This cross-sectional study examined how the progression and consistency of students' understanding of thermal concepts in everyday contexts changes across the grade levels. A total of 656 Turkish students from Grade 8 (age 13-14), Grade 10 (age 15-16), and the first year of college (age 19-20) participated in the study. The data were analysed using statistical procedures (descriptive and inferential). Findings indicated a substantial progression in the students' scientific understanding of thermal concepts across grade levels. In addition, the students' alternative conceptions about thermal concepts generally decreased in frequency across grade levels, but certain alternative conceptions were observed in every grade level to a similar extent. Even though the number of students who consistently used scientific ideas increased across grade levels, the number of students who consistently used non-scientific ideas decreased across grade levels. However, the number of students who used scientific and non-scientific ideas inconsistently generally increased as they progressed in the science curriculum. These findings can be associated with either fragmentation or alternative conceptions that result from the gradual enrichment processes students experience when they try to integrate scientific concepts into their conceptual frameworks.