12 opinions, one solution

Using a clever redevelopment strategy, the owner of the “Alte Schmitte” development in Lohn-Ammannsegg replaced the power-hungry electric heaters of the 1980s with efficient and economical geothermal heat pumps from CTA. Each flat now has a heat pump that is roughly the size of a wardrobe. For heating and hot water, these use heat from a geothermal probe field common to each building.

One common heat source, individual heat pumps per housing unit.

Straight out of the 80s

The freehold flats of the buildings in Lohn-Ammannsegg in the canton of Solothurn are typical of the 1980s. At that time, and influenced by the oil crisis, people searched for an alternative to fossil fuel heat generation. They found this alternative in decentralised electric heaters, which supplied each flat separately with heat for underfloor heating and for heating hot water.

Missing pipes and boiler rooms

This individual heat production presented some challenges for the heating renovation that was urgently needed in a number of flats. For a central heating solution, a common boiler room was missing, as were the necessary pipes leading to the flats.

Using the existing infrastructure and cutting costs

A common solution has to be found for a dozen flat owners. “As a straight replacement of the electronic heaters wasn’t possible, we had to come up with a redevelopment solution that perfectly fitted the given infrastructure and that would have as little impact as possible on the flat owners’ wallets”, explains Thomas Klaey of Klaey Haustechnik AG. Together with the flat owners who had committed to the scheme, the local installer and CTA developed a sustainable, renewable energy-based solution—it is technically forward-looking and uses the existing infrastructure, as well as rewarding the owners with significant cost savings.

Heat pump placed in the sloping roof.

Common geothermal probes, individual heat pumps

The geothermal probes were bored under the access roads and a top-insulated piping system laid to the individual buildings and housing units. A separate heat pump was installed for each flat at this heat source. The power-regulated heat pump adapts its output to the demand at any given time and therefore does not require a space-consuming energy storage system. It runs quietly and provides each flat individually with heat for heating and hot water. Existing underfloor heating and electrical installations can continue to be used without any adjustments.

Space-saving, ultraquiet, efficient

Key to the heating redevelopment are the Optiheat Inverta TWW geothermal heat pumps. Thanks to inverter technology, the output is adapted to the actual demand. It produces between 3 and 6.5 kW of heat and draws only as much energy from the geothermal probes as is currently needed. The unit, with an integrated boiler, provides heat for underfloor heating and hot water. It is also space-saving and ultraquiet—the unit has been installed directly into the flats (partially also in loft space and basement) and there it takes up about the same area as a wardrobe. This customised solution has worked well ever since the first winter. 

Significant operational cost savings

Today, the flats require only one quarter of the electricity for heating. So, in addition to the comfort of individually controllable heating, there are also significant cost savings. This also puts into perspective the investments, which were ultimately affordable for all owners thanks to cantonal subsidies.


  • Installer: Klaey Haustechnik AG, Lohn-Ammannsegg

  • Heat pump supplier: CTA AG


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