Indoor thermal environment and energy performance of radiant floor heating system coupled with pipe-embedded external wall utilizing low-grade heat

Pipe-embedded external walls (PEEWs) have attracted increasing attention due to their potential to reduce building energy consumption. However, radiant floor heating systems (RFHSs) coupled with PEEWs have rarely been studied, despite the proven effectiveness of RFHSs in lowering indoor design temperatures. To fill this gap, this study proposes an RFHS coupled with a PEEW and develops an experimentally validated three-dimensional steady-state CFD model, where a ground heat exchanger serves as a low-grade heat source. The indoor thermal environment and energy performance of the coupled system are compared with those of a conventional RFHS under varying outdoor temperatures (toa) and mean supply–return water temperatures in the embedded-pipe and floor loops (tep and tf). Results show that the PEEW improves thermal comfort and reduces energy consumption by utilizing low-grade heat. These advantages become more significant with higher tep and lower indoor air temperature (tair) and toa, confirming the suitability of the coupled configuration. A critical outdoor air temperature threshold is identified: below it, the embedded pipes raise the inner surface temperature of the PEEW and reduce radiant temperature asymmetry (RTA). At tair = 18 °C, tep = 15 °C, and toa = −15 °C, RTA decreases from 3.59 (conventional) to 3.04 °C (PEEW). Although the heat flux at the outer surface of the PEEW increases, the added heat is primarily supplied by low-grade energy, while the heat fluxes on the inner surface and the floor decrease. A case study in Taiyuan indicates an energy-saving rate of 21.20% at tair = 18 °C.