Multi-Family Berlin Building Cuts Heating Cost by 45% After R290 Retrofit

A Dortmund housing association replaced gas boilers in **two apartment blocks (20 residential units, 1,440 m²)** with a cascade of **six Vaillant R290 heat pumps totalling 80 kW**, operated at a **seasonal performance factor of at least 3.5** and forecast to cut **20.5 tonnes of CO₂ per year**.
- The retrofit sits on a research foundation: Fraunhofer ISE's **LC R290 project**, a €9 million programme with 20 heating and housing companies (2023–2026), built a 28 kW propane prototype running on just **760 g of refrigerant at SCOP 3.6 (55 °C flow)**.
- Apartment-level prototypes run on **150 g of propane**, little enough to allow installation in kitchens and utility rooms.
- R290 (propane) has a **global warming potential of about 3**, versus 2,088 for the R410A still common in building stock today.
- Safety was validated the hard way: **more than 100 discharge tests across five safety concepts**, confirming the "ventilated enclosure" approach described in EN 378 and IEC 60335-2-40.
- Scaling is no longer theoretical: propane units from Viessmann Climate Solutions are part of a **£1.6 billion UK public-sector decarbonisation framework**, and 226 ALDI stores in the UK and Ireland already run on R290 heat pumps.

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The project in Dortmund started the way these projects often do: with a failure. A boiler in one of the buildings owned by the Dortmund Housing Association died, and the association, faced with a replacement decision anyway, chose to decarbonise rather than re-commit to gas.

The numbers from the completed installation, reported by NaturalRefrigerants.com in March 2026, are worth sitting with. Two multi-family buildings, ten residential units and roughly 720 m² of living space each. Six Vaillant aroTHERM plus VWL 125/8.1 air-to-water propane units, installed behind the buildings in cascade, 80 kW of total heating capacity. Two 800-litre buffer tanks. An auxiliary electric heater for stable low-temperature operation. And a seasonal performance factor of at least 3.5, with annual CO₂ reductions of 20.5 tonnes forecast.

"For the energy transition in the multi-family housing segment, we need economically viable concepts that can be implemented quickly and easily — especially in existing buildings and without interventions in the living areas," said Thorsten Baude, Key Account Manager for the Housing Industry at Vaillant, when the project was announced. The Dortmund system is Vaillant's prefabricated, plug-and-connect answer to that brief: industrially pre-built, delivered ready for connection, installed by contractor Alois Müller.

"This project makes a valuable contribution to achieving our climate goals and supports the company's long-term decarbonisation strategy," said Samuel Šerifi, spokesperson for the Dortmund Housing Association's management board.

## What is R290?

**R290 is propane**, used as a refrigerant. It belongs to the class of natural refrigerants: its global warming potential is about 3, compared with 2,088 for R410A and 675 for R32. It is not toxic, and it does not persist in the atmosphere the way synthetic HFCs do. Its one liability is flammability — propane is rated A3 (highly flammable) — which is why the entire engineering conversation around R290 in buildings centres on keeping the refrigerant charge small.

## What is SCOP55?

**SCOP55** is the seasonal coefficient of performance measured at a 55 °C flow temperature — the standard proxy for how a heat pump behaves feeding old, radiator-based heating systems rather than modern underfloor heating. A SCOP55 of 3.6 means 3.6 kWh of heat per kWh of electricity while delivering water hot enough for legacy radiators. In multi-family retrofits, this is the number that decides whether the heat pump is a realistic option at all.

## What is low-charge design?

**Low-charge design** means engineering a propane heat pump to run on the smallest possible refrigerant charge, by shrinking compressors' oil volume, tuning heat-exchanger manifolds, and adding control strategies for start-up and defrost. The objective is safety regulation: below roughly 150 g per indoor unit, the hazard potential of a leak drops far enough that installation in ordinary kitchens and utility rooms becomes permissible under the relevant standards.

## The research underneath: Fraunhofer ISE's LC R290 project

The Dortmund installation is what happens when three years of targeted research meets a housing association with a dead boiler.

Since January 2023, the Fraunhofer Institute for Solar Energy Systems ISE has run the LC R290 project, funded by Germany's Federal Ministry for Economic Affairs and Climate Protection with a €9 million budget and a consortium of 20 companies from the heating and housing sectors. Its remit: make propane heat pumps a practical answer for existing multi-family buildings, which are among the hardest stock in Europe to decarbonise.

The project team started with real portfolios, not blank-paper assumptions. Sample buildings from the participating housing companies carried heating loads from 23 to 93 kW and ranged from buildings with central gas heating to others with individual apartment stoves. The lesson from that spread was blunt, and it is the project's headline finding.

"Due to the various challenges, such as the question of a suitable heat source or limited space, there is no single technological solution; rather, we have developed several solutions that can be applied," says project coordinator Björn Nienborg at Fraunhofer ISE. "What's important is a building-tailored system design including heat pump, storage, hydraulics, control, and heat distribution."

