In order to reduce energy use and improve indoor environmental quality (IEQ), new and retrofitted dwellings are increasingly equipped with complex heating and mechanical ventilation solutions, which occupants have to learn to use and live with. This work investigates the different ways in which occupants integrate these technologies into daily domestic practices, the associated difficulties and their potential consequences on comfort and building performance. Semi-structured interviews were carried out with occupants of 37 new and retrofitted low-energy dwellings around Copenhagen (Denmark), focusing on occupants' domestication of underfloor heating and mechanical ventilation. While most respondents were largely satisfied with the IEQ in their homes, the interviews highlighted a disconnection between expected and real uses and perceptions of these technologies. The design assumptions were sometimes unable to reflect occupants' expectations, including regarding personal control needs. Moreover, installation and operational failures were frequent and difficult for users to diagnose and correct, as they lacked relatable technical guidance. Consequently, occupants often resorted to alternative ways of taking control, which could prove detrimental for energy use and IEQ. The findings highlight the need for incorporating interdisciplinary insights on in-use performance in the design and installation of building systems, making them more robust, reliable and transparent.