Abstract: A flow-activated water heater includes a body defining water inlet chamber communicating with a heating chamber containing an electric resistance heater through an intermediate flow duct. A movable piston supported by a flexible membrane and provided with a small flow-restricting opening separates the inlet chamber from the duct. A pair of fixed contacts disposed exteriorly of one side of the body are connected to the heater and a pair of movable contacts defined by substantially semi-circular conductive plates fixedly secured at one end relative to said body exteriorly encircle the flow duct with the other end thereof movable into and out of engagement with the fixed contacts. The other ends are also joined to an external carrier movable in the longitudinal direction of the duct. A pivotable lever mechanism exterds through the wall of the body and has its inner end connected to the membrane and its outer end connected to the carrier.