[HCr21] Heating circuit, weather-compensated flow temperature control, room heating

Components and functions of the plant example:

Plant example HCr21 is a heating circuit for radiators with outside air temperature-controlled flow temperature control.

Function heating limit

Two heating limit functions are available. Both filtered outside temperatures are considered here.

• General heating limit (nested chart)
• The plant is enabled for Automatic mode (e.g. by the scheduler) if both filtered outside temperatures are below the value from [HtgLm]. The plant is not enabled for automatic mode if one of the two values exceeds the value from [HtgLm].
• Heating limit for operating mode Economy (nested chart)
• The plant is enabled for automatic mode in Economy if operating mode Economy is specified and both filtered outside temperatures are under the value from [HtgLmEco]. The plant is not enabled for Economy if one of the two values exceeds the value from [HtgLmEco].

BACnet objects

Outside air temperature-dependent flow control

Key

Heating curve

The setpoint for flow temperature is determined using a heating curve. Function block HCRV implements the heating curve.
[HCRV] Heating curve

BACnet objects

The scheduler [MSchedule] specifies a certain operating mode (Comfort, Pre-Comfort, Economy, or Protection). The setpoint for the desired room temperature applies accordingly.

The setpoint for room temperature is given to the heating curve and is subtracted internally from the nominal setpoint of 20 °C. The difference is added to the effective outside temperature so that the flow temperature setpoint is increased or reduced as per the desired room temperature.

For example, the setpoint in operating mode Economy is 15 °C. This results in a difference of 5K versus the internal, nominal room setpoint. This value is added to the effective outside temperature. This results in a downward shift of the calculated flow temperature setpoint compared to the corresponding flow temperature setpoint for Comfort (at the same effective outside temperature).

The setpoint for operating mode Pre-Comfort is generated as follows:
SpPcf = SpCmf - DSpPcf

BACnet objects

If plant power must be reduced, e.g. during hot water charging, connect input [YCtrMax] of the temperature controller with a limitation signal, e.g. via BACnet.

Room functions

The plant example is programmed in a way that enables operation without a room temperature.

The following functions require a room temperature:

• Room frost protection
• Room temperature is monitored. The room setpoint for protection [SpPrt] applies as the limit value. If violated, the plant starts in operating mode Comfort. The plant continues to operate until the value for [SpPrt] and a hysteresis (e.g. 2K) is exceeded.
• This function has a higher priority than automatic mode, the manual selection and operating mode switch.
• Room influence
• The room temperature is subtracted from the present room setpoint, multiplied by the value from the room influence factor [RInflFac] and the result is added to the present room setpoint for the heating curve, which influences the calculated flow setpoint.
• The calculated flow temperature setpoint is corrected downward if the room setpoint is exceeded. The calculated temperature setpoint is increased if the room setpoint is not achieved.
• Optimum start stop control (OSSC)
• This functionality is implemented with function block OSSC.
  [OSSC] Optimum start stop control

Plant operating modes:

• Protection (1)
• Economy (2)
• Pre-Comfort (3)
• Comfort (4)

The operating modes are reported on [MCalcVal] object 'Present operating mode'.

At the same time, the reason for the present operating mode is reported to another [MCalcVal] object:

• Exception (1)
• Low room temperature protection (2), resulting from a room temperature that is too low
• Operating mode switch (3), resulting from an active (not equal to Auto) operating mode switch
• Manual operating mode selection (4), resulting from an active (not equal to Auto) manual operating mode selection
• Heating limit (5)
• OSSC mode (6)
• Heating limit Economy (7)
• Scheduler (8), resulting from an active scheduler

Plant operating mode 'Protection' is similar to operating mode 'Off'. Temperature control and the pump are not enabled in 'Protection' and pin [In1] must be set to 1 on function block SELMS_BO if temperature control and the pump are active. Plant operating mode 'Protection' responds similarly to plant operating mode 'On', but at a reduced setpoint.
[SELMS_BO] Multistate selector for boolean value

There is a normal mode and local exception mode.

Normal operation

Sources for normal mode:

• Room frost protection
• [MI] Operating mode switch
• [MCnfVal] Manual operating mode selection
• Optimum start/stop control
• Heating limit
• [MSchedule] Scheduler

In normal mode, components are simultaneously switched on and off as per the operating mode.

Exception mode

Sources for exception mode:

• Pump fault
• Overtemperature detector

In exception mode, all outputs are switched off directly at priority 5 (the mixing valve is switched to bypass) and the present operating mode is set to 'Protection'.

Local exception mode (special case for overtemperature detector)

The pump remains on, for example, for 10 minutes if the overtemperature detector is triggered. The pump is then switched off. The outputs [AO] valve position and [BO] pump command are switched at priority 4.

The overrun described above may need to be adapted in the program depending on the application and installation. For example, it may have to be set to 0 for floor heating.

Start-up control (nested chart SttUpAlmHdl)

The following functionality is available for an automatic startup of the plant, e.g. after a loss of power and return of power:

1. Fixed switch-on delay (can be set on input pin [DlySttUp])
2. The plant remains off to ensure communications are reliable and cross-check references are triggered. 'Exception' is reported as the reason for the present operating mode.
2. Automatic acknowledge (2x) of possible pending alarms during a fixed switch-on delay.
3. Additional adjustable delay for staged plant ramp-up (can be set on input pin [DlyOn])

Alarm handling (nested chart SttUpAlmHdl)

The state of events on the plant is acquired by BACnet object 'Plant state'. The state is mapped on function block CMN_EVT.
[CMN_EVT] Common event

The following functions are available:

1. Automatic acknowledge (2x) after startup
2. Alarm acknowledgement with a value object
3. Alarm indication:
   - For pending alarms On
   - For unprocessed alarms flashing
4. Ability to connect the acknowledge button [BI]
5. Ability to connect to an alarm indication [BO], e.g. an alarm lamp through an output block [BO]
6. Ability to connect to other alarm handling functions on other plants, e.g. acknowledge button and alarm indication for multiple plants in the same control cabinet

Startup function and alarm handling for flat plants