Retrofit Existing Heating

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 Retrofit Existing Heating?

Retrofit existing heating is the process of replacing a fossil-fuel-based heating system — typically a gas boiler, oil boiler, or electric resistance heater — with a heat pump, while retaining as much of the existing building infrastructure as possible. The existing pipework, radiators, underfloor heating circuits, hot water cylinders, and control wiring are assessed, adapted, or replaced where necessary to make them compatible with heat pump operation.

A retrofit installation does not build a new heating system from scratch. It integrates the heat pump into an existing thermal envelope, hydraulic circuit, and control architecture. This distinguishes retrofit from new-build heat pump installation, where the system is designed from the ground up.

Retrofit existing heating is the dominant installation scenario in DACH markets (Austria, Germany, Switzerland, and German-speaking parts of Italy). The residential building stock in these regions is predominantly existing construction. Most homes were built before modern energy efficiency standards and were originally designed for high-temperature heating systems operating at 70°C–90°C flow temperatures.

Purpose of Retrofitting Existing Heating with a Heat Pump

The core purpose of retrofit is decarbonisation of space heating and domestic hot water. Heat pumps use electricity to move heat rather than burn fuel. This reduces direct CO₂ emissions from heating to zero at the point of use and reduces primary energy consumption when electricity comes from renewable sources.

Retrofit serves three operational purposes simultaneously. First, it replaces fossil fuel combustion with a low-carbon energy conversion process. Second, it preserves the capital investment already embedded in building infrastructure — pipework, emitter systems, and thermal mass. Third, it positions buildings within emerging regulatory frameworks that mandate fossil fuel phase-out across the EU and DACH region.

In practical terms, retrofit means a homeowner or building operator does not need to reopen walls, replace every radiator, or rebuild the hydraulic system if proper assessment and adaptation work is carried out before or during installation.

Why Retrofit Existing Heating Is Needed

Regulatory Drivers

The legal and policy environment in DACH markets has created a hard deadline for fossil fuel phase-out in buildings. Installers and building owners must understand these frameworks to plan retrofit timelines.

Germany (Deutschland)

• The Gebäudeenergiegesetz (GEG), amended in 2024, requires new heating systems installed from 2024 onward in new buildings to use 65% renewable energy. For existing buildings, the obligation applies when a heating system reaches end of life and is replaced.
• The Bundesimmissionsschutzgesetz (BImSchG) regulates emissions from heating installations, including noise and exhaust requirements that affect heat pump placement.
• KfW funding programs (BEG – Bundesförderung für effiziente Gebäude) provide investment grants for heat pump retrofits, with bonus rates for replacing oil heating systems.

Austria (Österreich)

• The Austrian Erneuerbare-Wärme-Gesetz (EWG) targets fossil fuel phase-out in heating. Oil and gas heating systems may no longer be installed in new buildings. Replacement requirements in existing buildings are phased in by federal state (Bundesland) energy laws.
• The KlimaBonus program offers direct subsidies for heat pump installation as part of Austria’s climate policy.
• OIB Richtlinie 6 (Energieeinsparung und Wärmeschutz) provides the energy efficiency framework governing building and installation standards.

Switzerland (Schweiz)

• The Mustervorschriften der Kantone im Energiebereich (MuKEn 2014) introduced the obligation that when a fossil fuel heating system is replaced, the new system must cover at least 10% of its energy demand from renewable sources. Many cantons have adopted stricter requirements.
• The Lärmschutzverordnung (LSV) regulates noise emissions relevant to air source heat pump outdoor units.
• Cantonal energy legislation (Kantonale Energiegesetze) determines specific retrofit obligations at regional level.

German-speaking Italy (Südtirol / Alto Adige)

• Provincial climate and energy plans align with Italian national targets under the Piano Nazionale Integrato per l’Energia e il Clima (PNIEC), which targets significant renewable heat penetration by 2030.
• KlimaHaus standards (equivalent to Austrian/German low-energy building certification) set energy performance benchmarks that influence retrofit design requirements.

