Heat generator

This function block controls the heat generator - the boiler, the heat pump or the combined heat and power unit. It accepts the heat request, derives the setpoint for the device from it and switches the burner with all protection times: minimum run time, minimum pause time and switch-on delay. In addition it provides stage 2 for two-stage devices, a power setting for modulating devices, the boiler circuit pump with run-on and the return temperature boost as boiler protection.

The block counts operating hours and starts and reports when maintenance is due. Both counters survive a restart of the controller.

Whatever the user shall change at runtime is an input. Enable, manual operation and the maintenance acknowledgement are therefore inputs and can be assigned to addresses. There is no dedicated control element - for the visualisation a text display on output ST and value displays on SV, LE, BT and AZ are sufficient.

Sequence
Request. The block expects the request in the form that all heat consumers supply: a bit on input AN and the desired temperature on input AT. Each form on its own works as well - if only a temperature greater than 0 is present and AN is not linked at all, that counts as a request too. If several consumers come together, a block that forms the maximum belongs in front of it.
Setpoint. The excess is added to the requested temperature; the result is limited to boiler setpoint minimum and maximum and appears on output SV. If the request does not report a temperature at all, boiler setpoint minimum applies. Without a request SV is 0.
Burner. The burner switches on when the boiler temperature is below the setpoint by the switching hysteresis, and off again when it exceeds the setpoint. In between are the protection times: after switching on it runs at least the minimum run time, after switching off it stays off at least the minimum pause time.
The minimum pause takes precedence over the request. A request arriving during the pause is not discarded: it remains pending and the burner starts by itself once the pause has elapsed. No new edge change is required.
Immediate switch-off reasons. A lock on input SP, a missing enable on FR, a missing boiler sensor and a device fault with the behaviour "switch off" bypass the minimum run time - protection against the device takes precedence over protection of the device.
Stage 2. With a two-stage device stage 2 is added when stage 1 has been running longer than the release delay and the boiler temperature still has not reached the setpoint. It then runs for at least the same time so that it does not cycle.
Power setting. With a modulating device a controller derives a power of 0 to 100 % on output LE from the deviation between setpoint and boiler temperature. It is limited to power minimum and maximum - a burner driven below its minimum power goes into fault. When the burner is off, LE is 0.
Boiler circuit pump. The pump runs with the burner and continues for the run-on time afterwards. The run-on dissipates the residual heat.
Return temperature boost. It protects the generator against dew point corrosion and knows three control types. With three-point valve a controller drives the mixing valve via outputs RA and RZ; with valve percent the same control value goes to RP as a percentage for continuous actuators; with pump switch-off the boiler circuit pump is cycled until the return is above the minimum temperature. Control only takes place while the pump is running - without circulation the return temperature says nothing. The default is off; then RA, RZ and RP stay silent.
Pump switch-off and minimum run time. The pump switch-off control type only switches the pump output, never the burner - its minimum run time is therefore not undercut. So that the boiler does not overheat without circulation, the pump restarts at the latest after the maximum switch-off time - and then keeps running until the return has recovered.
Switch-off limit via outdoor temperature. If it is switched on and the outdoor temperature is above the limit, the request is discarded. Domestic hot water must not be affected by this, otherwise there is no hot water in summer - therefore a value other than 0 on input WW lifts the limit. The default is off.
Operating hours, starts and maintenance. The block counts the run time of the burner in hours (output BT) and its starts (output AZ). Both values are retained across a restart of the controller. If a maintenance interval is set, output WA reports that maintenance is due as soon as that many operating hours or that many starts have accumulated since the last acknowledgement. For a generator the number of starts is the more meaningful wear figure than the operating hours, which is why maintenance can be tied to either. An edge from 0 to a value other than 0 on input QU acknowledges; the counters themselves keep running.
Fault. A message on input SM only applies once it has been pending longer than the configured delay. After that output SR is set, the status text spells out the fault and, depending on the setting, the generator switches off or keeps running. SR also reports a missing boiler sensor.
Missing sensor. Input KT is the only mandatory input. If it is not linked the block cannot switch: it reports a fault and leaves the burner off. An unassigned input returns 0 - the burner would run permanently.
Example
Gas condensing boiler, single-stage, serving one heating circuit and one domestic hot water charge. Boiler setpoint minimum 40 °, maximum 80 °, excess 5 K, switching hysteresis 5 K. Minimum run time 300 s, minimum pause time 300 s, pump run-on 300 s. Return temperature boost off - a condensing boiler wants a cold return. The heating circuit requests 45 °: SV becomes 50 °. If the boiler temperature drops below 45 °, the burner starts and stays in operation for at least five minutes, even if the 50 ° are already reached after two minutes. It then switches off and stays off for five minutes. If during that time the hot water charge requests 60 °, nothing happens at first - the status text names the remaining minimum pause - and once the pause has elapsed the burner starts by itself with SV 65 °.

