Heating circuit

This function block controls a complete heating circuit. It derives the weather-compensated flow setpoint from the heating curve, knows comfort and setback operation, shuts down automatically in summer, drives the circuit pump with run-on and minimum off time, controls the mixing valve and tells the heat generator which temperature it needs.

Up to now a heating circuit had to be assembled from individual blocks: heating+cooling curve, simple mixing valve, a time switch for the setback, a threshold switch for the summer shutdown and several assignments. That is six to ten blocks per circuit. The heating circuit combines all of this, and in addition it provides a heat request for the generator.

Whatever the user is supposed to change at run time is an input. Room setpoint, operating mode, time program and enable 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 VS, VL and MP are sufficient.

Operation
Type of circuit. On a mixed circuit the block drives the mixing valve through outputs VA and VZ. On a direct circuit these outputs stay silent and the mixing valve parameters disappear from the properties window; everything else — setpoint, pump, summer shutdown, frost protection, heat request — keeps working unchanged. Operating mode. Input BA sets the operating mode manually: 1 comfort, 2 setback, 3 frost protection, 4 off. Only with 0 (automatic) does the time program on input ZP decide between comfort and setback. The lock SP and a missing enable FR shut the circuit down regardless of BA. Flow setpoint. First the room setpoint is determined: in comfort operation the value on input SO, in setback operation either SO minus the configured setback or the separate setpoint on input SA. From that and the outdoor temperature the same curve as in the heating+cooling curve block calculates the flow setpoint. Slope, parallel shift and base point shape the curve, flow minimum and maximum limit the result. If input VG is connected and greater than 0 it overrides the curve — the limits still apply, they protect the circuit. Room influence. The room influence weights the deviation between room setpoint and measured room temperature. It only takes effect if input RT is connected. Without a room sensor the block stays with pure weather compensation — an unconnected input would read 0 °C, and that would produce an enormous overshoot. Summer shutdown. A sliding average is formed from the outdoor temperature, using the same method as the average over time block. The averaging period is the time constant: after this time the average has taken over about two thirds of a change. If it rises above the heating limit of the currently valid operating mode, the circuit shuts down: the pump stops, the flow setpoint goes to 0 and the heat request drops out. It only comes back when the average falls below the heating limit by the hysteresis and the minimum dwell time has elapsed. Without both, the system would switch back and forth several times a day during the transitional season. The average is stored and continues after a restart; if the controller was down longer than the averaging period it is discarded and learned again. Frost protection. If the outdoor temperature falls below the configured threshold, the circuit starts and holds the minimum flow temperature — even in operating mode Off, with the lock active and during summer shutdown. That is exactly what the operating mode "off" is for: it does not mean "dead". Without a connected outdoor sensor there is no frost protection. Fast setback. When switching to setback operation the pump stays off until the flow temperature has fallen to the reduced setpoint. The circuit cools down faster instead of distributing the stored heat. Without a connected input VL it has no effect. Fast heat-up. When switching to comfort operation the flow setpoint is raised by the configured amount for the configured time so that the room reaches temperature faster. With an increase of 0 it is omitted. Pump. The pump runs in comfort and setback operation and in frost protection. After the heat demand has ended it keeps running for the