A system is disclosed for cleaning the interiors of tubes in heat exchangers such as condensers and other devices having numerous substantially parallel tubes which periodically become fouled, scaled, or otherwise encumbered with deposits. An air-actuated seal resides on the end of a nozzle which is projected into the open end of a condenser tube or other heat exchanger tube to be cleaned. An air cylinder and piston retract the end of the nozzle on actuation of a single trigger valve on a manually operated water gun, which both begins high pressure water flow and operates the piston. On actuation of the trigger valve, the segment of the nozzle which has been inserted is retracted a short distance, crimping a flexible sleeve which surrounds it into a tight seal on the inner surface of the tube, thus preventing backflow or back spray. The system may employ projectiles designed to pass through the tubes under pressurized water, released by the gun, in close proximity to the interior surfaces of the tubes so that they may remove the deposits with a scraping or abrading action. The air-actuated sealing nozzle relieves the operator from arduous, repeated forcing of the nozzle into the tubes to maintain sealing contact with the interiors of the tubes. The system is readily adaptable to, and includes, a multi-nozzle arrangement actuated by a single trigger.