Energy-efficient and cost-effective in-house substations bypass for improving thermal and DHW (domestic hot water) comfort in bathrooms in low-energy buildings supplied by low-temperature district heating

Using a bypass to redirect a small flow through the in-house DH (district heating) substation directly to the return pipe is a commonly used but energy-inefficient solution to keep the DH network “warm” during non-heating seasons. Instead, this water can be redirected to the bathroom FH (floor heating) to cool down further and thus reduce the heat lost from bypass operation while tempering the bathroom floor and guaranteeing fast provision of DHW (domestic hot water). We used the commercial software IDA-ICE to model a reference building where we implemented various solutions for controlling the redirected bypass flow and evaluated their performance. The effect on the DH network was investigated using Termis software. Bypass flow redirected into bathroom FH during the non-heating period resulted in comparison to the reference case on average in a 0.6 °C–2.2 °C increase of the floor surface temperature and additional cooling of bypass water by 3.9 °C, reducing the heat loss from the DH network by 13% and covering 40% of the heat used in the bathroom FH. The use of the bypass flow in bathroom FH is a cost-effective solution exploiting the heat that would otherwise be lost in the DH network to improve comfort for customers at discounted price.