In this paper we derive a posteriori error estimates for theheat equation. The time discretizationstrategy is based on a θ-method and the mesh used for eachtime-slab is independent of the mesh used for the previoustime-slab. The novelty of this paper is an upper bound for theerror caused by the coarsening of the mesh used for computing thesolution in the previous time-slab. The technique applied forderiving this upper bound is independent of the problem and can begeneralized to other time dependent problems.
