Cascade Installation in Heat Pump Systems

Cascade installation is an advanced heat pump configuration designed to deliver high heating temperatures that single-stage systems often cannot achieve efficiently. By connecting two or more heat pumps in a series arrangement, the system distributes the temperature lift across multiple stages, improving performance, reliability, and energy efficiency. This approach enables heat pumps to support high-temperature radiator systems, domestic hot water production, and industrial heating applications while helping buildings meet modern decarbonisation and renewable energy requirements.

What is Cascade Installation in Heat Pump Systems

Cascade installation is a heat pump configuration that connects two or more heat pump units in series. Each unit processes the refrigerant output of the previous unit. This sequential arrangement allows the system to achieve final heating temperatures that a single heat pump unit cannot deliver alone.

In a cascade system, the first heat pump extracts heat from a low-temperature source and raises that heat to an intermediate temperature level. The second heat pump uses this intermediate temperature as its own heat source and elevates the energy further to a high supply temperature. The result is a thermodynamic chain that extends the operating range of heat pump technology beyond its single-stage limits.

Cascade installation is a distinct configuration category within heat pump installation planning. It differs from parallel installation, where multiple units share the load at the same temperature stage. It differs from buffer-integrated systems, where a single unit charges a thermal store. Cascade configuration specifically refers to the staged, series-connected refrigerant circuit arrangement.

What is the Purpose of Cascade Installation in Heat Pump Systems

The primary purpose of cascade installation is to generate high supply temperatures from low-temperature heat sources. A standard air-source or ground-source heat pump operates efficiently within a defined temperature lift range. When the required supply temperature exceeds that range, the coefficient of performance (COP) falls sharply, and the compressor operates under thermal stress. Cascade installation solves this problem by distributing the temperature lift across two thermodynamic stages.

Cascade configuration also enables heat pump technology to serve applications that have historically depended on fossil fuel boilers. Process heat, high-temperature domestic hot water, and legacy radiator systems all require supply temperatures above what a single heat pump stage can efficiently provide. By chaining two refrigerant circuits, the system produces these temperatures while maintaining operational efficiency.

A secondary purpose is system flexibility. Cascade configurations allow each stage to use a refrigerant and compressor type optimised for its specific temperature range. The low-temperature stage uses a refrigerant suited for cold evaporation conditions. The high-temperature stage uses a refrigerant suited for elevated condensation conditions. This division of thermodynamic labour produces better overall system performance than forcing a single refrigerant to cover the entire range.

Why Cascade Installation Is Needed

The Temperature Lift Problem

Heat pump efficiency decreases as the temperature difference between the heat source and the supply temperature increases. This difference is called the temperature lift. Every 10 K of additional lift reduces COP by approximately 2–3% in standard configurations. A large lift collapses efficiency and raises operating costs to a level where fossil fuels become economically competitive.

Standard monovalent heat pumps target supply temperatures up to approximately 55–60°C. Many existing buildings in Austria, Germany, and Switzerland were designed for radiator systems operating at 70/55°C or even 80/60°C flow/return profiles. Retrofitting these buildings with a single heat pump stage creates a temperature gap that cascade installation directly addresses.

Regulatory and Decarbonisation Context

The German Gebäudeenergiegesetz (GEG) requires new heating systems to fulfil minimum renewable energy fractions. The Austrian KlimaBonus and the Swiss MuKEn 2014 framework create financial incentives for eliminating fossil fuel systems. These regulations accelerate the transition from gas and oil boilers to heat pump systems. Many of the buildings targeted by these programmes carry legacy heating infrastructure that demands high supply temperatures. Cascade installation makes that transition technically feasible without complete hydraulic system replacement.

Industrial and Commercial Heat Demand

Commercial buildings, hotels, healthcare facilities, and light industrial processes require domestic hot water at 60–65°C or higher to comply with Legionella prevention standards under EN 806 and ÖNORM B 5019. Process heat applications require temperatures up to 80°C or beyond. A single heat pump stage cannot efficiently reach these temperatures. Cascade configuration extends heat pump applicability into sectors that would otherwise remain dependent on combustion-based systems.

Key Features of Cascade Installation

Cascade installation is defined by four core technical features: the intermediate heat exchanger, the staged refrigerant circuit, the sequential temperature staging, and the independent compressor control.

Intermediate Heat Exchanger

Definition. The intermediate heat exchanger (IHX) is the thermal interface between the first and second refrigerant circuits in a cascade system.

