Easier to install and more efficient geothermal systems for retrofitting buildings Proposals shall target easy to install and efficient underground coupling systems for retrofitting existing types of buildings or adaptable to existing types of buildings, including historical buildings, to make geothermal energy a standard source of heat and cold in building renovation. The difficulties in drilling in built environments must be taken into consideration and properly addressed. Proposals might address the need for improved and more cost-efficient heat pumps to optimize the use of the energy generated by the proposed geothermal system. Synergies may be considered with activities initiated under the Energy Efficiency call topics EE-10-2016 and EE-11-2016.TRL 7 shall be achieved at the end of the project (please see part G of the General Annexes).This topic will contribute to the PPP on Energy-efficient Buildings.Opening the project's test sites, pilot and demonstration facilities, or research infrastructures for practice oriented education, training or knowledge exchange is encouraged.The Commission considers that proposals requesting a contribution from the EU of between EUR 5 to 8 million would allow this specific challenge to be addressed appropriately. Nonetheless, this does not preclude submission and selection of proposals requesting other amounts. The cost and efficiency of existing geothermal systems, mostly based on vertical wells, to provide heating and cooling in buildings being retrofitted or renovated are not very competitive in particular when digging is difficult. The challenge is to demonstrate the cost-effectiveness and efficiency of geothermal systems for heating and cooling in individual installations being retrofitted. The action will result in the demonstration of geothermal systems, to be used in existing buildings, that make geothermal energy a viable and cost-competitive source of energy for heating and cooling. The demonstrated systems will be easy to install in built environments and have a proved efficiency in different geological conditions. The action will increase the commercial attractiveness of geothermal energy for heating and cooling and therefore increase the penetration of this renewable energy source.