Mixing circuits
- Ruben Gielen
- Dennis De Beuckeleer (Unlicensed)
Mixing distribution circuits are sometimes referred to as variable temperature (VT) circuits. They mix the supply flow with a part of the return flow to decrease the downstream temperature.
Mixing circuits are historically commonly used whenever the design flow of the downstream system differs from the designed supply flow temperature. By mixing the flow, the heat flow towards the end units decreases.
Mixing circuits are characterized by a variable flow temperature & constant flow rate at the secondary gate and a constant flow temperature & variable flow rate at the primary gate. When operating in partial load, the return temperature of a mixing circuit decreases quasi-linear with the heat flow. Because of this, mixing circuits generally perform better than dividing circuits, but worse than throttle circuits within hydronic systems.
The main benefits of mixing circuits are their stable controllability for temperature control and the constant secondary flow rate. Their main drawback is that they usually have a delay in heat supply. This can have an impact on thermal comfort during start-up periods or within systems with long distances between the distribution unit and the plant room.
A more detailed explanation on the comparison of different hydronic distribution circuits can be found here: Distribution circuits.
Mixing base circuit
Underlying components
A mixing BC usually contains the following kinds of components:
A pump: Used to circulate flow through the circuit and to increase the flow pressure. More information on pump components can be found here: Pumps
A control valve: Used to control the flow in the mix. Depending on the kind of mixing, this will be a 2 way or 3 way valve. More information on control valve components can be found here: Control valves
A balance valve for the pump: Used for static balancing the (secondary) system on full load conditions. More information on balance valve components can be found here: Balance valves
A balance valve on the bypass: Used for static balancing the system on partial load conditions. This balance valve will not be available on all mixing BCs.
A temperature sensor: Used for controlling the secondary flow system’s temperature. Changing this parameter will change the sensor’s integration time.
Different types of mixing BCs
Currently, there are nine different mixing circuit BCs. The main difference in working principles is always found in the way that the mixing point is controlled. When using a 3 way valve on the supply side, the mix will be controlled ‘actively’. When using a 2 way valve instead, the mix will be controlled ‘passively’. The reason for the name is based on the needed pump. When the pump installed at the mixing BC will pump from the boiler through, it is considered an active arrangement (1 pump needed). If the pump is separated from a primary pump, it is considered a passive arrangement.
Premix
The premix BC is used to lower the flow temperature using a static balance valve. It therefore protects the circuit further downstream from too high temperatures without the need for control valves. The premix BC has the same parameter as the active mixing BC further below.
Active mixing
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).
The active mixing BC itself has only one parameter:
Primary supply temperature: This is an optional parameter. Whenever the primary supply temperature further upstream of the BC is higher than the designed flow temperature downstream of the active mix, this must be manually corrected by the user. By giving the BC a higher primary temperature than the propagated secondary temperature, the full load valve position of the three-way control valve will be restricted to protect the flow downstream against too high temperatures. An example is given further below.
System check
As this mixing circuit is a so-called 'active' mixing circuit, the embedded pump can extract hot water from the primary side by itself. In fact, this circuit must be connected to a pressureless 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 cases where there are multiple types of end units (radiators, floor heating, etc.) the supply temperature in the regime may differ. 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.
Mixing circuit with primary bypass
This BC operates similarly to the ‘active mixing’ BC, but is now a source of constant flow at the primary gate due to the primary bypass. In case of long distances, this BC is used to reduce the waiting time for hot water to arrive at the 3-way mixing valve. The circuit has the same parameter as the active mixing BC.
Mixing circuit with premix
This BC operates similarly to the ‘active mixing’ BC. The premix balance valve ensures that the supply temperature to the end unit is lower than the supply temperature leaving the 3-way valve by mixing cold return water from the end unit. Due to the premix, the 3 way control valve usually has a higher rangability to control the flow temperature in the mix.
Operating principle
Take a look at the example circuit below. A boiler at high temperature (90 °C) provides heat to a radiator zone at lower temperature (60 °C). A mixing circuit is used to lower the flow temperature. In the upper system, the active mixing BC is used. In the lower system, the active mixing BC with premix is used. The parameter ‘Primary supply temperature’ is set to 90 °C.
When simulating the model, both systems deliver the same heat flows to the radiator unit and have temperature behaviour at the end unit. However, the system using the premix can open its 3 way control valve further than the system using just the active mixing circuit.
Using a premix therefore gives you a higher practical range of controllability. Note that the end units will never receive the 90°C of the primary supply flow. Increased rangeability thus comes with a decreased range in flow temperature.
Passive Mixing
Operating principle
The passive 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 2-way valve is completely opened whereas in partial load conditions, the 2-way valve is positioned in the function of the process value (measured water temperature) and setpoint (heating curve).
The passive mixing BC has the same parameter as the active mixing BC.
System check
As this mixing circuit is a so-called 'passive' circuit, the circuit needs a primary pump to extract hot water from the primary side. If the software encounters a missing pump on the primary side of the passive mixing circuit, it will report this as a problem while running the system check.
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 cases where there are multiple types of end units (radiators, floor heating, ...) the supply temperature in the regime may differ. The bypass in the passive mixing circuit guarantees a maximum supply temperature (45°C) for the floor heating, because of the mixing point on the supply pipe. Note that the primary temperature difference from the floor heating is larger than that of the radiators, which will influence the design flow.
Passive mixing with PICV
This BC operates similarly to the ‘passive mixing’ BC. The 2 way control valve and balance valve for primary balancing are now replaced by a pressure independent control valve. More information on PICVs can be found here: Balance valves
Passive mixing circuit with CV and DPR
This BC operates similarly to the ‘passive mixing’ BC. The balance valve for primary balancing is now replaced by a differential pressure regulator and a balance valve. More information on differential pressure regulators can be found here: Control valves
Mixing injection with primary dividing
This BC operates similarly to the ‘passive mixing’ BC, but is now a source of constant flow at the primary gate due to the primary dividing circuit. Combining the mixing & dividing circuits is mainly done to combine the advantages of the mixing circuit (constant flow at the secondary side & stable controllability) with the advantages of the dividing circuit (no delay on heat supply). The downside is an increased return temperature in partial load.