Honeywell SDC Service Manual page 125

SDC / DHC Control Functions
Operation with room temperature sensing
As long as the room temperature is above the set room setpoint,
the heating circuit pumps are running if outside temperatures are
below the set freezing limit.
If the room temperature drops below the set room setpoint,
heating is resumed.
Switch-off occurs again when the room temperature exceeds the
set room setpoint by 1 K. If at this moment the outside
temperature is still below the set freezing limit, only the heating
circuit pumps remain active.
NOTE If not every heating circuit is operated with room temperature
sensing, different frost protection functions can be assigned to the
individual heating circuits. If, for example, a mixed heating circuit
is operated with room temperature sensing and the direct heating
circuit is not, the latter's heating curve and room temperature
setpoint are to be set as low as possible.
In connection with a second outside sensor, the frost protection
function is activated as soon as one of the two outside
temperatures drops below the frost protection limit. In case of a
faulty outside sensor, frost protection is activated continuously.
ATTENTION
In connection with a room sensor, the thermostat function is
not active with active frost protection.
See also 5.2.3 "System Parameters" menu, Pg. 71
Cycle operation
The frost protection function is activated as soon as the
temperature drops below the set frost protection limit ("System
Parameters" menu, parameter 05). The frost protection function
becomes effective when frost protection is active and there is no
demand by the heating circuit.
EN2H-0221GE51 R0812 125