Optimization of a Passive Solar Heating System for Rural Household Toilets in Cold Regions Using TRNSYS

To address the poor thermal insulation and freeze resistance of rural outdoor toilets in cold regions—key obstacles to achieving the UN Sustainable Development Goal (SDG) 6.2 and popularizing rural sanitary toilets—this study fills the literature gap of insufficient research on passive solar heating systems tailored for rural toilets in cold climates. Using TRNSYS simulation, Plackett–Burman key factor screening, single-factor experiments, and Box–Behnken response surface methodology, we optimized the system with building envelope thermal parameters and Beijing’s typical meteorological year data as inputs, taking January’s average indoor temperature as the core evaluation index. Results indicated six parameters (solar wall area, air cavity thickness, vent area ratio, vent spacing, exterior wall insulation thickness, and heat-gain window-to-wall ratio) significantly influence indoor temperature ( p < 0.05). The optimal configuration was as follows: solar wall area 3.45 m 2 , window-to-wall ratio 30%, exterior wall insulation thickness 200 mm, vent spacing 1800 mm, air cavity thickness 43 mm, and vent area ratio 5.7%. Post-optimization, the average temperature during the heating season reached 10.81 °C (79.5% higher than baseline), with January’s average, maximum, and minimum temperatures at 7.95 °C, 20.47 °C, and −1.42 °C, respectively. This solution effectively prevents freezing of flushing fixtures due to prolonged low temperatures, providing scientific support for the application of passive rural toilets in China’s cold regions.