Honeywell SDC Service Manual page 126

Control Functions
126
• With the frost protection setting "cycle operation", there is no
continuous demand for the heat generator, in contrast to
continuous operation.
• With system frost protection active, the heating circuit pumps
are switched on and the mixed heating circuit valves are
closed.
• As long as the measured flow temperature of the mixed
heating circuits or the heat generator temperature in the direct
heating circuit, respectively, does not drop below the current
setpoint room temperature (RT
value is forwarded to the heat generator.
• When the flow temperature drops below the current setpoint
room temperature, heating is activated.
• Once the setpoint flow temperature has reached the setpoint
room temperature and the set time ("System Parameters"
menu, parameter 19) has passed, the demand value to the
heat generator is retracted and the mixed heating circuit valve
closes while the pumps continue running.
• If no data are detected from the outside sensor (e.g. because
the sensor is defective), only the pumps are switched on while
heating is disabled.
• The set minimum and maximum limits are taken into account
while heating.
• When the heat generator is activated, the set start-up
protection conditions of the heat generator are applied. This
can mean that the heating circuit pumps are switched off
temporarily.
Frost protection function in case of heat generator fault
If system malfunction message 30-3 or 31-3 occurs (e.g. switch-
on failure of the burner(s) due to fuel shortage or burner
malfunction), priority pump switch-off functions such as boiler
start-up protection, DHW priority etc. are disabled if frost
protection is active.
The heating water circulated in the heating circuits adopts the
overall mean room temperature and reduces or delays any
freezing.
or RT ), no demand
Frost Night
EN2H-0221GE51 R0812
SDC / DHC