You can set a CEL expression for the heat source such as: HeatIn = HCoef * (Treq – T) where Treq is the required temperature and HCoef is a positive valued power density coefficient (units = [W m^-3 K^-1]). The size of HCoef determines how fast your heat source reacts to temperature changes. You want HCoef to be large enough to give you the behaviour you want but not so large that it makes the solver struggle. Setting the source coefficient to be -HCoef should help significantly with convergence.