Technical Drivers

Beyond regulation, the technical case for retrofit is rooted in system efficiency and long-term operating cost. Heat pumps deliver a Coefficient of Performance (COP) between 2.5 and 5.0 in typical operating conditions. For every 1 kWh of electricity consumed, the system delivers 2.5 to 5.0 kWh of thermal energy. A gas boiler operates at a maximum efficiency of approximately 0.98 (98%), burning one unit of fuel to produce less than one unit of heat. The thermodynamic advantage of a heat pump is structural, not marginal.

The growing availability of renewable electricity — through grid decarbonisation, solar PV, and smart tariffs — amplifies this advantage over time. A gas boiler locked into gas infrastructure cannot benefit from falling renewable electricity costs. A heat pump can.

Key Features of Retrofit Existing Heating

Retrofit existing heating is defined by six core features. Each feature determines the technical scope, cost, and performance outcome of the installation.

Hydraulic System Assessment

Definition: Hydraulic system assessment is the systematic evaluation of the existing pipework, emitter circuit, and heat distribution network to determine compatibility with heat pump operation.

Purpose: Heat pumps operate most efficiently at low flow temperatures — typically 35°C to 55°C, compared to 70°C to 90°C for a gas boiler. The existing distribution system must be capable of delivering adequate heat output at these reduced temperatures.

Benefits: A thorough hydraulic assessment prevents undersized heat distribution, avoids unnecessary emitter replacement, and ensures the heat pump operates within its optimal efficiency range. It reduces installation cost and risk by identifying specific upgrade requirements rather than assuming a full system replacement is necessary.

Practical Application: The installer calculates the heat loss of each room according to DIN EN 12831 (or ÖNORM EN 12831 in Austria). The heat output of each existing radiator is then evaluated at the target flow temperature. Where existing radiators have sufficient oversizing relative to the original design, they can deliver adequate heat at 45°C or 50°C without replacement. Where they cannot, targeted replacement or supplementary emitter installation is required.

Low-Temperature Emitter Compatibility

Definition: Low-temperature emitter compatibility describes the capacity of radiators, underfloor heating circuits, or fan coil units to deliver the required room heat load at the lower flow temperatures characteristic of heat pump systems.

Purpose: Emitters designed for high-temperature systems are physically larger than required. This apparent disadvantage becomes an advantage in retrofit: larger radiators deliver more heat at lower temperatures than their nameplate rating suggests when derated to heat pump operating conditions.

Benefits: Many existing radiator installations in DACH building stock — particularly panel radiators installed during the 1990s through 2010s — have significant oversizing relative to the actual heat load. This oversizing directly enables low-temperature heat pump retrofit without full emitter replacement.

Practical Application: The Voith-Kübler method and EN 442-2 radiator derating curves are applied to calculate actual radiator output at reduced flow temperatures. A radiator rated at 1,000 W at 75/65°C flow/return and 20°C room temperature delivers approximately 480–520 W at 50/40°C. If the room heat loss at design conditions is 450 W or below, no replacement is necessary.

Flow Temperature Optimisation

Definition: Flow temperature optimisation is the process of setting and controlling the heat pump’s heating curve (Heizkurve) to deliver the minimum flow temperature necessary to meet the building’s heat demand at any given outdoor temperature.

Purpose: The lower the flow temperature, the higher the heat pump’s COP. Every 1°C reduction in flow temperature improves heating COP by approximately 2–3%. Flow temperature optimisation is therefore the primary lever for maximising seasonal system efficiency (SCOP/SPF).

Benefits: Optimised flow temperature management reduces annual electricity consumption, lowers operating costs, and extends compressor service life by reducing cycling frequency and pressure ratios. It also reduces thermal stress on pipework and fittings.

Practical Application: iDM heat pumps with Navigator 2.0 control systems implement weather-compensated heating curves with automatic adaptation. The control system measures outdoor temperature continuously and adjusts the flow temperature set point along the configured curve. During commissioning, the installer sets the design flow temperature at design outdoor temperature (typically −12°C to −15°C in DACH climates) and the reduced flow temperature at milder conditions. The Navigator 2.0 system learns building thermal behaviour over time and refines curve parameters automatically.

