At atmospheric pressure, the usual flow conditions in the cold wall horizontal rectangular thermal CVD reactors correspond to steady longitudinal thermoconvective rolls that make non-uniform vapour depositions, in shape of longitudinal parallel ridges. In order to get more uniform depositions, the pressure is generally lowered under the atmospheric pressure to promote forced convection flows, instead of mixed convection ones. In the present paper, using three-dimensional direct numerical simulations, we propose and analyse a method to get uniform deposition without lowering the pressure in the reactor. It consists in adequately exciting the parallel thermoconvective rolls at channel inlet to make them unsteady, periodic and sinuous in order to get a uniform time average of the deposition. This method is shown to be adapted for the horizontal and rectangular APCVD reactors with large longitudinal and transversal aspect ratios, when the Reynolds number of the gas flow is O(100), whatever the value of the surface Damkohler number. This situation is encountered in the online or scrolling APCVD reactors used to deposit coatings on float glass in the flat glass industry for instance. The simulations are based on simplified models for the transport equations (Boussinesq model) and the kinetics of the heterogeneous reactions (deposition model of silicon from hydrogen and silane: SiH4 -> Si + 2H(2)). (c) 2007 Elsevier B.V. All rights reserved.