PACS 03.75.Kk – Dynamic properties of condensates PACS 64.60.Ht – Dynamic critical phenomena PACS 05.30.-d – Quantum statistical mechanics Abstract.- We consider a quantum quench in a system of free bosons, starting from a thermal initial state. As in the case where the system is initially in the ground state, any finite subsystem eventually reaches a stationary thermal state with a momentum-dependent effective temperature. Remarkably enough, we find that this can, in some cases, even be lower than the initial temperature. We also study lattice effects and discuss more general types of quenches. Introduction. – The problem of a quantum quench, i.e. an instantaneous change in the parameters that determine the dynamics of a quantum system, has been recently studied in various theoretical models. These include systems of free bosons or fermions in which the quenched