Inputs
AN	Request	Heat request as a bit. Other than 0 requests heat, 0 does not. Optional. Not linked: input AT alone counts - if a temperature greater than 0 is present there, that is taken as a request.
AT	Request temperature	Desired flow temperature of the consumer in °C. The excess is added on top. Optional. Not linked or 0: the parameter boiler setpoint minimum applies.
KT	Boiler temperature*	Actual temperature of the heat generator in °C. Mandatory input. Not linked: the block reports a fault and leaves the burner off - an unassigned input returns 0 and the burner would run permanently.
RL	Return temperature	Actual temperature of the boiler return in °C, the measured value of the return temperature boost. Optional. Not linked: there is no return temperature boost, regardless of the control type set.
AU	Outdoor temperature	Current outdoor temperature in °C, only needed for the switch-off limit. Optional. Not linked: the switch-off limit has no effect, even if it is switched on.
FR	Enable	0 locks the block, other than 0 enables it. The lock takes effect immediately and bypasses the minimum run time. Optional. Not linked: the block counts as enabled.
SP	Lock	Other than 0 locks the block, 0 enables it. Acts like a missing enable and likewise immediately. Optional. Not linked: never locked.
SM	Device fault	Collective fault message of the heat generator. Other than 0 reports a fault; it only applies once it has been pending longer than the configured delay. Optional. Not linked: a device fault is never detected.
HB	Manual operation	Other than 0 requests the generator regardless of the request. The protection times remain in force - they protect the device and are not bypassed manually either. Optional. Not linked: no manual operation.
WW	Hot water request	Other than 0 lifts the switch-off limit via the outdoor temperature so that hot water is still available in summer. The input has no other influence on the sequence. The value comes from the output of the same short name on the heat request block; which request counts as hot water is set there per request via its kind. That way the generator hangs on a single block. Optional. Not linked: the switch-off limit applies to every request.
QU	Maintenance acknowledgement	The change from 0 to a value other than 0 is evaluated. It resets the maintenance message; operating hours and start counter keep running unchanged. Optional. Not linked: a reported maintenance stays pending.
Outputs
BR	Burner	Enable of the heat generator. 1 while the burner shall run, otherwise 0.
S2	Stage 2	Second burner stage. Only with the two-stage device, otherwise permanently 0.
LE	Power	Power setting 0 to 100 % for modulating devices. 0 with single-stage and two-stage devices and while the burner is off.
SV	Device setpoint	Setpoint given to the generator - requested temperature plus excess, limited to boiler setpoint minimum and maximum. 0 without a request.
PU	Boiler circuit pump	1 while the pump shall run. Runs with the burner and continues for the run-on time; the pump switch-off control type additionally cycles this output.
RA	Return boost open	Travel pulse valve open. Only with the three-point valve control type, otherwise permanently 0.
RZ	Return boost close	Travel pulse valve close. Only with the three-point valve control type, otherwise permanently 0.
RP	Return boost percent	Control value 0 to 100 % for a continuous actuator. Only with the valve percent control type, otherwise permanently 0.
ST	Status text	Plain text for a text display in the visualisation, for example "Anforderung 55.0 C (Kessel 48.2 C)", "Mindestpause 180 s", "Bedarf gedeckt 55.0 C (Kessel 58.0 C)", "Stufe 2, Anforderung 60.0 C", "Mindestlaufzeit 120 s (Kessel 52.0 C)" (minimum run time) or "Stoerung: Geraet meldet". The text is always in German, it comes from the controller. The address must be of type character string.
SR	Fault	1 with a pending device fault or a missing boiler sensor, otherwise 0.
BT	Operating hours	Run time of the burner in hours. The value is retained across a restart of the controller.
AZ	Number of starts	Number of burner starts. The value is retained across a restart of the controller.