Purpose. The IHX transfers heat from the condenser of the first-stage heat pump to the evaporator of the second-stage heat pump. This transfer occurs without mixing the two refrigerant circuits.

Benefits. The IHX allows each stage to use a chemically and thermodynamically incompatible refrigerant suited to its operating range. It provides thermal isolation between circuits, preventing pressure differentials in one stage from affecting the other.

Practical Application. In an iDM TERRA SW cascade configuration, the IHX is sized to maintain a minimum approach temperature of 3–5 K between the two refrigerant circuits. Approach temperature directly affects the overall COP of the cascade system. Undersizing the IHX increases approach temperature, raises compressor head pressure on stage two, and reduces system efficiency.

Staged Refrigerant Circuit

Definition. A staged refrigerant circuit is a thermodynamic arrangement where each heat pump stage operates with its own compressor, expansion valve, and heat exchanger set, but the energy output of stage one feeds the energy input of stage two.

Purpose. Staging the circuit allows the refrigerant selection and pressure ratios in each stage to be independently optimised. The low-stage circuit uses refrigerants with low evaporation temperatures, such as R290 (propane) or R744 (CO₂). The high-stage circuit uses refrigerants suited for elevated condensation, such as R134a or R1234ze(E).

Benefits. Independent circuit optimisation lowers the compression ratio in each compressor relative to what a single stage would need. Lower compression ratios reduce mechanical stress, improve volumetric efficiency, and extend compressor service life.

Practical Application. In CO₂ transcritical cascade systems, the low stage uses R744 operating in a transcritical cycle that reaches evaporation temperatures down to −40°C. The high stage then processes this intermediate-temperature heat and delivers supply temperatures above 80°C. This configuration applies directly to industrial hot water generation in food processing and pharmaceutical manufacturing in the DACH region.

Sequential Temperature Staging

Definition. Sequential temperature staging is the process by which the supply temperature is raised incrementally across two or more heat pump stages rather than in a single compression step.

Purpose. Sequential staging distributes the total required temperature lift across multiple smaller lifts. Each stage operates within its optimal thermodynamic efficiency window rather than at the extreme edges of its performance envelope.

Benefits. Distributing the lift increases the total system COP compared to forcing a single stage to cover the entire temperature range. It also allows the system to continue partial operation if one stage enters a fault state, improving system reliability.

Practical Application. A cascade system serving a 70°C radiator circuit from a 5°C ground source might use a first stage to raise temperature from 5°C to 35°C and a second stage to raise temperature from 35°C to 70°C. The combined COP of the two staged lifts exceeds the COP that a single stage would achieve across the full 65 K lift.

Independent Compressor Control

Definition. Independent compressor control is the ability to modulate the output of each stage compressor separately, without requiring synchronised operation of both units.

Purpose. Independent control allows the system to match heating output to demand at each stage independently. This prevents over-cycling, reduces part-load inefficiency, and allows load-following behaviour across a wide operating range.

Benefits. Variable-speed compressors in each stage, combined with independent control logic, allow the cascade system to operate efficiently at partial loads. Many cascade systems achieve their best COP values at 50–70% of rated capacity, rather than at full load.

Practical Application. The iDM Navigator 2.0 control system manages cascade stage sequencing, interstage temperature monitoring, and compressor modulation across both stages from a single interface. This integration reduces installation complexity and enables remote monitoring of each stage’s operating parameters via smart grid interfaces.

Detailed Feature Explanations

Thermodynamic Efficiency in Cascade Systems

The total COP of a cascade system is calculated differently from a single-stage COP. The thermal energy input to stage two is the thermal energy output of stage one. This dependency means the total system COP is a function of the product of both stage COPs, adjusted for the fraction of thermal output each stage delivers to the final load.

For engineering planning purposes, a cascade system from a 0°C source to a 70°C supply temperature typically achieves a seasonal COP (SCOP) in the range of 2.8–3.5, depending on source stability, IHX approach temperature, and compressor efficiency in each stage. This compares to a theoretical single-stage COP of approximately 1.8–2.2 for the same operating conditions.

EN 14825 governs the testing and calculation of heat pump efficiency across part-load conditions. Cascade systems operating under DACH incentive schemes must demonstrate SCOP performance figures calculated in accordance with this standard to qualify for BAFA subsidy in Germany or KlimaBonus funding in Austria.

Refrigerant Selection by Stage

Refrigerant selection in cascade systems is governed by two criteria: operating temperature range and regulatory compliance under the EU F-Gas Regulation (EU 517/2014) and its successor framework. Low-stage refrigerants must evaporate efficiently at temperatures below −20°C. High-stage refrigerants must condense efficiently at temperatures above 70°C.

