A novel concept for energy-efficient floor heating systems with minimal hot water return temperatures

The study proposed and investigated a new concept for hydronic floor heating in dwellings with the aim of reducing hot water temperatures toward a more robust and energy efficient operation. Modern heating systems often rely on low return temperatures to improve operation efficiencies through reduced heat losses from return pipes, greater utilisation of condensation heat from boiler flue gases or from the increased COP of heat pumps. Our study investigated the potential of using an apartment heating substation (or ‘flat station’) to supply space heating through two mixing loops using hot water supply temperatures of 30°C to bathrooms and 24°C to all non-bathrooms. The concept sought to minimise hot water supply temperatures to utilise a self-regulating effect while ensuring low return temperatures. In the first iteration of the concept, the high-temperature return water from the bathrooms was cascaded to the non-bathrooms to heat these rooms and provide further cooling of the hot water. The calculated energy-weighted return temperature under this original concept was 25.6 °C for the example case of a new energy-efficient apartment building. However, there was limited potential to utilise the cascaded coupling, so considering the complexity of its configuration and controls, the authors simplified the proposed concept to two mixing loops without a cascaded coupling. The calculated return temperature with the updated concept was 25.7 °C. The control of the floor heating included some aspect of self-regulation because the heat transfer strongly depended on the indoor temperature. Based on the results of this preliminary investigation, the concept may provide a robust and energy-efficient option for configuring floor-heating systems in situations that rely on low hot-water return temperatures.