Apparatus and methods for alerting a user of topical negative pressure therapy apparatus to a non-flow condition of aspirant fluid in the apparatus are described, the apparatus comprising: a device having vacuum pump means and a waste canister connected to the device; the waste canister being operably connected to a wound dressing by aspiration conduit means for aspirating fluid from the wound; the aspiration conduit means, the waste canister and the device providing a fluid flow path therethrough and the vacuum pump means providing for fluid flow therethrough; the device having first pressure sensing means at an upstream position between an outlet port for aspirated fluid from said waste canister and said pump means; said waste canister having further provision selected from one of, selectable vent means for admission of ambient air to said waste canister and selectable second pressure sensing means for sensing pressure in said waste canister; and, a control system for receiving signals from said sensors and initiating control signals in response and activating an appropriate user alarm.