Bivalent Systems in Heat Pump Installation

Retrofitting existing heating with a heat pump means upgrading an older gas, oil, or electric heating system into a cleaner, more efficient low-temperature heating solution without rebuilding the entire system. In homes across Austria, Germany, Switzerland, and South Tyrol, this usually involves assessing existing radiators, pipework, hot water storage, electrical supply, and controls to make sure the building can work efficiently with a modern heat pump. This guide explains how retrofit heat pump installation works, why it matters for energy savings and fossil-fuel phase-out, and which technical factors determine a successful retrofit project.

What is Bivalent System in Heat Pump Installation

A bivalent system is a heating configuration that combines a heat pump with a secondary heat generator to meet a building’s total thermal demand. The two sources operate in a coordinated sequence or in parallel, governed by outdoor temperature and load thresholds. One generator handles base load conditions; the other activates when demand exceeds what the primary source can efficiently deliver alone.

In heat pump installation, the bivalent system is distinguished from a monovalent system—which relies solely on the heat pump—by the deliberate integration of a supplementary generator. This second source is typically a gas or oil boiler, a district heating connection, an electric resistance heater, or a wood-pellet boiler. The system is unified through a shared hydraulic circuit and a central control unit that arbitrates source selection.

The defining characteristic of a bivalent system is the bivalence point: the outdoor temperature threshold at which the secondary generator is activated. Below this point, the heat pump alone is insufficient, either energetically or economically, to meet the building’s heat load.

What is the Purpose of Bivalent System in Heat Pump Installation

The primary purpose of a bivalent system is to ensure continuous, reliable heating across all outdoor temperature conditions without oversizing the heat pump. Heat pumps lose efficiency as outdoor temperatures fall. A heat pump sized to cover 100 percent of peak winter demand in the coldest climates would be oversized for the majority of the heating season, resulting in unnecessary capital cost and poor seasonal efficiency.

Bivalent systems resolve this by sizing the heat pump to cover base load—typically between 60 and 80 percent of annual heat demand—while assigning the secondary generator to peak load conditions that occur on a limited number of days each year. This approach optimises the overall system economy: the heat pump operates in its most efficient temperature range for the greatest number of operating hours, while the secondary generator handles the short-duration extreme-cold periods.

The secondary purpose is system resilience. A bivalent configuration provides operational redundancy. If the heat pump requires maintenance or fails temporarily, the secondary generator can sustain heating independently. This is particularly relevant for buildings with critical heating requirements or occupants with medical dependencies.

Why Bivalent Systems Are Needed

The Physics of Heat Pump Operation at Low Outdoor Temperatures

Heat pumps extract thermal energy from a source medium—outdoor air, ground, or groundwater—and upgrade it to usable heating temperatures via a refrigeration cycle. The efficiency of this process, expressed as the Coefficient of Performance (COP), is directly proportional to the temperature difference between source and sink. As outdoor air temperature falls, the COP of an air-source heat pump decreases. At extreme temperatures—typically between -10°C and -20°C depending on system design—some air-source heat pumps reach their operational limits or consume more energy than alternative heating methods.

Ground-source and groundwater heat pumps are less affected because subsurface temperatures remain relatively stable year-round. However, they are not immune to undersizing risks during extended cold periods or in buildings with high specific heat demand.

The Economics of Peak Sizing

Sizing a heat pump to meet 100 percent of peak heating demand carries a significant cost penalty. Peak demand occurs during only a small fraction of total annual operating hours. A heat pump sized for -15°C peak conditions would be substantially larger, more expensive, and less efficient during mild-weather operation than one sized for base load. The bivalent approach delivers better return on investment by matching heat pump capacity to the economically optimal operating range.

Building Stock and Retrofit Realities

A large portion of the DACH building stock consists of older structures with high specific heat demand and existing fossil fuel boilers. In these retrofit contexts, full replacement with a monovalent heat pump may require significant building envelope improvements before the heat pump can be sized appropriately. Bivalent installation allows the heat pump to be integrated into existing hydronic systems—often running at higher flow temperatures—while the existing boiler remains as a supplementary source during the transition. This staged approach is consistent with the requirements of the German Gebäudeenergiegesetz (GEG) and the Austrian OIB Richtlinie 6 for phased decarbonisation.