Domestic Hot Water Integration

Definition: Domestic hot water (DHW) integration is the configuration of the heat pump to produce hot water for sanitary use, either through a dedicated DHW heat exchanger, a combined buffer and DHW cylinder, or a separate indirect-fired storage cylinder.

Purpose: In retrofit installations, the existing DHW cylinder must be assessed for compatibility with heat pump output temperatures. Many older cylinders were sized for gas or oil boilers producing water at 80°C or above. Heat pumps operating with a monovalent configuration may require a larger storage volume or supplementary electric immersion element to meet peak DHW demand.

Benefits: Integrating DHW production into the heat pump system eliminates the need for a separate gas or electric DHW appliance, reduces installation complexity, and enables solar PV integration for low-cost DHW heating during daylight hours.

Practical Application: ÖNORM EN 15450 (Austria) and DIN EN 15450 (Germany) provide design guidelines for DHW systems integrated with heat pumps. iDM TERRA and AERO series systems support DHW temperatures up to 65°C (with electric boost to 70°C for legionella protection cycles) through their integrated control systems. The Navigator 2.0 controller can schedule legionella protection cycles automatically in compliance with hygiene regulations.

Electrical Infrastructure Upgrade

Definition: Electrical infrastructure upgrade involves assessing and adapting the building’s electrical supply — meter, consumer unit, cabling, and earthing — to support heat pump operation.

Purpose: Heat pumps require a reliable three-phase or single-phase electrical connection depending on rated capacity. Outdoor unit cables must meet voltage drop requirements. Smart grid-capable systems require data infrastructure. Time-of-use tariff integration requires compatible metering.

Benefits: A properly dimensioned electrical installation ensures heat pump reliability, enables smart grid-ready operation, supports grid-connected PV systems, and complies with national wiring regulations (ÖVE/ÖNORM E 8001 in Austria, DIN VDE 0100 in Germany, NIN in Switzerland).

Practical Application: For most residential heat pumps in the 5–15 kW range, a 16A or 25A three-phase supply is sufficient. Heat pump-specific tariffs (Wärmepumpentarif) available from Austrian and German grid operators reduce operating costs significantly and may require separate metering. The installer coordinates with the network operator (Netzbetreiber) for connection approval and metering configuration.

Control System Integration

Definition: Control system integration is the configuration of the heat pump’s control unit to manage the heat distribution system, DHW production, weather compensation, smart grid signals, and any residual auxiliary heating components.

Purpose: A retrofit installation frequently inherits existing zone controls, thermostatic radiator valves (TRVs), and room thermostats. These must be coordinated with the heat pump controller to avoid conflicting demands and short-cycling.

Benefits: Properly integrated controls maximise system SCOP, enable remote monitoring and diagnostics, support demand response participation, and provide the homeowner with a single interface for the entire heating system.

Practical Application: iDM’s Navigator 2.0 control system integrates weather compensation, zone management, DHW scheduling, and Smart Grid-Ready (SG Ready) signal handling in a single platform. The SG Ready interface allows the system to respond to grid operator signals — increasing or reducing heat pump load in response to grid conditions or renewable electricity price signals, pre-heating the building when electricity is cheap and green.

Types of Retrofit Existing Heating Installations

Retrofit installations are categorised by the source of environmental heat the heat pump extracts. Each type carries specific civil engineering requirements, efficiency profiles, and suitability criteria.

Air Source Heat Pump Retrofit (Luftwärmepumpe)

An air source heat pump (ASHP) extracts heat from ambient outdoor air. The outdoor unit contains a fan, heat exchanger coil, and in most configurations the compressor. The indoor unit contains the hydraulic connections, expansion vessel, controls, and buffer storage.

