Heating renovation in Hansaallee - a multi-million euro project for climate protection
Instead of replacing the natural gas boilers in the local heating network with new ones, the buildings at Hansaallee 128a to 146 (Frankfurt am Main) were converted to heat pump heating systems in the fall of 2024. To ensure that the heating heat pumps installed in the respective basements work efficiently, so-called eXergy machines generate the higher temperatures for hot water production. The new combined solution offers benefits for residents and the climate: heating costs are halved and local CO2 emissions are reduced to zero.
Uncertain natural gas prices, dependence on imports and climate protection - there are many reasons for turning away from natural gas. They were also decisive factors in the heating renovation of the buildings at Hansaallee 128a to 146 in Frankfurt am Main. Around 20 years after the last modernization, the owners' association had the local heating network shut down and switched the heating of the 129 residential units to heat pumps installed in the basements of the new buildings.
Energy costs for heating and hot water halved
This gives the residents reason to be happy, as replacing the natural gas heating system alone would not have resulted in any cost savings and switching to a low-temperature local heating network with a flow temperature of 40 °C and in-house reheaters for hot water would only have reduced heating costs by 15 %. By contrast, switching to the heat pump solution that has now been implemented halves the heating costs, reports Roberto Resch: „Instead of a total of over 200,000 euros for natural gas, only around 100,000 euros are now due for electricity each year,“ says the Managing Director of Valo Immobilienmanagement Rhein Main GmbH (Frankfurt am Main), which supported the project.
The savings are possible thanks to the well thought-out planning of the heating technology by the specialist energy efficiency planners IngKH (Eschborn). „The buildings from the 1990s offered good conditions,“ reports Manfred Reinl. „Relatively good thermal insulation and underfloor heating make low flow temperatures possible and the basement rooms with the local heating transfer points offered just enough space to install the heat pumps and heat storage units.“ As the central heating units are located in the houses, heat losses in the network are now history; over a tenth of the energy was lost in the heating network.
Central heating with heat pumps in every building
Reinl and his team have planned air-to-water heat pumps with an output of 15 kW each for the heating. Nine of these are installed in the largest of the buildings and three in each of the eight smaller apartment buildings. This ensures safety during maintenance work and repairs. The roof-mounted outdoor units of the heat pumps are quiet enough not to disturb people's sleep.
Heat booster relieves the heating heat pump
The heat pumps work at the ideal operating point, meaning they only need to supply around 40 °C in the flow. Another technology provides the 65 °C that is usual in apartment buildings for a hygienic hot water supply and circulation: the eXergy machine. The eXergiemaschine is a water-to-water heat pump developed by varmeco and its Swiss partner BMS-Energietechnik. It is suitable for higher source temperatures and temperature strokes - ideal for reheating the 40 °C heat to 65 °C, as it only requires a fifth of the energy that an electric heating rod would need for this temperature stroke.
Division into two temperature zones
Two heat storage tanks are installed in each heating system to decouple heat supply and consumption. One of these serves as a low-temperature buffer and stores heat at around 40 °C; it is charged by the heating heat pumps and supplies the heating circuits. The second storage tank, the high-temperature storage tank, has a temperature of over 65 °C at the top. This cylinder is charged by the eXergy machine and supplies the so-called fresh water heaters, in which drinking water is heated according to demand using the flow-through principle. The fresh water heaters also maintain a constant temperature during hot water circulation.
Combined solution ensures a lower electricity bill
As the hot water requirement here is lower than the heating requirement, the eXergy machines (eXm) have lower outputs than the heat pumps. An eXm-Pro with a thermal output of 20 kW is installed in the large house, while the wall-mounted 5 kW eXm-compact is sufficient in the smaller buildings. The eXergy machines are heat-controlled: They start up when the temperature at the top of the cylinder falls below the set point of 65 °C. „As the eXergy machines manage the temperature rise to 65 °C with a COP of around five and the heat pumps run at the ideal operating point, this combined solution works much more energy-efficiently than a solution with heating heat pumps alone,“ reports Reinl. Switching the heating system from fossil fuels to electricity naturally increased the demand for electricity. To ensure that winter heating loads could be reliably met, the house connections of three buildings had to be reinforced, while the connections of the other buildings had sufficient reserve capacity.
The more efficient technology comes at a price. Over 1.5 million euros were needed to renovate the heating and hot water technology. „Around two thirds of the sum was raised by the owners“ association itself, while the rest was contributed by the Federal Office of Economics and Export Control (BAFA), as the new, climate-neutral solution was eligible for funding," says Resch. A project worth millions for climate protection - and for sustainable, economical heating of the 129 apartments.
How the eXergy machine works
The eXergy machine, which varmeco and its Swiss partner BMS-Energietechnik have developed, represents a Optimized temperature stratification in the buffer tank (heating storage tank). For this purpose, a water-to-water heat pump with a nominal heat output Qth of 3 to 40 kW) operates inside the appliance, which is designed for a large temperature spread of around 50 K in the buffer cylinder and also works at source temperatures of 55 °C and above. During operation, the eXergy machine draws water from the center of the storage system via two circuits. One circuit feeds water to the condenser of the heat pump, where it is heated before entering the hot part of the cylinder. The other circuit runs via the evaporator and then feeds the cooled water into the lower, cold storage tank area.
The eXergy machine is in the 3- and 5-kW version is available as a compact wall-mounted appliance (see image) and in the output classes 5 to 40 kW heat output as a floor-standing appliance. An even larger eXergy machine for hospitals, apartment blocks or commercial use, for example, is currently under development.