Regulatory and Subsidy Frameworks

Several DACH subsidy programmes explicitly accommodate or require bivalent configurations. The Austrian KlimaBonus (Bundesförderung erneuerbares Heizen) funds heat pump installations in combination with existing boilers as part of a hybrid system pathway. The German Bundesamt für Wirtschaft und Ausfuhrkontrolle (BAFA) heat pump programme permits hybrid heat pump systems with gas backup under specific efficiency conditions. Switzerland’s Gebäudeprogramm supports heat pump integration within existing hydronic infrastructure. Installers and building owners must understand bivalent system types to correctly configure, document, and apply for these subsidies.

Key Features of Bivalent Systems

Bivalent systems share a consistent set of technical features that determine performance, efficiency, and regulatory compliance. These features apply across air-source, ground-source, and groundwater heat pump configurations, with variations in implementation.

Key features include:

• Bivalence point definition and control logic
• Operating mode selection (alternative, parallel, partially parallel)
• Buffer storage integration
• Control unit arbitration and communication protocols
• Hydraulic separation and flow temperature management
• Energy metering and subsidy documentation capability

Each feature carries operational significance. A correctly set bivalence point determines whether the system achieves its designed Seasonal Coefficient of Performance (SCOP). Inadequate buffer storage leads to short-cycling. Poorly configured control logic causes simultaneous operation of competing generators, increasing energy cost.

Detailed Explanation of Key Features

Bivalence Point

Definition: The bivalence point is the outdoor air temperature at which the heat pump’s heating capacity equals the building’s heat loss at that temperature. Below this temperature, the heat pump output alone is insufficient to maintain indoor set-point temperature.

Purpose: The bivalence point determines the thermal boundary between heat pump primary operation and secondary generator engagement. It is the central parameter around which the bivalent system is designed.

Benefits: A correctly calculated bivalence point maximises heat pump operating hours, optimises annual SCOP, and ensures the secondary generator is not activated prematurely—protecting both energy efficiency and subsidy compliance.

Practical Application: The bivalence point is calculated using the building’s heat load curve, derived from EN 12831 (Heizlastberechnung) and the design outdoor temperature (Norm-Außentemperatur) for the installation location. For Austrian locations, design outdoor temperatures are specified in ÖNORM EN 12831-1 and ÖNORM B 8135. For German locations, DIN EN 12831-1 applies. The bivalence point for most Central European retrofits is set between -5°C and +2°C, reflecting a heat pump share of 60–80 percent of annual energy demand.

Operating Modes

Definition: Bivalent systems operate in one of three control modes depending on how the heat pump and secondary generator interact: alternative, parallel, or partially parallel.

Purpose: The operating mode determines which generator is active at any outdoor temperature condition and whether both generators can operate simultaneously. Mode selection affects system efficiency, hydraulic design, and control complexity.

Benefits: Selecting the correct operating mode for the installation context reduces energy waste, simplifies hydraulic design, and ensures compatibility with existing equipment.

Practical Application:

• Bivalent-Alternative Mode: The heat pump operates alone above the bivalence point. Below it, the heat pump shuts down and the secondary generator takes over entirely. This mode is appropriate where the two generators have incompatible flow temperature requirements or where simultaneous operation cannot be hydraulically managed. It is common in gas boiler retrofits with high-temperature hydronic systems.
• Bivalent-Parallel Mode: Both generators operate simultaneously below the bivalence point. The heat pump provides its maximum available output, and the secondary generator supplements the deficit. This mode maximises heat pump utilisation and is preferred where low flow temperatures are achievable. It requires a control system capable of coordinating both sources dynamically and a hydraulic layout that supports simultaneous flow.
• Bivalent-Partially Parallel Mode: A hybrid of the above. The heat pump operates alone above the bivalence point and in parallel with the secondary generator between the bivalence point and a lower cut-off temperature. Below the cut-off, the secondary generator operates alone. This mode is applicable where the heat pump has a defined minimum operating temperature limit.

Buffer Storage (Pufferspeicher)

Definition: A buffer storage vessel is a hydraulically integrated thermal storage unit that decouples heat pump operation from immediate heating circuit demand. It stores surplus heat and releases it during demand peaks.

Purpose: Buffer storage prevents heat pump short-cycling—a condition where frequent on/off switching reduces compressor lifespan and degrades efficiency. It also absorbs surplus output during partial load conditions and provides volume for hydraulic decoupling in combined systems.

Benefits: Correct buffer storage sizing extends compressor service life, reduces wear-related maintenance costs, improves SCOP, and is required by many manufacturers as a warranty condition for bivalent configurations.