run-on time so that the residual heat of the boiler is carried away. After switching off it may only restart once the minimum off time has elapsed — otherwise it cycles at the switching threshold. The operating hours are counted, stored and made available on output BT. Mixing valve. The mixing valve is driven with the same pulse generation as in the simple mixing valve block: a controller forms the manipulated variable, from which travel pulses with a minimum pause and a limitable pulse length are derived. The direction of travel is never switched directly, there is always a short pause in between. The outputs are only sent on a change. When the circuit is at rest the mixing valve closes once over its full travel time. In cooling operation (input KU) the direction of action is reversed. Actuator feedback. If inputs RA or RZ are connected, the block checks whether the actuator has accepted the switching command. If the feedback does not match the output, the output is sent again a limited number of times, the error counter FZ is incremented and the fault output SR is set; if the feedback catches up, the fault clears by itself. Only the direction whose input is connected is monitored. Heat request. As long as the circuit is heating, output AN signals the demand and output AT the required flow temperature at the generator — that is the flow setpoint plus the configured increase, which covers heat exchanger losses and mixing valve reserve. Together these two outputs form the request format used by all consumers; they can therefore be collected and passed on to a generator. The form of the message can be restricted to the bit or to the temperature alone. In cooling operation no heat is requested. Anti-blocking function. On the configured weekday and at the configured hour a free-running cycle takes place: the pump runs for the configured time, then the mixing valve travels fully open and fully closed once. Both parts can be switched off individually. The cycle only starts with the circuit at rest and is aborted immediately by any heat demand. Adjusting during operation. Whether the heating curve fits only shows after a heating season; whether the heating limit sits right, only after a transitional period. Five quantities therefore have an additional input: heating curve (HK), parallel shift (HP), room influence (RE) and the two heating limits (HG, HA). A connected input takes precedence over the parameter of the same name. Exactly those values that can only be judged in operation can thus be assigned to addresses and adjusted from the visualisation — without the Studio and without the installer. These values survive a restart of the controller by themselves: whatever is on a group address is stored by the controller and restored during start-up, before the function blocks come up. Writing happens hourly and on an orderly shutdown — so with a hard power failure the last hour can be lost. Addresses with an active initial value are deliberately not stored; they always start with the configured value. When it does not get warmer. If the flow setpoint sits at the flow maximum limit, a steeper curve no longer helps — the limit cuts the increase off again. The status text reports this with "Vorlauf begrenzt" (flow limited). The curve is then not too flat, the limit is too low — and that one belongs to the installer: it protects the circuit, for example the screed of an underfloor heating system. Missing sensor. If input AU is not connected — or, on a mixed circuit, input VL — the block cannot work: output SR is set, the status text reports the missing sensor and a message is issued.
Example
Underfloor heating with a mixing valve: slope 0.6, flow minimum 20 °C, flow maximum 40 °C, comfort room setpoint 21 °C, setback 4 K, comfort heating limit 18 °C. At an outdoor temperature of 0 °C a flow setpoint of about 33 °C results, correspondingly less in setback operation. If the sliding outdoor average rises above 18 °C in May, the circuit shuts down and reports "Sommer aus"; it only comes back once the average falls below 17 °C and the minimum dwell time of two hours has elapsed.

