Standard Booster WSHP vs Flexible ST2P-range W/W | Pure Thermal

Pure Thermal Technical Bulletin

Subject: System Comparison.

                Comparing Standard Booster Water Source Heat Pumps to more Flexible ST2P range Stage 2 Water/Water units.

Issue date: May 2024

Subject area: Two Stage Heat Pump Applications

Detail reviewed: Standard Booster WSHP vs Flexible ST2P-range W/W


Standard booster Water Source Heat Pumps (WSHP) can offer application restrictions due to three main areas:

  • Standard Booster units can utilise high GWP refrigerants such as R134a (GWP of 1430)

The Global Warming Potential of these refrigerants is above a safe ceiling of 700 GWP. This can be a selection barrier on low carbon and low GWP applications and also projects that require future proofing with respect to F-Gas phase out restrictions.

  • Standard Booster units operate with a narrow delta T of 7°C to 10°C on the condenser/hot delivery side of the Heat Pump.

This renders standard booster WSHP to be less suitable for retrofit applications which require a wider delta T to maximise the system efficiency based on current system operating temperatures.

  • Standard Booster units achieve a 78°C leaving temperature which can restrict retrofit application.

This means that applications requiring above 78°C are restricted by the leaving temperature of the heat pump. For example, retrofit 82/71°C applications can call for 85°C heat pump flow temperatures on the heat pumps side of a break/plate heat exchanger which booster units are unable to provide.


To overcome these application restrictions Pure Thermal are able to provide options for Stage 2 heat pump units that include standard booster units and also more flexible Low GWP, wide DT and high leaving temperature FLEXIBLE ST2-P heat pumps.

Application flexibility – Stage 2 Heat Pump units:

  • Pure Thermal provide:

Standard Booster units – Bronze range

BUT Pure Thermal ALSO provide:

Flexible STAGE 2 units – ST2-P range units

Flexible STAGE 2 units – ST2P range

  • Low GWP Refrigerant:

The Pure Thermal Flexible Stage 2 ST2P range Water/Water units utilise Low GWP Refrigerants with options to deliver GWP 1 system designs.
A low GWP enables a Two Stage System (or single stage water source) to be applied within more demanding applications where low carbon and F-Gas future proofing are design priorities.

  • Wide DT capability 50 °C to 10 °C:

The Pure Thermal Flexible Stage 2 ST2P range units are capable of operating on a wide delta T up to 50 °C to provide new build and retrofit application flexibility.

A low return temperature and wide DT can increase heat pump efficiency and increase heat pump capacity. This capability enables the ST2-P range units to operate with the widest DT possible to enable straightforward retrofit application for both Heating and Hot Water applications.

  • High Temperature Capability 95 °C:

The Pure Thermal Flexible Stage 2 ST2P range units are capable of operating with up to 95°C output temperatures.

This design flexibility means that a Two Stage Low GWP Heat Pump System can be applied within the most demanding applications and removes any operating temperature restrictions that may be associated with standard booster units.


These three key factors enable a Two Stage Heat Pump System to deliver higher system efficiencies and provide application flexibility:

Standard Booster + Flexible ST2P Detailed image





This flexible high-performance approach means that the heat pump has the capability to match the exact requirements of an application whilst providing a future proofed system design.

Low GWP 1 + Wide DT 50 °C to 10 °C + High Temperature 95 °C

Contact the Pure Thermal team at or on 0203 488 4618

Flexible ST2P Operating Envelope P60













Standard Booster Operating Envelope
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