Gas bubble evolution under gas supersaturation can alter cooling performance in PEM electrolysers, yet data for pressure-induced dissolved-gas supersaturation in non-boiling water flow are lacking. Experiments were therefore performed in a heated rectangular channel (6 mm × 50 mm, q˙=147,6kW/m²) over 4 000 ≤ Re ≤ 27 000. Dissolved-gas supersaturation and undersaturation were generated by rapid pressure drops and rises, respectively. Wall temperatures, inferred from embedded thermocouples, stayed below 80 °C, eliminating boiling. Dissolved-gas super-saturation increased the Nusselt number by up to 22 % for Re < 12 000; the benefit vanished at higher Re. Gas under-saturation reduced Nu by up to 9 %. The trends are attributed to gas-bubble-induced turbulence in the core flow and boundary-layer agitation at the wall, and they agree with earlier thermal-gas-super-saturation data obtained in the same Re range. In addition, tests on laser-structured and hydrophobically coated surfaces revealed that surface functionalization strongly affects bubble activity and heat transfer.