Practical Application: Buffer storage volume is determined by the heat pump’s thermal output, the heating circuit’s minimum flow rate, and the building’s thermal inertia. A commonly applied rule of thumb in DACH practice is 20–50 litres per kilowatt of heat pump output for buffer storage, though precise sizing requires hydraulic calculations per VDI 2078 and VDI 6001. Austrian installer guidance under ÖNORM H 5151 specifies minimum storage configurations for heat pump systems integrated with secondary generators.

Control Unit and Arbitration Logic

Definition: The control unit is the electronic management system that monitors outdoor temperature, flow temperatures, set-point requirements, and source availability, then activates or deactivates the heat pump and secondary generator according to programmed logic.

Purpose: The control unit is the operational brain of the bivalent system. It enforces the bivalence point, manages mode transitions, prevents simultaneous conflicting operation, and logs operational data for subsidy reporting.

Benefits: A well-configured control unit optimises the balance between heat pump and secondary generator operation, maximises heat pump run time, prevents energy waste from uncoordinated switching, and enables remote monitoring and fault detection.

Practical Application: Modern heat pump controllers—such as the iDM Navigator 2.0—communicate with secondary generators via standardised interfaces including OpenTherm, Modbus RTU, or proprietary bus protocols. The control unit sets the bivalence temperature threshold, manages set-point handover between sources, and can integrate Smart Grid Ready (SG Ready) signals to shift heat pump operation toward low-cost or low-carbon grid periods. Under the German VDE-AR-N 4100 and Austrian E-Netzbetreiber framework, SG Ready-capable systems are eligible for specific grid interaction tariffs.

Hydraulic Separation and Flow Temperature Management

Definition: Hydraulic separation refers to the design of the heating circuit to prevent direct pressure interactions between the heat pump, secondary generator, and distribution circuits. Flow temperature management ensures each generator operates within its optimal temperature range.

Purpose: Heat pumps operate most efficiently at low flow temperatures (35–55°C). Many secondary generators—particularly gas boilers in existing buildings—are designed for higher flow temperatures (65–85°C). Hydraulic separation allows each generator to serve its respective part of the circuit without thermal interference.

Benefits: Correct hydraulic design prevents heat pump operation at thermally unfavourable conditions, protects the secondary generator from condensation-related damage in low-temperature operation, and enables correct metering of each generator’s energy contribution.

Practical Application: The hydraulic interface between heat pump and secondary generator is commonly realised using a hydraulic separator (Weiche), a low-loss header (Verteiler), or a stratified buffer tank with separate connection ports for each generator. In Austrian installations, ÖNORM EN 14336 governs the hydraulic installation of heating systems. German installations follow DIN 18380 (Heizanlagen und zentrale Wassererwärmungsanlagen) for commissioning and hydraulic integrity testing.

Types of Bivalent Systems

Bivalent systems are classified by the combination of heat pump type and secondary generator type. Each combination has distinct application profiles, efficiency characteristics, and regulatory considerations.

Air-Source Heat Pump with Gas or Oil Boiler (Hybrid Heat Pump)

This is the most prevalent bivalent configuration in DACH retrofit installations. The air-source heat pump handles base load; the fossil fuel boiler activates during cold periods or high-demand peaks. This configuration is explicitly addressed in the German GEG under the hybrid heating system provisions. The system qualifies for the 65 percent renewable energy share requirement when the heat pump delivers the majority of annual energy demand.

Applicable iDM products: iDM TERRA AL (air-to-water heat pump series) combined with an existing boiler via the iDM Navigator 2.0 controller.

Air-Source Heat Pump with Electric Resistance Heater

The secondary generator in this configuration is an electric immersion heater or electric boiler, typically integrated directly into the hydraulic system or a combined unit. This configuration avoids fossil fuel infrastructure but has higher peak operating costs when electricity prices are elevated. It is common in new builds and multi-family buildings without gas connections.

Ground-Source Heat Pump with Gas or Oil Boiler

Less common due to the greater investment in ground-source infrastructure, this configuration is used in large commercial or industrial buildings with high peak thermal demand that exceeds the capacity of a single ground-source unit. The ground source provides stable efficiency throughout the heating season; the boiler handles short-duration demand spikes.

Applicable iDM products: iDM TERRA SW (brine-to-water heat pump series) with secondary boiler integration via iDM Navigator 2.0.

Heat Pump with District Heating (Fernwärme)

In urban areas of Austria, Germany, and Switzerland with district heating networks, a heat pump can be integrated as a base load source while the district heating connection serves as the secondary high-temperature source. This configuration is growing in relevance as district heating networks expand and decarbonise. Hydraulic integration requires a heat exchanger to separate district heating pressure zones from the building’s internal circuit.

