Industrial heat exchangers are regularly designed for forced convection dominant flow, but natural convection may impact heat transfer for certain fluids or under turndown conditions. In this study, we measured mixed convection heat transfer for p-xylene and propylene glycol liquids in vertical flow over ideal horizontal plain and low-finned tube banks. Our study shows that aiding natural convection may either significantly enhance or penalize forced convection heat transfer under certain conditions; in addition, our limited data for opposing natural convection affirm that it may significantly penalize or have little impact on heat transfer, depending on the tube geometry and conditions. When natural convection enhances heat transfer, a regime-based correlation predicts coefficients as much as twice as high as the forced convection coefficient. However, when natural convection penalizes heat transfer, the correlation predicts coefficients as low as half the value of the forced convection coefficient. Further study of mixed convection regimes will support development of a generalized correlation.