Honeywell SUITEPRO TB8575 - Manual (Page 16)

TB6575/TB8575 SUITEPRO™ DIGITAL FAN COIL THERMOSTATS
62-0278—07 16
2 Pipes with Auto Changeover
For this application the system switch only provides "Off"
and "Auto". When in "Auto" mode the water temperature
will indicate if the thermostat should operate in heating or
cooling.
Operation:
After exiting the installer setup, the thermostat will perform
a 5 minute purge. During this initial 5 minute purge, the
valve (W/Y) will energize and the fan will be de-energized.
After the 5 minute purge, the thermostat will go into the
appropriate mode as described in Table 6.
Table 6. Logic for 2 Pipes with Auto Changeover
*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 manual fan will
be available. It will stay in this operation until the next
purge cycle occurs.
2 Pipes with Auxiliary Heat
For this application, when there is hot water in the pipes,
the system switch provides "Off" and "Heat". When there
is cold water in the pipes, the system switch provides
"Off", "Heat", and "Cool".
Operation:
After exiting the installer setup, the thermostat will perform
a 5 minute purge. During this time, the valve (W/Y) will
energize and the fan and auxiliary heat (Y/A) will be de-
energized. After the 5 minute purge, the thermostat will
go into the appropriate mode as described by Table 7.
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 hours 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 1 , and if the
power is reset.
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.
1 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".
Pipe Temperature after purge System Mode
>Threshold for Heating (IS code #7) Heat
Between Thresholds
After Purge Occurs
Fan Only*
< Threshold for Cooling (IS code #6) Cool
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