This article covers three common Pump Group configurations: Direct, Mixing, and Thermostatic Mixing. It explains how they work, where they are used, their main components and key parameters, and the supply temperature, system flow, Kv, circulation pump, connections, and installation conditions that must be confirmed during selection. It also reviews common installation and operating issues for reference.
1. What Is a Pump Group?
A Pump Group integrates a circulation pump, valves, temperature monitoring, and, where required, mixing control into one module to establish an independent heating circuit. Different configurations suit different supply temperatures and control requirements.
1) Pump Group Structure and Operating Principle
A Pump Group consists of Supply and Return flow paths and integrates a circulation pump, isolation ball valves, thermometers, a check valve, and an EPP insulation casing. Depending on the control requirement, it may also include a Mixing Valve or Thermostatic Mixing Valve.
This product series includes the 8350 Economic Pump Group, 8351 Mixing Pump Group, and 8352 Thermostatic Mixing Pump Group. All three models are DN25, use G1″ F × G1-1/2″ M connections, and accept a 130 mm circulation pump.
Thermostatic Mixing Pump Group Structure
2) Role of a Pump Group in a Heating Circulation System
The circulation pump overcomes resistance from piping, valves, manifolds, and heating terminals so hot water can circulate continuously between the heat source and the terminal circuit. The Pump Group also provides circuit isolation, temperature monitoring, backflow prevention, and mixing control where required. One heat source can supply multiple heating circuits with different temperature requirements through separate Pump Groups.
2. What Are the Main Types of Pump Group?
Pump Groups mainly differ in whether they mix water and how they control the supply temperature. The three common configurations provide direct circulation, adjustable mixing, and automatic thermostatic mixing.
Pump Group Types
1) Economic / Direct Pump Group
The Economic Pump Group is a direct-circulation configuration. It does not independently mix or regulate the supply temperature, so water from the heat source flows essentially directly into the terminal circuit. It is better suited to systems where the heat-source outlet temperature already meets the terminal requirement. The 8350 has a design-simulation Kv of 6.5.
8350 Economic Pump Group
2) Mixing Pump Group
The Mixing Pump Group adds a mixing valve that blends part of the high-temperature supply water with lower-temperature return water before sending it to the terminal circuit. This configuration is required when boiler supply temperature is significantly higher than the temperature required by underfloor heating or another low-temperature circuit. The 8351 has a design-simulation Kv of 5.5, lower than the 8350 because the mixing arrangement adds hydraulic resistance.
8351 Mixing Pump Group
3) Thermostatic Mixing Pump Group
The Thermostatic Mixing Pump Group also provides mixing, but its thermostatic mixing valve automatically adjusts the hot-to-cold water ratio according to the actual outlet temperature. The 8352 has a system temperature setting range of 25–65°C and suits underfloor heating and other circuits that require stable low-temperature supply control.
8352 Thermostatic Mixing Pump Group
4) What Are the Differences Between the Three Pump Groups?
| Type | Mixing Function | Temperature Control | Best-Suited Circuit |
| Economic / Direct | No | Determined by heat source | Circuit that does not require separate temperature reduction |
| Mixing | Yes | External / manual adjustment | Circuit requiring lower supply temperature |
| Thermostatic Mixing | Yes | Automatic thermostatic control | Low-temperature circuit requiring a stable set water temperature |
The first selection step is to determine whether the water temperature supplied by the heat source matches the temperature required by the terminal circuit.
3. Where Are Pump Groups Mainly Used?
Pump Groups are commonly used with radiators, underfloor heating, boilers, heat pumps, and multi-zone heating systems. The design water temperature and flow requirement of each terminal determine the required Pump Group configuration.
1) Radiator Heating Systems
Radiators generally allow a higher supply temperature than underfloor heating. If the heat source can provide water at the radiator design temperature, a Direct Pump Group can establish an independent circulation circuit. Modern low-temperature radiators must still be assessed against the actual design temperature; not every radiator circuit should automatically use a Direct configuration.
2) Underfloor Heating Systems
Underfloor Heating is a typical application for a Mixing Pump Group. Assume the heat source supplies water at 70°C, the underfloor heating return is 30°C, and the target supply is 40°C. Under a simplified energy balance that ignores heat loss, the hot-water proportion is approximately:
(40 − 30) ÷ (70 − 30) = 25%, while return water accounts for about 75%. This shows that an underfloor heating mixing circuit may need to recirculate a large proportion of return water.