Heat Pump with Solid Biomass or Pellet Boiler

This combination is common in rural Austrian and German installations where wood pellet or log boilers are already installed. The heat pump provides efficient base load heating; the biomass boiler operates during cold periods or serves as domestic hot water support. Both generators are renewable, making this combination fully compliant with GEG renewable energy requirements and eligible for KlimaBonus funding in Austria.

Use Cases

Retrofit in Existing Residential Buildings

A single-family home in Graz, Austria, built in 1975, has a gas boiler and a radiator system designed for 75/65°C flow temperatures. A full monovalent heat pump replacement would require either radiator upgrades or a heat pump capable of delivering high flow temperatures—both adding cost. A bivalent installation allows the iDM TERRA AL to operate at reduced flow temperatures for the majority of the heating season, while the existing gas boiler activates during the coldest weeks. The annual share of heat pump energy typically reaches 70–80 percent in this scenario, satisfying KlimaBonus requirements.

New Build in a Climate Zone with Extreme Winter Temperatures

A new residential development in the Vorarlberg Alps at 1,200 metres altitude faces design outdoor temperatures of -16°C. Sizing a monovalent air-source heat pump for this extreme would result in a large, expensive unit with poor part-load efficiency during mild periods. A bivalent configuration—air-source heat pump for base load and an electric heater for the coldest days—provides reliable coverage without capital cost penalties. Ground-source systems in this context may achieve monovalent operation due to stable ground temperatures.

Multi-Family Residential Building with District Heating Connection

A six-unit apartment block in Munich has an existing district heating connection. A bivalent configuration integrating a heat pump for base load reduces district heating consumption, lowers energy costs, and meets the GEG’s renewable energy requirements for the building. The district heating connection provides backup capacity and covers domestic hot water during summer without requiring the heat pump to operate at reduced efficiency.

Commercial or Light Industrial Building

A manufacturing facility in Zurich, Switzerland requires both space heating and process heat at moderate temperatures. A ground-source heat pump provides base load space heating efficiently. The secondary generator—a gas boiler—serves as peak load support and provides process heat at temperatures above the heat pump’s operating range. This configuration is consistent with SIA 384/1 (Swiss standard for heating systems in buildings) and qualifies under Swiss cantonal energy regulations for building permit approval.

Staged Decarbonisation in Public Buildings

Municipal buildings across Germany and Austria face mandatory decarbonisation timelines under the GEG and Austrian Erneuerbares-Wärme-Gesetz. Bivalent installations allow public building operators to introduce heat pumps while existing boiler infrastructure remains in service, managing capital expenditure across budget cycles. Each year, as heat pump share of annual energy increases through optimisation, the fossil fuel contribution diminishes on a documented trajectory.

Benefits

Energy Efficiency Benefits

A correctly designed bivalent system achieves a higher annual SCOP than a monovalent heat pump sized for peak demand. The heat pump operates in its most efficient outdoor temperature range—typically above 0°C—for the majority of annual operating hours. Peak demand, which constitutes only 5–15 percent of annual hours in most DACH climates, is handled by the secondary generator. This results in lower annual primary energy consumption compared to either a standalone fossil boiler or an oversized heat pump.

Economic Benefits

The capital cost of a bivalent system is typically lower than a monovalent heat pump sized for full peak coverage. The heat pump unit itself is smaller and less expensive. Where an existing boiler is retained as the secondary generator, the investment is further reduced. Operating costs are optimised through heat pump prioritisation during periods of high COP and secondary generator operation during periods when heat pump efficiency would be marginal.

Regulatory Compliance Benefits

Bivalent systems provide a documented pathway to compliance with the GEG’s 65 percent renewable energy requirement for heating systems. Austrian OIB Richtlinie 6 and the Erneuerbares-Wärme-Gesetz mandate renewable heating shares in new and substantially renovated buildings; bivalent configurations enable compliance in building stock that cannot yet support full monovalent heat pump operation. Swiss MuKEn 2014 cantonal implementation similarly recognises bivalent systems as a compliant decarbonisation path.

Operational Resilience Benefits

Bivalent systems provide inherent redundancy. If the heat pump enters a fault state or requires scheduled maintenance, the secondary generator maintains building heating. This is particularly relevant for healthcare facilities, elderly care homes, and buildings in high-altitude locations where heating failure during winter carries serious consequences.

