[Bo24] Boiler, modulating

Controls a boiler with its own external control unit.

An interface to the control unit of the boiler (Bu24) enables it, modulates its power and reads optional signals for fault and feedback.

The program has settings for the nominal power of the boiler and for the transient state times (a signal that the boiler is in a transient state that blocks the logic for generating further step-up and step-down pulses for the power control and compensation) when switching on or off (e.g., 5 minutes when switching on and 2 minutes when switching off).

The switch-on and switch-off sequence for the valves, the pump, and the interface to the control unit are defined in the function block CMDSEQ_B.

This program block has the following optional functions:

Option: Mixing valve (1xAO, 1xAI)

Controls the position of a mixing valve (Vlv24).

A PID controller changes the position of a mixing valve to maintain the return temperature (this is the boiler inlet temperature) at a given setpoint.

This option can only be applied successfully together with the optional return temperature.

There is a configuration value "Setpoint minimum return temperature" (SpTRtMin).

A parameter defines the temperature difference between the flow temperature and the return temperature. This is the temperature increase that the boiler can maintain at its nominal flow rate = DeltaT, e.g., 8 K.

Another parameter defines the relationship between the present return temperature setpoint and the flow temperature setpoint (the boiler outlet temperature setpoint):

0: The return temperature setpoint follows the flow temperature setpoint (PrSpT).

The resulting setpoint for the return temperature is calculated as follows: maximum (SpTRtMin, (PrSpT - DeltaT).

1: The return temperature setpoint shifts the flow temperature setpoint (PrSpT).

The setpoint for the return temperature is SpTRtMin.

The present flow temperature setpoint is calculated as follows: maximum (value at the pin "Sp", (SpTRtMin + DeltaT).

Option: Pump (1xBO, 4xBI)

Controls a pump (Pu21). Different alternatives are possible, e.g., TwnPu21.

The pump runs when the boiler is running, and the optional shutoff valve is open.

Option: Shutoff valve (1xBO, 2xBI)

Controls a shutoff valve that opens when the boiler is running (Vlv21).

Option: Deactivate frost protection in case of gas detection

Reads the status of the gas detector and stops any equipment that is turned on due to frost protection.

When gas is detected, all the equipment is turned off (valves are closed, pumps are off, and the control unit of the boiler is disabled).

Option: Frost protection

If the flow temperature (outlet) and/or the return temperature (inlet) of the boiler fall below a certain value, e.g., 8 °C, the shutoff valve opens and the pump starts to ensure water circulation.

If the temperatures fall further, e.g., to 5 °C, the interface to the control of the boiler enables the boiler and the boiler starts.

Option: Keep valves open and pump running for specified time on overtemperature

In case of overtemperature, the shutoff valve is kept open and the pump continues to run for a specified time, e.g., 2 minutes.

Option: Local temperature control for flow temperature limitation

Switches off the enable command for the control unit of the boiler if the flow temperature (boiler outlet temperature) exceeds the boiler flow temperature tolerance, e.g., by 4 K.

Option: Operating mode switch (1xMI=2xBI)

Reads the state of an operating mode switch, normally at the front of the electrical panel, and has the states "Auto", "Off", and "On".

Option: Overtemperature detector (1xBI)

Reads the signal from an overtemperature detector and generates an alarm.

The program logic stops the enable command to the control unit of the boiler. When the option "Keep valves open and pump running for specified time on overtemperature" is active, the pump stops and the shutoff valve closes with a delay. Otherwise, the pump stops and the shutoff valve closes immediately.

Option: Return temperature (1xAI)

Creates an analog input for the return temperature and reads its signal. It can generate an alarm (high and low limit).

Option: Safety temperature limiter (1xBI)

Reads the signal from a safety temperature limiter and generates an alarm.

The program logic stops the enable command to the control unit of the boiler. When the option "Keep valves open and pump running for specified time on overtemperature" is active, the pump stops and the shutoff valve closes with a delay. Otherwise, the pump stops and the shutoff valve closes immediately.

Option: Setpoint shift with external signal for operating mode "Fixed"

The boiler with its own external control unit has an operating mode "Fixed". The operating mode "Fixed" comes after the mode "On". It ensures that this boiler stays at its nominal power and that another energy generator that has been switched on after it is modulating maintains the main flow temperature.

A PID controller in another program block (HGenCtl22) compares the main flow temperature to its setpoint and in case of a deviation increases its output (in %). The boiler can read this signal and if its operating mode is "Fixed", it results in a local setpoint increase, e.g., 0-100 % increases the setpoint of the boiler by 0-3 K. This keeps the boiler at its nominal power and prevents its modulation.

See also

• Predefined I/Os
• Trend types
• Module families

See also

• Predefined I/Os
• Trend types
• Module families