Kiel diagram showing the heavy-metal hot subdwarfs (black dots, Naslim et al. 2013, 2020; Jeffery et al. 2017; Østensen et al. 2020; Dorsch et al. 2019, 2020) and our model. The dashed light line is the evolution before the star settles onto the horizontal branch. Light blue circles are distributed every 200 Kyr. The full thicker line is the evolution during the quiescent He-core burning phase. Black crosses are placed every 20 Myr in this part of the track. The red dot corresponds to the location of EC 22536−5304 (Dorsch et al. 2021).