Environmental Benefits

By maximising heat pump operating hours and minimising fossil fuel boiler use, bivalent systems reduce CO₂ emissions relative to fossil-only heating. In combination with increasing grid decarbonisation in Austria, Germany, and Switzerland, the heat pump’s environmental performance improves year-over-year as the share of renewable electricity in the national grid increases.

Selection Criteria

Selecting the appropriate bivalent system configuration requires systematic evaluation of the building, climate, existing infrastructure, and regulatory context. The following criteria apply to DACH installations.

Building Heat Load and Heat Loss Curve

The building’s design heat load, calculated per EN 12831, is the primary sizing input. The heat loss curve—the relationship between outdoor temperature and heat demand—determines the bivalence point and the heat pump’s required capacity. Buildings with poor insulation have steep heat loss curves, requiring higher secondary generator capacity. Well-insulated new builds may achieve low bivalence points, approaching monovalent operation.

Existing Hydronic System and Flow Temperatures

Existing radiator systems designed for high flow temperatures (>65°C) limit heat pump efficiency. The installer must evaluate whether low-temperature operation is achievable—through partial radiator upgrades, oversizing of existing radiators, or operating the heat pump at elevated condensing temperatures with reduced COP. If high flow temperatures are unavoidable, the bivalent-alternative mode with the boiler handling peak load at high temperatures is often the pragmatic solution.

Climate Zone and Design Outdoor Temperature

The design outdoor temperature for the installation location (Norm-Außentemperatur) determines the frequency and duration of conditions below the bivalence point. Locations with mild winters—such as the Vienna Basin, the Rhine Valley in Vorarlberg, or the Bavarian lowlands—have fewer extreme cold hours; the secondary generator’s annual contribution will be small. Alpine or continental locations with colder winters require the secondary generator to contribute a larger share of annual energy.

Secondary Generator Type and Compatibility

The choice of secondary generator must be evaluated for compatibility with the heat pump’s hydraulic and control architecture. Gas and oil boilers require flue gas infrastructure. Electric heaters require sufficient electrical service capacity. Biomass boilers require fuel storage and handling infrastructure. District heating connections require heat exchanger and pressure management. Each secondary generator type has specific installation requirements under the relevant DACH regulatory frameworks.

Subsidy Programme Requirements

Austrian KlimaBonus, German BAFA and Investitionszuschuss, and Swiss cantonal programmes each have specific requirements for bivalent system configurations, including minimum heat pump efficiency ratings (minimum SCOP or COP values), heat pump energy share documentation, and approved product categories. The installer must confirm that the selected heat pump model and system configuration meet the requirements of the applicable funding programme before installation proceeds.

Heat Pump Model Capabilities

Not all heat pump models support bivalent integration at the control level. The installer must confirm that the selected heat pump’s controller supports external generator activation via a standardised interface, can log heat pump energy output separately from the secondary generator, and can manage bivalence point control as a configurable parameter. iDM heat pumps with the Navigator 2.0 controller support these functions natively, including SG Ready interface for grid-integrated operation.

Comparisons

Bivalent System vs. Monovalent System

A monovalent system relies entirely on the heat pump for all heating demand. It requires the heat pump to be sized for the design peak load and to operate at low outdoor temperatures where COP may be reduced. Monovalent operation is appropriate for well-insulated new builds with low flow temperature heating systems, ground-source or groundwater heat pumps with stable source temperatures, and locations with relatively mild design outdoor temperatures.

Bivalent systems are preferred when peak sizing of the heat pump would be uneconomical, when existing hydronic infrastructure constrains heat pump operation, or when regulatory pathways require staged integration of renewables. The monovalent system eliminates fossil fuel dependency entirely; the bivalent system trades complete decarbonisation for optimised capital cost and operational flexibility.

Bivalent-Alternative vs. Bivalent-Parallel

In bivalent-alternative mode, the heat pump shuts down when the secondary generator activates. This simplifies hydraulic and control design but wastes available heat pump capacity at low temperatures. In bivalent-parallel mode, the heat pump continues to contribute its maximum output while the secondary generator covers the deficit. Parallel mode extracts more annual energy from the heat pump—improving economics and subsidy compliance—at the cost of greater hydraulic and control complexity.

Parallel mode is the preferred configuration where the heat pump can operate efficiently at low temperatures, the control system supports coordinated source management, and the hydraulic design accommodates simultaneous flow from both sources. Alternative mode is appropriate for high-temperature retrofits where the heat pump cannot efficiently serve the distribution circuit at low outdoor temperatures.

