Designing interactive technology to support and reduce domestic energy consumption has occupied HCI researchers for decades. This paper extends this body of HCI research by using postphenomenology as an analytical lens to understand how the digital energy assistant' Watts' mediates domestic district heating. We base our inquiry on empirical insights from a two-year study. We conducted informal meetings, focus groups with seven households before Watts' distribution, two rounds of semi-structured interviews with seven household representatives using Watts, and 304 questionnaire respondents. We illustrate findings in three postphenomenological dimensions - practical, ontological, and epistemological - showing a two-sidedness of technology-mediated domestic district heating. For instance, Watts enables some to reduce heating by magnifying certain new aspects of consumption, but also constrains others from doing the same by concealing the "raw" consumption data. Finally, we discuss four tensions of Watts mediation and how postphenomenology can complement HCI studies on domestic energy consumption.