The effect of flow asymmetry was observed experimentally in vertical parallel channels of a continuous direct ohmic heater. While downward-oriented flow is evenly distributed at isothermal conditions, the flow rate in parallel channels differs in the case of heating due to buoyancy effects. Three different configurations of parallel flow (with and without internal heating in channels) are analyzed, giving critical values of Gr/Re or Ri = Gr/Re-2 and ensuring stable and even distribution of flow into parallel channels of vertically oriented heat exchangers or heaters. Experimental verification was based upon flow visualization (injection of a colored tracer and monitoring the tracer by a camera), measurement of temperature profiles, and stimulus response experiments using KCl as a tracer for conductivity methods (2 Pt conductivity probes) and Tc99 as a radioisotope tracer (collimated scintillation detectors).