In a mixing circuit, the Pump Group circulation pump supplies the entire secondary-side flow required by the underfloor heating circuit. This flow is often different from the primary-side flow from the heat source. To prevent hydraulic interaction between the primary and secondary sides, which can cause hydraulic imbalance or protective shutdown of a heat pump/boiler, a Mixing Pump Group in a multi-circuit system is generally recommended for use with a Hydraulic Separator or a hydraulic manifold with bypass to provide hydraulic decoupling between the primary and secondary circuits.
Underfloor Heating System Diagram
3) Boilers, Heat Pumps, and Other Hydronic Circulation Systems
Pump Groups can also be used with Boilers, Heat Pumps, Buffer Tanks, and multi-zone heating systems. For example, if one system includes both Radiators and Underfloor Heating, each circuit can use a different Pump Group so it operates at its own design flow and water temperature.
Multi-Zone Heating System
4) Which Pump Group Should Be Used for Different Heating Circuits?
If the circuit does not need to change the heat-source supply temperature, use a Direct Pump Group as the preferred option.
If the circuit needs to reduce or adjust the supply temperature, use a Mixing Pump Group as the preferred option.
If the circuit must automatically maintain a set supply temperature, use a Thermostatic Mixing Pump Group as the preferred option.
4. What Are the Main Components of a Pump Group?
A Pump Group’s hydraulic performance and control capability depend on the circulation pump, ball valves, thermometers, check valve, mixing valve, and insulation structure.
1) 130 mm Circulation Pump
This series accepts circulation pumps with a 130 mm installation length. The reference pump shown in the product data is APM25-6-130T. The 130 mm dimension is the installation length between the pump’s two connection ends, not the overall height of the Pump Group.
2) Supply/Return Ball Valves and Thermometers
The supply and return sides normally each include an isolation ball valve and thermometer. The thermometers in this series have a 0–120°C range. Two thermometers allow direct observation of the Supply-to-Return temperature difference. For example, with 45°C supply and 35°C return, ΔT is about 10 K, which helps assess heat transfer and circulation performance in the circuit.
3) Check Valve
The Check Valve restricts reverse flow in unintended directions. This is particularly useful when multiple Pump Groups operate in parallel because it reduces cross-flow through a stopped circuit caused by other circulation pumps. In this series, the check function is integrated into the blue-handle ball valve and is enabled by default. Rotating the handle 45° disables the check function.
Check Valve Open Operation
4) Mixing Valve and Thermostatic Mixing Valve
A Mixing Valve adjusts outlet temperature by changing the mixing ratio between heat-source supply water and return water. A Thermostatic Mixing Valve adjusts this ratio automatically to maintain the set temperature. When selecting a thermostatic version, confirm the Temperature Setting Range; for the 8352, it is 25–65°C.
5) Safety Thermostat / Overtemperature Safety Switch (Optional Component)
In an underfloor heating system, scale buildup or aging can cause the mixing valve cartridge to seize. High-temperature water may then flow directly into the floor-heating pipework, potentially damaging PE-RT/PEX piping or deforming wood flooring. Professional underfloor-heating Pump Group configurations generally recommend an optional overtemperature safety thermostat (Limit Thermostat / Safety Switch) downstream of the mixing valve on the supply side. If the monitored supply temperature exceeds the safety setpoint, such as 55°C, the thermostat automatically cuts power to the circulation pump and provides a physical layer of overtemperature protection.
6) EPP Insulation Casing
This series uses EPP 45 g/L insulation to enclose the circulation pump, valves, and fittings in a compact module. The casing reduces direct heat loss and protects internal components. Its structure can also be reconfigured for different Pump Group layouts and Supply/Return orientation changes.
5. Key Pump Group Specifications and Parameters
Common selection parameters include DN, connections, circulation-pump installation length, Kv, working pressure, temperature, and medium. These parameters define the mechanical installation and hydraulic performance.
1) DN25 and G1″ × G1-1/2″ Connections
DN25 describes the nominal bore of the Pump Group; it does not mean every connection is 1″. The 8350, 8351, and 8352 actually use G1″ Female × G1-1/2″ Male connections. When purchasing, do not specify only “DN25 Pump Group”; also confirm the upstream, downstream, and circulation-pump connections.
