The prompt hot water and water conservation system includes a hot water supply pipe connected to a hot water heater outlet. A discharge faucet is connected to the supply pipe. A return pipe is connected to the supply pipe and to a hot water tank drain. The return pipe carries cooled water to the drain for reheating. Reheated water circulates into the supply pipe. A flow control device, in the return pipe, includes a chamber that houses a sphere. Return water flow moves the sphere toward a stop and permits unimpeded movement of water into the tank drain for reheating. When the faucet is opened, water flows from the tank drain into the flow control device. The sphere is moved to contact a small end of the chamber and substantially stop flow through the return pipe. Water is circulated through the system by increased density of cooled water.