Inputs
AU	Outdoor temperature*	Current outdoor temperature in °C. Basis of the heating curve, of the sliding average for the summer shutdown and of the frost protection. Not connected: the fault output is set, the status text reports the missing sensor and a message is issued. Control then runs with an outdoor temperature of 20 °C, so the circuit stays at the lower flow limit, and there is no frost protection. An internal link to an upstream block counts as connected.
VL	Flow temperature	Measured flow temperature of the circuit in °C. Actual value of the mixing valve control and the end condition of the fast setback. Mandatory on a mixed circuit; not connected: the fault output is set and the mixing valve stays still. On a direct circuit the input is only needed for the fast setback; if it is not connected there, the fast setback is omitted and no fault is reported.
RT	Room temperature	Optional. Measured room temperature in °C for the room influence. Not connected: the room influence has no effect, the setpoint results from pure weather compensation. The connection is checked, not the value.
SO	Room setpoint comfort	Optional. Required room temperature in comfort operation in °C; it is also the base point of the heating curve. Not connected or less than or equal to 0: 20 °C is used.
SA	Room setpoint setback	Optional. Separate room setpoint for setback operation in °C. Only takes effect if the parameter "type of setback" is set to "separate room setpoint (SA)". Not connected or less than or equal to 0: the comfort setpoint minus the configured setback applies.
BA	Operating mode	Optional. 0 = automatic (time program ZP decides), 1 = comfort, 2 = setback, 3 = frost protection, 4 = off. With 1 to 4 ZP is not evaluated. Frost protection also acts with 4. Not connected: automatic.
ZP	Time program	Optional. Only evaluated with operating mode 0 (automatic): 0 switches to setback, other than 0 to comfort. The usual source is a weekly time switch. Not connected: comfort operation.
FR	Enable	Optional. Other than 0 enables the circuit, 0 blocks it: the pump stops, the flow setpoint goes to 0, the heat request drops out. Frost protection still acts. Not connected: the circuit is enabled.
SP	Lock	Optional. Other than 0 blocks the circuit like a missing enable; 0 releases it. Frost protection still acts. Not connected: never blocked.
KU	Cooling operation	Optional. Other than 0 switches to cooling operation: the setpoint comes from the cooling curve, the direction of action of the mixing valve is reversed, the summer shutdown is omitted and no heat is requested. The input is usually supplied by the heating/cooling changeover block. Not connected: heating operation.
VG	Flow setpoint preset	Optional. A value greater than 0 overrides the curve and becomes the flow setpoint directly; flow minimum and maximum still apply. Intended for a higher-level program that sets the flow temperature itself. Not connected or 0: the curve applies.
RA	Feedback open	Optional. Feedback of the actuator for the open direction. If it does not take on the value of output VA within the monitoring time, the output is sent again and a fault is reported. Not connected: this direction is not monitored.
RZ	Feedback close	Optional. The same for the close direction and output VZ. Not connected: this direction is not monitored.
HK	Heating curve	Optional. Slope of the curve, in cooling operation that of the cooling curve. A value other than 0 takes precedence over the parameter "slope". A 0 counts as "value not yet received": the parameter then still applies — a slope of 0 would be meaningless and the circuit would run practically without a curve. Not connected: the parameter applies.
HP	Parallel shift	Optional. Shifts the whole curve up or down. If the input is connected it takes precedence over the parameter — including with the value 0, because 0 means "no shift" here and is a valid value. Not connected: the parameter applies.
RE	Room influence	Optional. Weighting of the deviation between room setpoint and room temperature in percent. If the input is connected it takes precedence over the parameter — including with the value 0, because 0 means "pure weather compensation". It still only takes effect if input RT is connected as well. Not connected: the parameter applies.
HG	Heating limit comfort	Optional. If the sliding outdoor average is above this value, the circuit shuts down in comfort operation. A value other than 0 takes precedence over the parameter; a 0 counts as "value not yet received". Not connected: the parameter applies.
HA	Heating limit setback	Optional. The same limit for setback operation, otherwise like HG. Not connected: the parameter applies.
Outputs
PU	Pump	1 as long as the circuit pump should run — in comfort and setback operation, in frost protection, during the run-on time and during the anti-blocking cycle. Otherwise 0.
VA	Mixing valve open	Travel pulse in the open direction. On a direct circuit the output is not written.
VZ	Mixing valve close	Travel pulse in the close direction. The two directions are never active at the same time. On a direct circuit the output is not written.
MP	Mixing valve position	Calculated mixing valve position in percent, 0 to 100. An estimate derived from the travel pulses, not a measurement.
