A lithography apparatus includes: a projection system configured to project a desired image onto a substrate; an active module that generates a time-varying heat load; a temperature conditioning system configured to maintain a component of the lithography apparatus at a predetermined target temperature; and a heat buffer including a phase change material in thermal contact with the active module, the phase change material having a phase change temperature such that the phase change material is caused to undergo a phase change by the time-varying heat load, and wherein the phase change material is stationary relative to the projection system during critical operations of the lithography apparatus.