Common low-stage refrigerants include R290 (propane, GWP = 3), R744 (CO₂, GWP = 1), and R1270 (propylene, GWP = 2). These natural refrigerants dominate new cascade designs due to their low global warming potential. The F-Gas Regulation has progressively restricted high-GWP refrigerants in new equipment. Installers working in Germany, Austria, and Switzerland must verify that the selected refrigerants comply with the current phase-down schedule.

High-stage refrigerants include R134a, R1234ze(E), and R513A. These refrigerants have condensation properties suited to producing supply temperatures above 65°C. In equipment certified to EN 378, the maximum operating pressures for each stage must be declared by the manufacturer and verified during commissioning.

Hydraulic Integration at the Intermediate Stage

The IHX connects the two refrigerant circuits hydraulically and thermally. In the hydraulic design of the wider heating system, the intermediate stage temperature also determines the operating window of any buffer storage connected between the two heat pump stages.

Some cascade configurations include an intermediate buffer vessel between stage one and stage two. This buffer decouples the cycling behaviour of the two stages and prevents short-cycling when heating demand fluctuates rapidly. In others, the IHX is direct-coupled, and stage control logic manages synchronisation without intermediate storage.

VDI 4645 provides the planning framework for heat pump system integration in Germany, including hydraulic schemes for cascade arrangements. ÖNORM H 5151 governs the hydraulic planning requirements in Austria. EN 14336 defines the commissioning procedures that apply to both single-stage and cascade heat pump installations.

Types of Cascade Configurations

Two-Stage Air-to-Water Cascade

Description. Two air-source heat pump units connected in series. Stage one extracts heat from outdoor air and produces an intermediate water temperature. Stage two takes this intermediate temperature as its heat source and delivers high-temperature water to the heating distribution system.

Application. High-temperature retrofit of air-source systems in urban buildings where ground source drilling is not feasible. Common in apartment building upgrades in Vienna, Munich, and Zurich.

Regulatory Relevance. Noise emissions from two outdoor units must comply with TA Lärm in Germany and ÖNORM S 5021 in Austria. Unit placement must account for combined acoustic output from both units in the cascade.

Ground-Source Cascade (Brine-to-Water / Water-to-Water)

Description. A ground-source primary stage extracts heat from borehole or groundwater sources at stable low temperatures. A second stage uses this intermediate temperature to generate high supply temperatures. Ground-source primary stages benefit from consistent source temperatures year-round, which stabilises the operating conditions of the entire cascade.

Application. Commercial buildings, hotels, and healthcare facilities requiring simultaneous high-temperature heating and cooling. Common in new construction projects under Austria’s strict OIB Richtlinie 6 energy performance requirements.

Regulatory Relevance. Groundwater cascade systems require permits under Austrian water law (WRG) and Swiss cantonal water protection regulations. Borehole field sizing must comply with VDI 4640 in Germany.

CO₂ Transcritical Cascade

Description. The low stage uses CO₂ (R744) in a transcritical refrigerant cycle. The high stage processes the resulting intermediate heat to produce supply temperatures above 80°C. CO₂ transcritical cascades achieve the lowest GWP of any cascade refrigerant configuration.

Application. Industrial process heat in the food industry, dairy processing, and pharmaceutical manufacturing. Also applicable to large-scale domestic hot water systems in hotels and hospitals where Legionella prevention demands minimum 65°C recirculation temperatures.

Regulatory Relevance. CO₂ transcritical systems operate at significantly higher pressures than conventional refrigerant systems. EN 378 pressure equipment requirements apply. Technicians must hold refrigerant handling certifications under EU F-Gas Regulation Article 10.

Hybrid Cascade with Fossil Backup

Description. A heat pump cascade forms the primary heating system. A gas or oil boiler connects as a backup stage for extreme outdoor temperatures or peak demand. The control system selects the most energy-efficient combination of stages for each operating condition.

Application. Transitional retrofit projects in existing buildings where full heat pump coverage is not yet economically viable. Applicable in German buildings where GEG requires at least 65% renewable energy fraction but allows hybrid configurations to fulfil this requirement.

Regulatory Relevance. GEG Section 71 permits hybrid heat pump systems. BAFA subsidy conditions for hybrid systems specify minimum heat pump capacity relative to design heat load. Swiss MuKEn 2014 permits hybrid systems where the heat pump covers the base load and combustion covers peak demand.