VS	Flow setpoint	Valid flow setpoint in °C. 0 if the circuit is off, blocked or in summer shutdown.
AN	Heat request	1 as long as the circuit needs heat, then for the run-on time of the request. Stays at 0 with the form "temperature only".
AT	Request temperature	Required flow temperature at the generator in °C: flow setpoint plus the increase. Stays at 0 with the form "bit only".
ST	Status text	Plain text of the current state for a text display, for example "Komfort 21.0", "Absenkung 17.0, Schnellabsenkung", "Sommer aus (MW 18.4 C)", "Komfort 21.0, Vorlauf begrenzt", "Frostschutz", "Aus, Freifahrlauf". The address must be of the character string type.
SR	Fault	1 as long as a sensor is missing or a feedback of the actuator is absent. Otherwise 0.
BT	Pump operating hours	Running time of the circuit pump in hours. The value is stored and continued after a restart.
FZ	Error counter	Number of missing actuator feedbacks since the start.
Parameters
	Circuit	Mixed or direct. On a direct circuit the mixing valve outputs stay silent and the mixing valve parameters are hidden. Default: mixed.
	Slope	Slope of the heating curve. The larger the value, the more the flow temperature rises as the outdoor temperature falls. Typical values: underfloor heating 0.4 to 0.8, radiators 1.2 to 1.8. Default 1.2. A connected input HK takes precedence.
	Parallel shift	Shifts the whole curve up or down by this amount. Default 0 K. A connected input HP takes precedence.
	Base point	Required flow temperature at an outdoor temperature of 20 °C. Default 0 — then the curve stays unchanged and its base point lies on the room setpoint.
	Flow minimum / maximum	Lower and upper limit of the flow setpoint. The maximum protects the circuit, for example the screed of an underfloor heating system, and also limits the preset on input VG. Default 20 °C and 50 °C.
	Room influence	Weighting of the deviation between room setpoint and room temperature in percent. Only takes effect with input RT connected. Default 0 — that is pure weather compensation, the previous behaviour of a heating curve without a room sensor. A connected input RE takes precedence.
	Type of setback	Either by an amount in kelvin or to the separate room setpoint on input SA. Default: by amount.
	Setback	Amount by which the room setpoint is reduced in setback operation. Default 4 K.
	Fast setback	When switching to setback the pump stays off until the flow temperature has fallen to the reduced setpoint. Default off.
	Fast heat-up increase / duration	When switching to comfort operation the flow setpoint is raised by this amount for the configured time. Default 0 K, that is no fast heat-up.
	Heating limit comfort / setback	If the sliding outdoor average is above the limit of the currently valid operating mode, the circuit shuts down. Default 18 °C and 15 °C. A connected input HG/HA takes precedence.
	Averaging period	Time constant of the sliding outdoor average in hours. Default 24 h.
	Heating limit hysteresis	The circuit only comes back once the average falls below the heating limit by this amount. Default 1 K.
	Minimum dwell time	Minimum time the summer shutdown is held before it may change again. Default 120 min.
	Frost protection outdoor temperature	If the outdoor temperature is below this value, the circuit starts even in operating mode off, with the lock active and during summer shutdown. Default 3 °C.
	Frost protection flow	Flow setpoint held by the circuit in frost protection. Default 15 °C.
	Pump run-on time	The pump keeps running for this time after the heat demand has ended. Default 300 s.
	Pump minimum off time	After switching off, the pump may only restart once this time has elapsed. Default 60 s.
	Anti-blocking function pump / mixing valve	What the free-running cycle moves. Each can be switched on individually. Default: both off.
	Anti-blocking weekday / hour / duration	When the free-running cycle starts and how long the pump runs during it. The mixing valve then travels fully open and fully closed once over its travel time. Default Sunday, 10 o'clock, 60 s.
	Mixing valve travel time	Travel time of the actuator from end position to end position. Default 120 s.
	Mixing valve gain P / integral I	Controller parameters of the mixing valve control. Default 10.0 and 0.0.
	Mixing valve PWM time / min. pause / max. pulse	Length of a pulse cycle, shortest pause between two pulses and longest individual travel pulse. Default 60 s, 15 s and 0 s (no limit).
	Mixing valve hysteresis	The valve only travels above this deviation between setpoint and actual value. Default 1 °.
	Feedback monitoring time	Time within which the feedback must take on the value of the output. Only takes effect with the feedback input connected. Default 10 s.
	Repeat send time	Interval at which the output is sent again after a fault. Default 30 s.
	Max. repetitions	Number of repetitions per fault. 0 = unlimited. Default 3.
	Request increase	Added to the flow setpoint and reported to the generator as the request. Covers heat exchanger losses and mixing valve reserve. Default 5 K.
	Request run-on	The request stays active for this time after the heat demand has ended. Default 0 s.
	Form of the message	Bit and temperature, bit only or temperature only. Default: bit and temperature.

Heating circuit

Operation

Example

Inputs

Outputs

Parameters