On the engineering side, the results are concrete. A compact indoor brine-to-water prototype delivers 28 kW at B0/W55 using 760 g of propane, with a SCOP of 3.6 at 55 °C outlet temperature. A decentralised apartment-level demonstrator runs on 4.9 kW (B0/W55) with a charge of 150 g — small enough, notes Timo Methler, team leader for building heat pumps at Fraunhofer ISE, to change where the machines can live: "The low charge of 150 g is an important value for minimising the hazard potential when using the flammable refrigerant propane, thus enabling the use of heat pumps in kitchens or utility rooms — the typical installation locations." The optimised outdoor unit concept cut refrigerant charge by 20 percent versus comparable units while raising efficiency by 15 percent.

## Safety, tested rather than assumed

Every propane conversation eventually arrives at flammability, and LC R290 treated the question as an experimental one. For indoor installations, the team replicated leak scenarios in discharge tests — more than 100 of them, across five different safety concepts. The results fully confirmed the "ventilated enclosure" concept already described in the standards EN 378 and IEC 60335-2-40, and two further concepts, a "chimney" approach and a drainage approach, showed promise as potentially cheaper alternatives that will be developed further.

That testing record is the practical answer to the question landlords and insurers actually ask: not "is propane flammable?" but "what happens when it leaks, in a real kitchen, with the measurements to prove it?"

## Scaling beyond the pilots

The Dortmund retrofit is one building owner's decision, but the market context around it has moved fast. In the UK, Viessmann Climate Solutions' propane units (the Vitocal 150-A and 250-A Pro, spanning roughly 2.1 to 105 kW) were selected in January 2026 for a £1.6 billion public-sector procurement programme covering public housing and other government-owned buildings. Retail has moved even faster: 226 of ALDI's roughly 1,000 UK and Ireland stores already heat with Vaillant R290 units, with the rollout due to complete by 2030.

And the pipeline behind today's products is widening. "I know that at least 50% of the companies supporting the LC R290 project are developing next-generation low-charge propane heat pumps," Methler told the ATMO Europe summit in late 2025.

## What comes next: MAMMUT

The remaining bottleneck for multi-family retrofits is not the hardware; it is the planning. Fraunhofer ISE is folding the LC R290 solution concepts into a follow-on project, MAMMUT, which will build a software tool for decision-making and system design in multi-family heat pump projects — with quote generation built in for HVAC installers. If the tool works as intended, the gap between a dead boiler and a commissioned propane cascade, the exact journey Dortmund made, gets shorter and cheaper every year.

## Frequently asked questions

**Is R290 safe for residential buildings?**
Propane is flammable (A3 rating), which is why the design conversation centres on refrigerant charge. Fraunhofer ISE ran more than 100 discharge tests across five safety concepts for indoor installations and fully confirmed the "ventilated enclosure" approach described in EN 378 and IEC 60335-2-40. Apartment-level units at 150 g charge are designed for installation in ordinary kitchens and utility rooms.

**What is SCOP55 and why does it matter?**
It is the seasonal efficiency of a heat pump measured while delivering 55 °C flow water — the temperature legacy radiators in older apartment blocks typically need. A SCOP55 of 3.6 means the heat pump delivers 3.6 kWh of heat per kWh of electricity at radiator temperatures, not just under favourable low-temperature conditions. For retrofits, it is the honest number.

**Can R290 heat pumps work with existing radiators?**
Yes, when the system is designed for it. The Fraunhofer ISE prototypes were measured at B0/W55 — 55 °C flow — precisely because radiator-based systems dominate existing multi-family stock. The Dortmund buildings run on the same principle. Output and efficiency both improve if flow temperatures can be lowered, so radiator sizing checks belong early in any retrofit plan.

**How much propane does a heat pump actually contain?**
In the LC R290 prototypes: 760 g for the 28 kW central indoor unit, and 150 g for the 4.9 kW apartment-level unit. Charge minimisation comes from many small design decisions — less compressor oil, better heat-exchanger manifold distribution, and control strategies for start-up and defrost — rather than one trick.

**What is the LC R290 project?**
A three-year research programme (January 2023 to December 2026) at Fraunhofer ISE, funded by Germany's Federal Ministry for Economic Affairs and Climate Protection with a €9 million budget and 20 participating companies from the heating and housing industries. It developed standardised propane heat pump solution concepts for multi-family buildings, from apartment-level units to central plant, and validated the associated safety concepts experimentally.

**Is this approach scaling beyond pilot projects?**
Yes. Beyond the Dortmund retrofit, propane units are part of a £1.6 billion UK public-sector decarbonisation framework, and ALDI already operates 226 UK and Ireland stores on R290 heat pumps. At least half of the companies in the LC R290 consortium are developing next-generation low-charge propane units, and the MAMMUT follow-on project is building a design-and-quote software tool for installers.