2) 130 mm Circulation Pump Installation Dimension
The 130 mm dimension only confirms whether the circulation pump fits mechanically; it does not determine whether the pump’s hydraulic performance suits the system. Two pumps may both have a 130 mm installation length, but different Q-H curves can produce completely different actual flow rates after installation.
3) Kv Flow Coefficient
Kv indicates the flow capacity of the valve assembly at a given pressure differential. For water, Pump Group pressure drop can be estimated approximately as: Δp (bar) ≈ [Q (m³/h) ÷ Kv]². For example, the 8350 has a Kv of 6.5. At a system flow of 2 m³/h: Δp ≈ (2 ÷ 6.5)² ≈ 0.095 bar ≈ 9.5 kPa.
This result means the Pump Group itself already consumes about 9.5 kPa of differential pressure. When selecting the circulation pump, also include resistance from piping, elbows, manifolds, and terminal equipment. If the medium contains a relatively high glycol concentration, account for changes in density and viscosity rather than applying pure-water conditions directly. The Kv values of the 8350 and 8351 are 6.5 and 5.5, respectively.
Pump Group Pressure-Drop Estimate
4) Working Pressure, Temperature, and Glycol Concentration
The main operating parameters for this product series are as follows:
| Parameter | Product Specification |
| Working Medium | Water / Glycol-based solutions |
| Maximum Glycol Percentage | 30% |
| Maximum Working Pressure | 10 bar |
| Working Temperature | 5–95°C |
| Thermometer Scale | 0–120°C |
These values come from the specifications of this product series and should not be treated as universal industry parameters for all Pump Groups.
5) Temperature Adjustment Range of the Thermostatic Mixing Version
The 8352 has a System Temperature Setting Range of 25–65°C. This range covers an underfloor-heating design supply temperature of 35–45°C. If the terminal circuit requires a temperature outside this range, select a different temperature-control valve or Pump Group.
6. How to Select the Right Pump Group
Select a Pump Group by checking temperature control, terminal design temperature, flow, Kv, circulation pump, and system operating conditions step by step.
1) Direct, Mixing, or Thermostatic Mixing?
If the heat-source water temperature directly meets the terminal requirement, give priority to Direct. If the heat-source temperature is higher than the terminal requirement and the supply must be reduced and adjusted, select Mixing. If the Pump Group must also maintain a stable set water temperature automatically, select Thermostatic Mixing. A larger circulation pump cannot correct a temperature-control problem caused by selecting the wrong Pump Group type.
2) Select by Heating Terminal and Required Supply Temperature
For example, assume a system is designed for 70°C boiler supply, 60°C Radiator supply, and 40°C Underfloor Heating supply. The Radiator circuit may use heat-source water directly or rely on heat-source control, while the underfloor-heating circuit requires further temperature reduction. Even with the same heat source, the two terminal circuits therefore use different Pump Groups.
3) Select by System Flow and Kv
For water, the design flow of a heating circuit can be estimated as: Q (m³/h) ≈ P (kW) ÷ [1.163 × ΔT (K)]. For example, for a 20 kW circuit with a design Supply/Return temperature difference of 10 K: Q ≈ 20 ÷ (1.163 × 10) ≈ 1.72 m³/h.
After determining the design flow, calculate the Pump Group pressure drop from Kv and include it in the total system resistance. This is more reliable than simply assuming “20 kW requires DN25.” For glycol solutions, further correct the flow and pressure loss using the actual fluid properties.
4) Confirm the Circulation Pump and Connection Specifications
For the circulation pump, confirm at least the Installation Length, Connection, Design Flow, and Required Head. For example, if the system requires 5 m of head at the design flow but a candidate pump can provide only 3 m at that flow point, the most remote circuit may still have insufficient flow even when the 130 mm installation length and connections fit perfectly. Confirm the actual operating point from the Pump Curve rather than judging by pump model or installation length.
5) Confirm Working Pressure, Temperature, and Medium
Finally, verify Maximum Working Pressure, Temperature, Glycol Concentration, and Seal Material. This series is designed for water or glycol solutions up to 30%, with a maximum working pressure of 10 bar and a working temperature of 5–95°C. Seal materials are EPDM or PTFE. For actual OEM projects, confirm material and fluid compatibility against the system operating conditions.
7. What Should Be Considered When Installing a Pump Group?
Correct Pump Group operation after installation depends on Supply/Return direction, check-valve status, orientation changes, and system commissioning.
