A combined numerical and experimental investigation is presented showing the effect of heat on the stability of a horizontal vortex street for ReD=75 and RiD between 1 and 2. Detailed 2D-HiRes PV experiments are compared with 2D numerical results. The results show that an early transition to 3D of the vortex street takes place for RiD>1. Furthermore, the location at which the 2D wake becomes essentially 3D turns out to be dependent on RiD. For an increasing addition of heat (increasing RiD), this location is shifted into the direction of the cylinder. By applying 3D measuring techniques, such as 3D-PTV and 3D visualisation techniques, the transition process and the behaviour downstream of the transition point are investigated.