Suitability: ASHP retrofit is the most accessible type for existing buildings because it requires no groundwork. Outdoor unit placement follows regulations on noise (TA Lärm in Germany, ÖNORM EN ISO 9614-2 and local Bauordnung in Austria, LSV in Switzerland) and setback distances from property boundaries.

Relevant iDM Products: iDM AERO AL series (air-to-water, modulating inverter compressor) and iDM TERRA AL (air source with high DHW capability) are designed for retrofit conditions, operating reliably down to −20°C ambient temperature.

Efficiency Profile: Seasonal COP (SCOP) in DACH climates typically 2.8–4.0 depending on flow temperature and climate zone.

Ground Source Heat Pump Retrofit (Erdwärmepumpe)

A ground source heat pump (GSHP) extracts geothermal energy from the ground through a brine loop — either horizontal ground collectors (Flächenkollektor) or vertical borehole heat exchangers (Erdwärmesonde).

Suitability: GSHP retrofit is appropriate where garden space is available for horizontal collectors or where borehole drilling is permitted. Borehole installation requires regulatory approval under Wasserrecht (water law) in Austria and Germany, and Gewässerschutzgesetz in Switzerland. Drilling depth and method are subject to cantonal and federal geological constraints.

Relevant iDM Products: iDM TERRA SW (brine-to-water heat pump) is designed for ground source applications. The TERRA SW series operates at higher COPs than air source systems due to the more stable ground temperature compared to ambient air.

Efficiency Profile: SCOP in DACH climates typically 3.5–5.0 depending on brine inlet temperature and flow temperature.

Hybrid Heat Pump Retrofit (Hybride Wärmepumpe)

A hybrid heat pump system combines a heat pump with an existing gas or oil boiler. The heat pump covers the base load; the boiler provides peak demand coverage when outdoor temperatures fall below the heat pump’s bivalence point (Bivalenzpunkt).

Suitability: Hybrid retrofit is used when the existing heating distribution system cannot be adapted for low-temperature operation and full heat pump coverage would require extensive emitter replacement. It is also applicable where building envelope thermal performance is insufficient to support a monovalent heat pump in peak winter conditions without oversized equipment.

Regulatory Note: Under Germany’s GEG 2024, hybrid systems using a gas boiler are subject to the 65% renewable energy requirement. The heat pump component must cover sufficient energy share to meet this threshold. In Austria, EWG provisions set comparable minimum renewable share requirements.

Efficiency Profile: Overall system efficiency depends on the split between heat pump and boiler operating hours. A well-designed hybrid system achieves 60–80% of heat pump operating hours annually.

Exhaust Air Heat Pump Retrofit (Abluft-Wärmepumpe)

An exhaust air heat pump extracts heat from the warm exhaust air stream of a mechanical ventilation system. The heat pump produces both space heating and DHW from ventilation exhaust heat.

Suitability: Exhaust air heat pumps are suitable for well-insulated retrofit buildings with low specific heat demand (typically below 40–60 kWh/m²a) and mechanical ventilation systems. They are common in Passive House and near-Passive House retrofit scenarios.

Limitation: The available heat source is limited by the exhaust air volume flow rate. Buildings with high heat demand relative to floor area are not suited to exhaust air heat pump systems without supplementary heat sources.

Use Cases for Retrofit Existing Heating

Residential Single-Family Homes (Einfamilienhäuser)

The dominant retrofit scenario in DACH markets. A gas or oil boiler installed in the 1990s or 2000s reaches end of life. The homeowner replaces the system under GEG, EWG, or cantonal energy law obligations. The existing wet distribution system — radiators or underfloor heating — is assessed for low-temperature compatibility. An air source heat pump is installed with minimal civil engineering. KfW BEG or KlimaBonus funding supports the investment.

Multi-Apartment Buildings (Mehrfamilienhäuser)

Central heating retrofit in apartment buildings involves higher hydraulic complexity. Zone balancing, staircase riser redesign, and individual metering (Heizkostenabrechnung, required under German Heizkostenverordnung and Austrian equivalent regulations) must be integrated with the heat pump system. Ground source systems with borehole arrays are common where garden or parking area is available.