Use Cases

Radiator Retrofit in Existing Residential Buildings

Many residential buildings constructed before 1990 in Austria, Germany, and Switzerland use high-temperature radiator systems designed for 75/65°C or 80/60°C flow/return profiles. Replacing the entire radiator distribution system is costly and disruptive. Cascade installation allows a heat pump system to match these legacy supply temperature requirements without radiator replacement, reducing retrofit project cost and duration.

The economic case is strengthened by KlimaBonus (Austria), BEW subsidy (Germany), and cantonal energy subsidies in Switzerland, which partially fund cascade heat pump installations in retrofit applications.

Domestic Hot Water Generation at Scale

Multi-unit residential buildings, hotels, and hospitals require domestic hot water at 60–65°C to comply with Legionella prevention standards. EN 806 and ÖNORM B 5019 specify minimum storage and circulation temperatures. A single-stage heat pump often cannot efficiently maintain these temperatures across periods of high draw-off demand. Cascade installation delivers the sustained high supply temperatures necessary for compliant hot water systems.

Industrial Process Heat

Light industrial processes including food preparation, laundry, surface treatment, and pharmaceutical manufacturing require process water or steam at temperatures between 70°C and 120°C. CO₂ transcritical cascade configurations supply process heat up to 90°C with COP values that compete economically with gas-fired systems at current energy price ratios in the DACH region.

District Heating Substations

Municipal district heating networks in Austria and Germany increasingly integrate heat pump substations that boost return temperatures from network circuits and deliver supply temperatures for secondary distribution. Cascade configurations allow these substations to handle the temperature lift from district return conditions (typically 30–50°C) to secondary supply conditions (70–80°C) with high efficiency.

Benefits

System Efficiency Benefits

• Higher achievable supply temperatures at better COP than single-stage alternatives
• Each stage operates within its optimal thermodynamic efficiency window
• Natural refrigerants (R290, R744) in low-stage circuits reduce F-Gas regulatory exposure
• SCOP values in the range of 2.8–3.5 for high-temperature applications versus 1.8–2.2 for equivalent single-stage operation

Installation and Retrofit Benefits

• Enables heat pump technology in buildings with existing high-temperature radiator systems
• Reduces or eliminates the need for full hydraulic system replacement in retrofits
• Modular structure allows phased installation: first stage first, second stage added when budget allows
• Compatible with existing buffer vessels and distribution infrastructure in many cases

Operational and Resilience Benefits

• Partial load operation remains efficient due to independent compressor modulation at each stage
• If one stage enters a fault state, the system can often continue operation at reduced temperature
• Two-stage architecture distributes mechanical wear across two compressors, extending service life
• Smart grid integration at each stage level allows demand response and electricity cost optimisation

Regulatory and Financial Benefits

• Enables compliance with GEG renewable energy fraction requirements in Germany
• Qualifies for BAFA BEW subsidy in Germany and KlimaBonus in Austria when installed in eligible building types
• Satisfies OIB Richtlinie 6 and MuKEn 2014 requirements for nearly zero-energy building standards
• Supports decarbonisation targets without requiring replacement of all legacy heating infrastructure

Selection Criteria

Required Supply Temperature

The primary selection criterion is the maximum supply temperature the heating system demands. If the design supply temperature exceeds 60°C on a regular basis, cascade installation should be evaluated. If the design supply temperature exceeds 70°C, cascade installation is typically necessary for efficient operation.

Systems with variable supply temperatures — for example, weather-compensated control where summer temperatures stay below 50°C but winter peaks reach 70°C — can use cascade configurations with stage deactivation at low load. This avoids running both stages unnecessarily during mild conditions.

Heat Source Temperature and Stability

Ground-source primary stages are preferred when a stable source temperature is critical for consistent high-stage performance. Air-source primary stages are more variable and require the cascade control system to manage wider fluctuations in intermediate temperature.

For outdoor temperatures regularly below −10°C, which occur across alpine regions in Tyrol, Vorarlberg, Salzburg, and Bavaria, ground-source cascade configurations provide more reliable high-temperature output than air-source cascade systems.

Refrigerant Compatibility and F-Gas Phase-Down

The EU F-Gas phase-down schedule restricts the availability of high-GWP refrigerants in new equipment from 2025 onward. Cascade configurations using natural refrigerants (R290, R744) in both stages are the most future-proof choice. Equipment manufacturers, including iDM, provide cascade product lines designed around natural refrigerant circuits to protect against refrigerant supply disruption.