WA	Maintenance due	1 as soon as one of the configured maintenance intervals is reached since the last acknowledgement, otherwise 0.
Parameters
	Device	Single-stage: only output BR is switched. Two-stage: additionally stage 2 via S2. Modulating: additionally a power setting via LE. Default single-stage.
	Boiler setpoint minimum	Lower limit of the setpoint. Also applies when the request does not report a temperature at all. Default 40 °.
	Boiler setpoint maximum	Upper limit of the setpoint. Protects the generator against an excessive request. Default 80 °.
	Excess	Is added to the requested temperature. Covers heat exchanger losses and the reserve of the mixers. Default 5 K.
	Switching hysteresis	The burner switches on when the boiler temperature is below the setpoint by this amount, and off again when it exceeds the setpoint. Default 5 K.
	Minimum run time	The burner runs at least this long, even if the setpoint has already been reached. Lock, missing sensor and a fault with the behaviour "switch off" bypass this time. Default 300 s.
	Minimum pause time	The burner stays off at least this long after switching off. A request arriving during the pause is not discarded. Default 300 s.
	Switch-on delay	Additional waiting time before every start. Default 0 s, that is no delay.
	Release delay stage 2	Stage 1 must run this long before stage 2 is added. Stage 2 then runs for at least the same time. Only with the two-stage device. Default 300 s.
	Pump run-on time	The boiler circuit pump keeps running this long after the burner has switched off. Dissipates the residual heat. Default 300 s.
	Power minimum	Lower limit of the power setting. A burner driven below its minimum power goes into fault. Only with the modulating device. Default 30 %.
	Power maximum	Upper limit of the power setting. Only with the modulating device. Default 100 %.
	Power gain P	Proportional component of the power setting controller. Default 10.0.
	Power integral I	Integral component of the power setting controller. Default 0.0, that is a pure P controller.
	Return temperature boost	Control type: off, three-point valve via RA and RZ, valve percent via RP or pump switch-off via PU. Default off, then RA, RZ and RP stay silent.
	Return minimum	Temperature the return shall reach at least. Default 45 °.
	Return hysteresis	The valve is only driven from this deviation onwards. With the pump switch-off the pump only restarts once the return exceeds the minimum temperature by this amount. Default 2 K.
	Valve travel time	Travel time of the valve actuator from end position to end position. Default 120 s.
	Valve gain P	Proportional component of the return temperature boost controller. Default 10.0.
	Valve integral I	Integral component of the return temperature boost controller. Default 0.0.
	Valve PWM time	Duration of one complete cycle of travel pulse and pause. Default 60 s.
	Valve PWM min. pause	Smallest pause between two travel pulses. Default 15 s.
	Valve PWM max. pulse	Limits the duration of a single travel pulse. Default 0, that is no limit.
	Pump max. switch-off time	Only with the pump switch-off control type: after this time the pump restarts in any case so that the boiler does not overheat without circulation. It then stays on until the return has exceeded the minimum temperature again. Default 600 s, 0 = unlimited.
	Switch-off limit outdoor temperature	Switches the switch-off limit on. Default off, that is the previous behaviour.
	Switch-off limit	If the outdoor temperature is above this, the request is discarded. A pending hot water request on input WW lifts the limit. Default 20 °.
	Maintenance operating hours	After this many operating hours since the last acknowledgement WA reports that maintenance is due. Default 0, that is off.
	Maintenance number of starts	After this many burner starts since the last acknowledgement WA reports that maintenance is due. Default 0, that is off.
	Fault delay	The device fault message must be pending this long before it applies. Default 5 s.
	Behaviour on fault	Switch off: the burner goes off immediately, even before the minimum run time has elapsed. Keep running: only a message is issued, the generator keeps working. Default switch off.

Heat generator

Sequence

Example

Inputs

Outputs

Parameters