Abstract The metrology and the impact of various parameters and operating conditions on the bulk‐to‐tube heat transfer coefficients in two‐phase bubble columns are investigated on a small‐scale mock‐up. It is shown that (1) quasi‐adiabatic conditions can be reached in the column; (2) the bulk‐to‐tube heat transfer coefficients for each U ‐tube downward and upward sections may or may not differ significantly, depending on the way uncertainty of the measurements is estimated; (3) using the different measurements and uncertainty estimates for given conditions, a mean heat transfer coefficient over all tubes is estimated within ±5%. The consequences for bulk‐to‐tube heat transfer coefficient prediction in a larger column are discussed.