, the
temperature is always controlled to the setpoint. Switching
points are used to determine when to switch between
heating and cooling modes. If the current mode is heat
and the temperature drifts above the cool switch point
(Setpoint + Switching Differential), the thermostat will
switch to heat operation and will heat the space until
setpoint is reached.
For this application, the setpoint setting and switching
points are illustrated in Fig. 29. The switching differential
is defined via IS code 10.
Fig. 29. 4 Pipe Auto Changeover with Single Setpoint
and Switching Points
Heat/Cool Setpoint Method
In 4 pipe auto changeover with heat and cool setpoints,
the system key is used to switch between the heating
setpoint and cooling setpoints. Use the Up and Down
arrow buttons to change the setpoint.
For this application, the setpoint settings and deadband
are illustrated in Fig. 30. The deadband is changed via IS
code 10.
Fig. 30. 4 Pipe Auto Changeover setpoints and
deadband.
Pipe
Temperature
After Purge Heat Mode Cool Mode
> Threshold
for Heating
(IS code 7)
Valve (W/Y)
energized on calls
for heat
Changeover.
Mode changes to
Heat.
Between
Thresholds
After Purge
Occurs
Aux Heat (Y/A)
energized on calls
for heat
Changeover. Mode
changes to Heat
because Cool is not
available.
< Threshold
for Cooling
(IS code 6)
Mode stays in Heat
but Aux Heat (Y/A)
energized on calls
for heat
Valve (W/Y)
energized on calls
for cool
M31330
HEAT SWITCHING POINT
COOL SWITCHING POINT
SWITCHING
DIFFERENTIAL
SETPOINT
DEADBAND
COOLING SETPOINT
HEATING SETPOINT
M27565