Direct numerical simulations of three-dimensional Rayleigh-Benard convection are performed with non-uniform temperature boundary conditions at the lower plate. The boundary conditions excite convective states in a form of counterrotating convective rolls that remain stable for at least ten eddy-turnover times even if their size is significantly different from the size of rolls encountered in natural convection at the same Rayleigh number. For longer times applied temperature forcing becomes ineffective and forced convective patterns lose their stability. Changing the size of the convective elements increases heat transfer by about 15–20% as compared with the case of natural convection.