Available Electrical Infrastructure

Cascade installations require electrical supply capacity for two compressor units. In some existing buildings, the main electrical supply cable requires upgrading before a cascade system can be commissioned. This infrastructure assessment must occur during the planning phase, before equipment selection, to avoid installation delays.

Physical Space Requirements

A cascade configuration places two heat pump units in the plant room or outdoor area. Space requirements for the two units, the IHX, associated pipework, and service access must be assessed against available space at the project site. For split-sited configurations where the two stages are located in separate plant areas, the refrigerant pipe run between stages must be designed within manufacturer-specified maximum lengths to avoid performance loss.

Comparisons

Cascade vs. Single-Stage High-Temperature Heat Pump

Cascade vs. Bivalent System

A bivalent system combines a heat pump with a secondary heat generator (typically a boiler) operating in parallel for peak demand. The heat pump and boiler share the same distribution circuit. In a cascade system, two heat pump stages are thermodynamically connected in series. The cascade system achieves high supply temperatures through staged heat pumping rather than by supplementing with combustion.

Bivalent systems are less expensive to install and suitable where gas or oil backup is acceptable under local regulatory conditions. Cascade systems are more appropriate where full decarbonisation is required and combustion-based backup is not permitted or desirable.

Cascade vs. Parallel Multi-Unit Installation

Parallel multi-unit systems connect two or more heat pump units to the same distribution circuit at the same temperature stage. This increases capacity without increasing supply temperature capability. Cascade installation increases supply temperature capability. These are different solutions to different problems: parallel installation for capacity, cascade installation for temperature lift.

Integration with Other Systems

Buffer Storage Integration

Cascade systems benefit from hydraulic separation between the heat pump stages and the heating distribution system via buffer storage. A correctly sized buffer vessel absorbs the thermal output of the cascade system during periods of low demand and releases it during peak demand periods. This prevents short-cycling of compressors in both stages and stabilises the intermediate temperature between stages.

VDI 4645 specifies minimum buffer storage volumes for heat pump systems in Germany as a function of heat pump output capacity. ÖNORM H 5151 provides equivalent guidance for Austrian installations. Buffer storage must be designed to accommodate the temperature stratification requirements of both cascade stages.

Solar Thermal Coupling

Solar thermal collectors can feed heat into the intermediate stage of a cascade system. When solar thermal output temperature matches the intermediate temperature between stage one and stage two, the solar heat displaces compressor energy in stage one. This reduces the electrical energy consumption of the cascade system during periods of solar availability. This configuration is particularly relevant for commercial and agricultural buildings in the sunnier regions of Austria’s Styria and Burgenland.

Smart Grid and Demand Response Integration

Cascade systems with variable-speed compressors and individual stage controls are well-suited to smart grid demand response. Electricity grid operators in Austria (APG), Germany (50Hertz, Amprion, TenneT, TransnetBW), and Switzerland (Swissgrid) increasingly offer incentive tariffs for flexible heat pump load. Cascade systems can participate by shifting stage one and stage two operation independently to periods of low grid tariff or high renewable electricity availability.

The iDM Navigator 2.0 control platform provides smart grid-ready (SG-Ready) interfaces as standard. This enables direct communication between the cascade control system and utility demand-response signals, automating load shifting without manual intervention.

Photovoltaic Integration

A cascade system’s two-stage architecture increases the total electrical load that can productively absorb on-site photovoltaic generation. When PV output is high, both stages can operate simultaneously to generate and store heat in buffer storage for later use. When PV output is low, only the stage with the highest priority load operates. This load-matching capability improves the self-consumption rate of PV systems paired with cascade heat pump installations.

District Heat Network Connection

In mixed-source configurations, a district heat network connection can feed the intermediate stage of a cascade system at network return temperature. The second stage then boosts this intermediate temperature to the required building supply temperature. This reduces the electrical input to the cascade system by using network heat as the low-temperature source, while allowing the building to maintain independent supply temperature control.

Cascade installation extends the capabilities of heat pump technology by efficiently delivering high supply temperatures from low-temperature energy sources. Through staged refrigerant circuits, sequential temperature lifting, and independent compressor control, cascade systems achieve higher efficiency, greater operational flexibility, and improved reliability compared to single-stage alternatives. As buildings, industries, and district heating networks transition away from fossil fuels, cascade heat pump systems provide a practical solution for high-temperature heating, domestic hot water production, and process heat applications. Their compatibility with natural refrigerants, smart energy systems, and modern decarbonisation regulations makes cascade installation a future-ready approach for sustainable heating in both retrofit and new-build projects.