Hydrothermal stabilizer

Plumbing apparatus is provided for reducing energy consumption of a hot water heater, the apparatus includes a tank and a buoyant piston movably disposed within the tank and having a perimeter slidably engaging an inside wall of the tank. Water entering a top of the tank pushes the buoyant piston towards a bottom of the tank and at a selected position, grooves are provide in an inside wall of the tank to enable entering water to pass the buoyant piston. When the apparatus is connected between a conventional hot water heater and a cold water source, the water heater is buffered from the cold water source for small draws of water from the hot water heater thus preventing such small draws of water from tripping the water heater thermostat.