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Boiler control functions
SMGB0066 rev.0 - 12/2009
Controller pack
Service Manual
4
4.6. BoIlEr ConTrol fUnCTIonS
4.6.1. Boiler Control
Configuration Specific
•
Boiler control is available only in configurations 3,4,5 (CONF3, CONF 4, CONF5)
function
•
The boiler control decides whether to switch the boiler on or off based on a proportional plus integral action (P+I) control
algorithm and the difference between the Boiler Setpoint and the Supply Water Temperature (TSUP).
The boiler will only be used when
the outdoor temperature is below the bivalence point (maximum outdoor temperature for boiler operation)
-
or the boiler has been manually released (after a heat pump fault).
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In addition the Supply-Return Difference Check is performed before allowing the boiler to switch on, unless the heat
pump is switched off or has an internal fault.
Special note
•
Parameter P33 should be set = OFF in configurations 3, 4, and 5. P33 relates to a function “Return Limit Offset” for
electric heater control. It is not recommended to use this function for boiler control. If P33 is not set to OFF, then the
boiler will be prevented from starting until this special condition is met.
note
The boiler is always enabled when there is DHW loading required, or when the Heat Pump is blocked by the
-
Tariff/Timer input.
Boiler Setpoint & Supply Setpoint Control offset
•
If the heat pump is switched off, or has an internal fault, the Boiler Setpoint = Supply Setpoint (SSUP)
If the heat pump is switched on, the water setpoint used for the boiler control depends on the value of the Supply
Setpoint. When the supply setpoint is low the boiler will only be used if the actual supply temperature is much lower than
the desired setpoint. To achieve this, the boiler setpoint is offset lower than the supply setpoint. When the supply setpoint
is higher than can be achieved by the heat pump alone, the boiler setpoint is equal to the supply setpoint. This function is
intended to help reduce the number of times the boiler is used, to emphasise energy saving operation.
If Supply Setpoint > Heat Pump maximum supply temperature – Heat Pump Sensor 0.5K,
then Boiler Setpoint = Supply Setpoint (SSUP)
If Supply Setpoint < Heat Pump maximum supply temperature – Heat Pump Sensor Offset - 0.5K,
then Boiler Setpoint = Supply Setpoint (SSUP) – Supply Setpoint Control Offset (EP31)
The heat pump maximum supply temperature depends on the outdoor temperature and the heat pump operating limits.
Control Algorithm
•
The P+I algorithm calculates a “Load factor” from 0% to 100%. The boiler is switched on when the Load Factor reaches
100% and switched off when the Load Factor reaches 0%.
Required Load Factor = PI function ( Boiler Setpoint – TSUP )
ON
BOILER
CONTROL
100%
0%
Required Load Factor