Commercial and Light Industrial Buildings

Office buildings, retail premises, and light industrial units with existing gas heating undergo retrofit as part of corporate sustainability reporting requirements under CSRD (Corporate Sustainability Reporting Directive, EU 2022) and national green procurement frameworks. The scale of these installations typically involves larger GSHP systems or multiple air source units in cascade configuration.

Buildings in Historic Protection Zones (Denkmalschutz)

Historic buildings in protected areas face additional constraints. External alterations to the facade or garden may require approval from the Denkmalschutzbehörde (monument protection authority). Outdoor unit placement and noise must meet both standard residential zoning requirements and heritage protection conditions. iDM AERO units with low-noise operating modes and compact outdoor units address this use case.

Benefits of Retrofit Existing Heating with a Heat Pump

Energy Efficiency and Cost Reduction

Heat pumps reduce heating energy costs by 40–70% compared to gas or oil heating at current energy price differentials in DACH markets. The seasonal performance factor (SPF or JAZ — Jahresarbeitszahl) measures actual annual system efficiency. A well-commissioned retrofit with iDM equipment typically achieves a JAZ of 3.0–4.5, meaning 3–4.5 units of heat delivered per unit of electricity consumed.

Carbon Emission Reduction

Direct CO₂ emissions from heating fall to zero at the point of use upon removal of the gas or oil boiler. Lifecycle emissions depend on the electricity grid carbon intensity. Austria’s electricity mix — with high hydropower share — provides particularly low lifecycle emissions for heat pump operation. Germany’s grid decarbonisation trajectory under Energiewende further improves the carbon case over system lifetime.

Future-Proof Regulatory Compliance

Buildings retrofitted with heat pumps comply with current and foreseeable future energy and climate legislation across DACH markets. This protects property values and avoids the risk of future mandatory retrofit obligations with associated compliance costs.

Energy Independence and Price Stability

Heat pumps reduce dependence on gas and oil imports, which are subject to geopolitical price volatility — as demonstrated by European energy markets in 2021–2023. Electricity prices are more stable and increasingly linked to domestic renewable generation rather than international commodity markets.

Compatibility with Solar PV

Retrofitted heat pump systems integrate directly with photovoltaic systems. The heat pump can absorb surplus PV generation through Smart Grid Ready control interfaces, producing heat or pre-heating DHW storage during periods of excess solar generation. This reduces grid export, maximises self-consumption, and reduces operating costs.

Increased Property Value

In DACH real estate markets, buildings with heat pump heating and high energy performance certificates (Energieausweis) command measurable price premiums. German and Austrian energy performance legislation requires disclosure of heating system type in property listings, making the heat source directly visible to buyers and tenants.

Selection Criteria for Retrofit Heat Pump Systems

Selecting the appropriate heat pump type and specification for a retrofit installation depends on five primary criteria.

Building Heat Loss

The design heat load (Normheizlast) calculated in accordance with DIN EN 12831 or ÖNORM EN 12831 determines the required heat pump capacity at design outdoor temperature. Undersizing results in insufficient heating coverage; oversizing leads to short-cycling, reduced efficiency, and higher capital cost. Accurate heat loss calculation is mandatory before equipment selection.

Key inputs include building geometry, wall and roof U-values, window specifications, infiltration rate, ventilation type, and the design outdoor temperature for the climatic location. The design outdoor temperature varies significantly within DACH geography: −12°C in Munich, −16°C in Vienna, −20°C or below in Alpine locations such as Innsbruck or Davos.

Existing Distribution System Flow Temperature

The maximum flow temperature the heat pump must achieve to deliver the building’s heat demand determines the heat pump’s operating range and directly affects efficiency. Systems retaining high-temperature emitters may require flow temperatures of 60°C or above, which demands a heat pump capable of high-temperature operation or necessitates emitter upgrades to reduce the required flow temperature.

iDM AERO and TERRA series heat pumps cover a flow temperature range of up to 65°C in standard operation, with high-temperature variants available for demanding retrofit conditions.

