Abstract: In high-pressure cleaning units with heating stages, i.e. with a possibility for circulating a partial flow of the forwardly pumped water through a constricted, heat-generating nozzle (74) in a return conduit to the pump's inlet side, the problem occurs that the entire water flow circulates at high pump pressure upon temporary interruption of ejection, which causes an inexpediently high heating of the water. By the invention, apparatus is provided for sensing this situation and thus block the return conduit (70,72), so that the pressure tends to rise; in this way, however, the cleaning unit's ordinary bypass valve (12) will sense that delivery from the pump (P) is totally blocked, whereby it connects the pump in bypass state in the ordinary way through a non-constricted bypass connection (14). In this way, ejection can be interrupted temporarily without any substantial heat accumulation in the circulating water, and another advantage is that the ejection conduit itself is pressure relieved in the situation.