ExampleHotWater (FB)¶

FUNCTION_BLOCK ExampleHotWater

Example hot water boiler control

This example function block implements a simple hot water boiler control to demonstrate how to implement a plant / an aggregate in ST using CODESYS Building Automation library function blocks. All other examples are implemented using CFC, because this way it’s a bit more easy to read / understand. No matter of this ST is also a good choice to implement plants / aggregates. Here we demonstrate, that an implementation using ST is also easy to read / understand if structured nicely and well documented.

This example involves:

2 point control of boiler temperature
steady control of boiler supply temperature
demand-based activation of loading pump
legionella prevention
optional unload protection using supply temperature sensor

Watch out for comments in the implementation where those aspects are covered.

supply temperature sensor
mixing valve
loading pump
boiler supply temperature sensor
upper boiler temperature sensor
lower boiler temperature sensor
circulation pump

2 point control of boiler temperature¶

A boiler load cycle is requested, if boiler temperature is below setpoint. The boiler load cycle is stopped, if boiler temperature exceeds setpoint plus hysteresis.

Legionella prevention¶

Boiler temperature is to be kept above 60°C for at least an hour the day. Circulation pump is enabled during this load cycle.

Optional unload protection¶

In case there is a supply temperature sensor ExampleHotWater checks the supply temperature before / during a load operation. In case the supply temperature is below boiler temperature the boiler will not be loaded.

Caveats¶

To keep complexity as low as possible this example omits - amongst others - the following real world application aspects worth mentioning:

configuration missing (configuration of m_legionellaPreventionDone, m_legionellaPreventionTimer, m_loadPump, m_circPump, m_mixingValve etc. is not or not completely exposed on the ExampleHotWater VAR_INPUT section)
input consistency checks (rHyst <= 0, implausible measurement values, …)
no legionella warning issued, if legionella prevention load cycle is missed due to unload conditions
sensors missing (both pumps - service / error indication, …)
xError / eErrorID not computed at all, see ExampleAirConditioning2 how to do this
sensor / actuator process I/O not mapped (FB sensor inputs, m_loadPump, m_circPump, m_mixingValve, …)

InOut:

Scope	Name	Type	Initial	Comment
Input	`rSetpt`	`REAL`	50.0	Hot water temperature setpoint
	`rHyst`	`REAL`	5.0	Hot water temperature hysteresis
	`rSupplyTemp`	`REAL`		Supply temperature
	`xSupplyTempSensor`	`BOOL`	FALSE	Optional supply temperature sensor exists
	`rBoilerSupplyTemp`	`REAL`		Boiler supply temperature
	`rBoilerTemp1`	`REAL`		Upper boiler temperature
	`rBoilerTemp2`	`REAL`		Lower boiler temperature
	`xCircPumpRequest`	`BOOL`	FALSE	Request circulation pump
	`tLegionella`	`TIME`	TIME#1440m0s0ms	Time to next legionella prevention boiler load cycle
	`xReset`	`BOOL`	FALSE	Reset
	`itfDateTimeProvider`	`Util.IDateTimeProvider`	Globals.g_dtpDateTimeProvider	Source for the current date and time information in milliseconds since 1.1.1970 00:00:00.000
Output	`rRequestedBoilerTemp`	`REAL`		Requested boiler temperature
	`xLegionellaPreventionDone`	`BOOL`		Legionella prevention has been done in the last 24 hours
	`xLoad`	`BOOL`		Boiler will be loaded actually.
	`xLoadPump`	`BOOL`		Loading pump running
	`xCircPump`	`BOOL`		Circulation pump running
	`rValve`	`REAL`		Mixing valve
	`tLegionellaRemaining`	`TIME`		Remaing time to next legionella prevention boiler load cycle