Decarbonisation of a Social Housing Building in Pasaia – Water-to-water Heat Pumps with Air Collection in a Dual Thermal Lift System

A social housing building in the Basque Country has benefitted from all the advantages of renewable energy by replacing its individual natural gas boilers with a modern dual thermal lift installation using aerothermal and geothermal heat pumps from Ecoforest. This innovation was recognised with the 2024 DecarBuilding European Award for Decarbonised Installations, granted by the European Heat Pump Association (EHPA).

H2020 HAPPENING Project

This installation was carried out as a case study for the H2020 HAPPENING project funded by the European Commission, which also includes two other demonstration sites in Austria and Italy. The system uses heat pumps as the main components to provide heating and domestic hot water (DHW), supported by photovoltaic panels and both thermal and electrical storage systems.

DHW and Heating Installation with Dual Thermal Lift

The Pasaia building, owned by the public social housing company Alokabide S.L., consists of 8 flats that previously had individual natural gas boilers. The new HAPPENING system centralises the production of warm water using two air-to-water heat pumps connected to a 2,000-litre tank. The first thermal lift occurs in this tank. From there, low-temperature water (20°C) is distributed to the individual dwellings, where a water-to-water heat pump carries out the second thermal lift to supply DHW and heating.

Benefits

Decoupling energy production from demand improves the coefficient of performance (COP) of the central air-to-water heat pumps. Since COP varies with outdoor temperature, this system allows for the production of warm water during the warmest hours of the day—when heat pump efficiency is highest. These hours also coincide with peak photovoltaic generation, maximising self-consumption. Additionally, distributing water at low temperatures significantly reduces thermal losses, which are a major issue in conventional central heating systems.

Hybridisation with Photovoltaic Energy

The building is also equipped with a rooftop photovoltaic system and batteries to maximise the building’s energy self-sufficiency, with a smart Energy Management System (EMS) managing any surplus energy. The Ecoforest heat pumps use this  pv surplus by storing it as thermal energy in the warm water tank, reducing electricity consumption from the grid.

Natural Refrigerant R290

Thanks to the use of heat pumps with natural refrigerant R290, the energy retrofit preserved the existing radiators, as they are capable of operating at temperatures above 60°C. This resulted in considerable savings in both time and investment.

The results confirm that this solution is an ideal alternative for the retrofit of multi-family residential buildings.