Available Heat Source

Ground source systems provide higher efficiency but require land area or drilling permits. Air source systems are universally applicable but subject to noise regulations governing outdoor unit placement. The selection between air and ground source depends on plot size, soil geology, regulatory approval timeline, and installation cost.

Electricity Supply Infrastructure

Three-phase supply availability, connection capacity, and tariff options affect both equipment selection and operating economics. Single-phase heat pumps up to approximately 7–8 kW are available for buildings without three-phase supply. Buildings with existing three-phase supply from industrial or agricultural use can accommodate larger heat pump capacities.

Funding Eligibility

In DACH markets, grant funding significantly affects the economic case for retrofit. Equipment specification must meet the efficiency thresholds required for funding eligibility. In Germany, BEG (Bundesförderung für effiziente Gebäude) requires a seasonal COP (Jahresarbeitszahl) of at least 1.7 as a minimum threshold, with higher efficiency attracting higher grant rates. Austrian KlimaBonus funding has comparable eligibility criteria. Swiss cantonal energy programs apply similar energy efficiency requirements.

Comparison: Retrofit vs New-Build Heat Pump Installation

Comparison: Air Source vs Ground Source Retrofit

Integration with Other Systems

Solar Photovoltaic (PV) Integration

Retrofit heat pump systems integrate with solar PV through the Smart Grid Ready (SG Ready) interface, now standard on iDM Navigator 2.0-equipped systems. The SG Ready protocol defines four operating states:

• State 1: Heat pump locked out (grid operator control).
• State 2: Normal operation.
• State 3: Increased operation recommended (surplus PV available or low-tariff period). Heat pump pre-heats buffer storage and DHW cylinder.
• State 4: Maximum operation (direct PV surplus signal). Heat pump runs at full capacity to absorb available PV generation.

This integration enables heat pump operation to shift toward periods of solar generation, reducing grid import, lowering electricity costs under dynamic tariffs, and increasing PV self-consumption.

Mechanical Ventilation and Heat Recovery (MVHR)

Retrofit buildings with mechanical ventilation systems (Lüftungsanlage mit Wärmerückgewinnung, WRG) benefit from reduced heating loads, as MVHR systems recover 75–90% of heat from exhaust air. The combined effect of MVHR and heat pump results in very low net heating energy demand and high overall system energy performance. This combination is particularly relevant for near-Passive House retrofit scenarios subject to KfW Effizienzhaus or Austrian Klima:aktiv certification pathways.

Building Automation and Energy Management Systems (EMS/BMS)

Larger residential and commercial retrofit installations integrate heat pumps with building automation systems via Modbus, BACnet, or KNX protocols. iDM Navigator 2.0 supports Modbus RTU and TCP/IP communication, enabling integration with third-party EMS platforms, smart home systems, and utility demand response programs (Netzsteuerung). This positions the building as a smart energy asset capable of participating in demand flexibility programs, which are expanding under EU energy market regulation.

Thermal Buffer Storage

Buffer storage (Pufferspeicher) decouples heat pump production from heat distribution demand. This allows the compressor to run in longer, more efficient cycles rather than short-cycling with each room thermostat call. Buffer storage is particularly important in retrofit installations where zone valves or TRVs create variable hydraulic demand. iDM recommends buffer storage sizing in accordance with VDI 4645 (Planung und Dimensionierung von Wärmepumpenanlagen) to ensure adequate thermal capacity relative to heat pump output.

Retrofitting existing heating with a heat pump is a practical path from gas, oil, or electric resistance heating to efficient, low-carbon space heating and domestic hot water. By assessing the building heat loss, hydraulic system, radiators, flow temperature, electrical supply, DHW storage, and control integration, installers can adapt much of the existing infrastructure instead of rebuilding the whole heating system. For homes and buildings in the DACH region, a well-planned heat pump retrofit improves energy efficiency, supports fossil-fuel phase-out, enables solar PV and smart-grid use, and helps future-proof the property under evolving heating regulations.