Bivalent System vs. Multivalent System

A multivalent system integrates three or more thermal sources—for example, a heat pump, a solar thermal system, and a gas boiler. While multivalent configurations can achieve higher annual renewable energy shares and greater operational flexibility, they require more complex hydraulic design, advanced control logic, and higher installation cost. Bivalent systems represent the practical optimum for most residential and light commercial applications in DACH markets.

Integration with Other Systems

Integration with iDM Heat Pump Systems

iDM Energiesysteme GmbH designs its heat pump range for bivalent integration as a first-class use case. The iDM Navigator 2.0 controller supports bivalent system configuration natively, including adjustable bivalence point, operating mode selection (alternative, parallel, partially parallel), secondary generator activation via relay or bus interface, and energy monitoring for heat pump and secondary generator separately.

The iDM TERRA AL (air-to-water) and iDM TERRA SW (brine-to-water) product lines are both engineered for bivalent installation in DACH residential and light commercial contexts. The iDM iPump A combines the heat pump and hydraulic components in a compact unit designed for simplified bivalent retrofit integration.

Integration with Domestic Hot Water Systems

In bivalent heating systems, domestic hot water preparation (Trinkwarmwasser) must be incorporated into the control architecture. The heat pump can serve as the primary domestic hot water source via a dedicated hot water storage tank or a combined buffer and hot water tank. The secondary generator may support domestic hot water during periods when the heat pump is operating in alternative mode. The control unit must manage priority switching between space heating and domestic hot water to prevent simultaneous full-load demand from both circuits.

Austrian ÖNORM B 5019 governs the design and installation of drinking water heating systems and establishes the thermal requirements for Legionella prevention, which influences storage temperature and heat-up strategies in bivalent configurations.

Integration with Smart Grid and Dynamic Tariffs

Bivalent systems are well-positioned for smart grid integration. The SG Ready interface—defined in the Bundesverband Wärmepumpe (BWP) SG Ready specification and recognised in Austrian and Swiss grid frameworks—allows the control unit to shift heat pump operation toward periods of surplus renewable electricity on the grid. During high-grid periods (SG Ready signal level 3 or 4), the heat pump operates at increased set-point temperatures, pre-heating the building or buffer storage. During low-grid or high-cost periods (signal level 1), the secondary generator can be called upon if needed.

Dynamic electricity tariffs from Austrian and German energy suppliers increasingly reward heat pump load flexibility. iDM Navigator 2.0 supports SG Ready signal integration, enabling time-of-use optimisation within the bivalent control framework.

Integration with Building Automation and Energy Management Systems

In commercial buildings, schools, and multi-unit residential developments, bivalent heat pump systems integrate with building automation systems (BAS) via BACnet, Modbus TCP, or KNX interfaces. The BAS manages set-point scheduling, occupancy-based load reduction, and fault monitoring across all building systems. The heat pump controller communicates operating status, current output, and alarm conditions to the central BAS, enabling coordinated energy management and remote facility oversight.

Swiss SIA 386 and German DIN EN 15232 classify building automation efficiency levels; bivalent heat pump systems with full BAS integration qualify for the highest efficiency classifications, which may reduce calculated primary energy demand in building energy certificates (Energieausweis).

Integration with Photovoltaic Systems

Bivalent heat pump systems can be coordinated with on-site photovoltaic (PV) generation. When PV output exceeds immediate building electrical demand, the heat pump is prioritised for operation to utilise surplus renewable electricity directly. This PV surplus integration reduces net grid electricity consumption and lowers operating costs. The control logic requires a PV surplus signal input to the heat pump controller, which shifts heat pump operation to align with solar production periods.

In Austrian and German net metering frameworks, self-consumption of PV electricity by the heat pump avoids feed-in at lower rates and displaces grid electricity at higher retail rates. This economic incentive strengthens the investment case for bivalent heat pump systems in buildings with PV installations.

Bivalent systems in heat pump installation connect a heat pump with a secondary heat generator, such as a boiler, electric heater, district heating connection, or biomass system, to deliver efficient and reliable heating across changing outdoor temperatures. Instead of sizing the heat pump for rare peak winter demand, the system uses a defined bivalence point, coordinated control logic, buffer storage, and hydraulic integration so the heat pump covers the efficient base load while the secondary source supports extreme-cold or high-demand periods. This makes bivalent heating especially useful for DACH retrofits, existing radiator systems, subsidy compliance, staged decarbonisation, and buildings that need both lower energy costs and dependable heat supply.