Active mixing circuit
Operating principle
The active mixing circuit controls the net thermal power passing through the circuit by mixing cold return water (coming from the end unit return pipes) with hot water from the primary production side. In full-load operation, the inlet gate of the 3-way mixing valve is completely opened whereas in partial load conditions the 3-way mixing valves are positioned in the function of the process value (measured water temperature) and setpoint (heating curve).
Individual components and parameters
Secondary pump
3-way mixing valve
Balance valve for pump
Balance valve on the 3-way valve bypass
System check
As this mixing circuit is a so-called 'active' mixing circuit, the embedded pump is able to extract hot water from the primary side by itself. In fact this circuit must be connected to a pressure less header/collector. If the software encounters a pump on the primary side of the active mixing circuit, it will report this as a problem while running the system check.
Note that when the primary pump is not removed the pump and valve optimization is no longer supported, it is still possible to simulate the system as a reference case but the user needs to fill in the parameters.
Design flow and temperature regime propagation
By running the action "compute design flows" all the design flows are calculated based on the power and temperature regime of the end units. In case where there are multiple types of end units (radiators, floor heating, etc.) the supply temperature in regime may differ. In order to guarantee the maximum supply temperature (45°C) of the floor heating the 3-way mixing valve needs to be limited in full load valve position. In the example below the 3-way mixing valve is limited to 60% valve position by filling in the primary supply temperature override. Note that the primary temperature difference from the floor heating is larger than that of the radiators, which will influence the design flow.