In this talk I will present recent results obtained in collaboration with Edoardo d'Angelo, Nicolo Drago and Nicola Pinamonti. Starting from the framework of perturbative algebraic quantum field theory, we have formulated a flow equation, similar in form to the Wetterich equation, which can be used on arbitrary globally hyperbolic spacetimes, for field theories that give rise to normally hyperbolic equations of motion. An advantage of our equation is that it can be formulated in an arbitrary Hadamard state, so non-existence of the unique vacuum is not an obstruction.