Method for the avoidance of dry fire of an electrical flow heater with a heating block in a hydraulic circuit, in which a circulation pump is located, and a fusible plug and a thermal switch which are electrically connected in series and connected to the heating block so as to be thermally conductive, wherein the thermal switch opens above a predetermined switching temperature T S and the fusible plug melts at a higher, predetermined temperature T Sch, wherein a current flow through the fusible plug and the thermal switch is measured and, in the event of a power failure, the heating block is switched off, wherein the circulation pump is switched on, then the heating block is switched on for a first time period Δt 1, and after switching off the heating block, a second time period Δt 2 is waited, wherein • either the heating block is switched on again after the second time period Δt 2 has passed, and o in the event that a current flow through the fusible plug and the thermal switch is measured immediately after the second time period Δt 2 has passed, the current flow through the heating block remains switched on, o while in the event that no current flow through the fusible plug and the thermal switch is measured immediately after the second time period Δt 2 has passed, but the current flow occurs within a third time period Δt 3, a too low volume flow is concluded, • or it is checked within the time period Δt 2 if a current flow through the fusible plug and the thermal switch occurs and o in the event that no current flow through the fusible plug and the thermal switch occurs within the second time period Δt 2, a dry fire is concluded, o while in the event that a current flow through the fusible plug and the thermal switch occurs during the entire second time period Δt 2, the current flow through the heating block is switched on again. • either the heating block is switched on again after the second time period Δt 2 has passed, and o in the event that a current flow through the fusible plug and the thermal switch is measured immediately after the second time period Δt 2 has passed, the current flow through the heating block remains switched on, o while in the event that no current flow through the fusible plug and the thermal switch is measured immediately after the second time period Δt 2 has passed, but the current flow occurs within a third time period Δt 3, a too low volume flow is concluded, o in the event that a current flow through the fusible plug and the thermal switch is measured immediately after the second time period Δt 2 has passed, the current flow through the heating block remains switched on, o while in the event that no current flow through the fusible plug and the thermal switch is measured immediately after the second time period Δt 2 has passed, but the current flow occurs within a third time period Δt 3, a too low volume flow is concluded, • or it is checked within the time period Δt 2 if a current flow through the fusible plug and the thermal switch occurs and o in the event that no current flow through the fusible plug and the thermal switch occurs within the second time period Δt 2, a dry fire is concluded, o while in the event that a current flow through the fusible plug and the thermal switch occurs during the entire second time period Δt 2, the current flow through the heating block is switched on again. o in the event that no current flow through the fusible plug and the thermal switch occurs within the second time period Δt 2, a dry fire is concluded, o while in the event that a current flow through the fusible plug and the thermal switch occurs during the entire second time period Δt 2, the current flow through the heating block is switched on again.