Pump Group Installation
1) Confirm the Supply and Return Directions Correctly
Reversing Supply and Return can affect the circulation pump, check valve, and mixing valve. Before installation, confirm the direction from the system schematic and Pump Group flow path. Do not judge only by “red thermometer on the right and blue on the left,” because this Pump Group series supports left/right reversal.
2) How Can the Supply and Return Positions Be Switched?
The 8350, 8351, and 8352 all allow the left/right Supply and Return layout to be adjusted to suit site piping. On the Mixing and Thermostatic Mixing versions, reversing the layout also requires reorienting the relevant mixing-valve components. The EPP base and Cover Plate can be rearranged accordingly. Do not simply rotate the entire Pump Group 180°.
3) How Is the Check Valve Function Enabled or Disabled?
The Check Valve in the blue-handle ball valve is active by default. To disable the check function, rotate the handle 45°. After commissioning or maintenance, confirm that the check function is restored to the operating state required by the project; otherwise, a multi-circuit system may develop unintended flow paths.
4) How Should Temperature and Circulation Be Checked After Installation?
After system startup, do more than confirm that the Pump is running. Check Supply Temperature, Return Temperature, ΔT, and actual terminal flow. If the circulation pump runs but the room does not heat and the Supply/Return temperature difference is abnormal, check trapped air, valve positions, mixing settings, and system flow before concluding that the circulation pump has failed.
8. Common Pump Group Problems and Causes
When a Pump Group operates abnormally, troubleshoot from four main areas: temperature, flow, check-valve status, and the circulation-pump operating point.
Pump Group Troubleshooting
1) Circulation Pump Runs but the Heating Circuit Does Not Heat
A running Pump only confirms that the motor is operating; the system may still be below design flow. Common causes include Air Lock, isolation valves that are not fully open, abnormal Check Valve status, system blockage, insufficient circulation-pump head, or an incorrect Mixing Valve setting. On site, first check whether hot water has reached the Pump Group inlet, then determine whether the problem is on the heat-source side or the circulation side.
2) Supply Temperature Is Too High or Too Low
If a Mixing Pump Group has an abnormal temperature, check the primary supply temperature, mixing-valve position, and Supply/Return direction. For a Thermostatic Mixing model, also check the setpoint and thermostatic cartridge movement. For example, if the setpoint is 40°C but the actual temperature is only 30°C, the thermostatic valve may be faulty, or the primary hot-water temperature and flow may be insufficient.
3) Abnormal Supply/Return Temperature Difference
An excessively large ΔT usually requires checking for low flow. An excessively small ΔT may result from excessive flow, insufficient load, or bypass flow. The two thermometers allow the Supply-to-Return temperature difference to be used as a practical indicator of overall circuit operation.
4) Reverse Flow in the System
When multiple Pump Groups operate in parallel, system differential pressure may drive water through a stopped circuit while other circulation pumps continue running. Check whether the Check Valve operates correctly, whether it remains disabled, and whether the Supply/Return direction is correct. If a stopped circuit still heats abnormally, reverse cross-flow should be one of the first conditions investigated.
5) Pump Group Noise or Unstable Circulation
Common causes include incomplete system venting, abnormal pump-inlet conditions, excessive pump speed, changes in system resistance, and an unsuitable circulation-pump operating point. Turning the Pump directly to maximum speed when a terminal does not heat is not good practice. If the actual cause is Air Lock or an incorrect valve position, higher pump speed may only increase flow noise. A practical troubleshooting sequence is: confirm the water-path status → vent the system → check valves → verify design flow and resistance → adjust the Pump Setting.
9. Summary
Match the Pump Group to the heat-source supply temperature, required terminal temperature, design flow, system resistance, and circulation-pump operating point. Direct, Mixing, and Thermostatic Mixing configurations address different temperature-control requirements, while Kv, the Pump Curve, and Supply/Return ΔT help determine whether the system can reach its design operating condition.
Plumberstar can provide Economic, Mixing, and Thermostatic Mixing Pump Groups for OEM customers, HVAC equipment suppliers, and hydronic-system customers, with support for connection, circulation-pump, valve, and insulation-casing configurations. If you provide the heat-source type, design Supply/Return temperatures, system flow, working pressure, and connection requirements, we can further assist in confirming a suitable Pump Group solution.
