Project: Gas hybrid with solar thermal energy

Performance coupled with environmental protection

New build apartment house

  • The apartment building with eight residential units needs a stable, efficient heat supply.

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  • The high number of residential units means that fluctuating hot water demand and changing peak loads are to be expected. Therefore, the two heat generators must be optimally matched to each other.

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Gas in a hybrid system

The construction of a new apartment block in a suburb of Karlsruhe is being planned. The sunny location could allow solar thermal technology to be employed, amongst other things. Read on to find out which heating system is chosen in the end.

A challenging project

The new building will be fairly large and contain eight residential units. Heating the building sufficiently will require a high level of output that must be reliably available at any time and on demand.

Challenges of the building project

The investment costs for the new apartment block are high so it is all the more important to plan the entire project with precision. The developer must also consider that the new tenants in the many apartments will require heat for the heating system and hot water at quite different times, and peak loads will therefore fluctuate. Therefore, it is vital that the heat generators are perfectly coordinated.

Project: Multi-family house gas hybrid heat pump

Reasons for choosing dual heat generators

The plot is already connected to the utilities and has a gas connection. For this reason, the developer decided to use this fuel to heat the building and supply it with hot water. Moreover, this type of heating system is excellent at compensating for the peak loads and varying hot water requirements mentioned above.

New buildings are legally required to cover at least part of their heating and hot water demands using renewable systems. The developer decides to use solar heat as a renewable heat generator. The roof is large enough and south-facing. The property is detached and not shaded by taller adjacent buildings. Therefore, the solar thermal system can be expected to produce an optimum yield.

01. Durable technology

Heat generators for apartment blocks and other large properties need more than just suitable output levels. They must also be able to handle long periods of continuous load and greatly varying output demands. 


Condensing boilers are robustly built and low-maintenance solutions. Modern solar collectors are even less demanding with regard to care and maintenance.


02. Environmentally friendly operation

Thanks to optimised fuel consumption, modern gas boilers emit far less environmentally harmful CO2 than older appliances. They easily comply with current strict emissions regulations. Combining them with a solar heating system makes the entire heating system even more eco-friendly.


03. Low operating costs

Naturally, the optimised fuel consumption of modern boilers has a beneficial effect on heating bills. Using the free heat provided by the solar heating system saves even more money. If solar heat alone is used to cover all DHW heating in summer, the boiler can be switched off altogether during this period to cut costs still further.

04. Flexible and space-saving installation

Modern heating boilers are available either as compact floor-mounted appliances or as wall-mounted units. If the building is connected to the gas mains, there is no need to install a gas tank. The solar collectors are banished entirely to the roof and do not take up any space at all either in the house or grounds.

The following products were selected

The components have to be well-coordinated in order for the hybrid heating system to work without fault. This is best achieved by sourcing all appliances from the same manufacturer. After consulting their heating systems experts, the developer decided to employ components made by the heating and air conditioning technology specialist WOLF.

WOLF Gas condensing boilers CGB-2 family

CGB-2 family of wall-mounted gas condensing boilers

The appliance selected was the 55-kW model from the CGB-2 series. It is both space-saving and compact.

The boiler is a modulating appliance that adapts automatically and perfectly to the changing output requirements.

WOLF flat-plate collectors F3-1 and CFK-1

F3-1 flat-plate collector

The collectors have proved themselves over many years and are very low-maintenance and efficient.

WOLF decentralised living space ventilation CWL-D-70

CWL-D-70 mechanical ventilation unit

The new building has excellent insulation and an air-tight building envelope so it made sense to integrate a ventilation system.

Every residential unit was equipped with decentralised appliances that create a healthy indoor environment controlled by the occupant and prevent mould formation. This also protects the fabric of the building, which reduces the need for repairs in the future and avoids the associated expense.

Powerful heating system

The developer has made a good decision. The powerful condensing boiler ensures that the heating system and hot water generation work perfectly. The solar heating system provides backup, improves the ecological footprint of the systems and, last but not least, makes the project eligible for subsidy.

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