A system for heating water which includes a first container and a second container, an air-tight connection between upper ends of the first and second containers, each container including a respective water heating element, and a water outlet port and a water inlet port, a manifold which is operatively connected to the respective inlet and outlet ports of each container and which connects, in a controlled manner, the respective inlet port of each container to a pressurized source of cold water, and each container, at least in a respective upper volume thereof, and in the air-tight connection, containing a gas, and whereby cold water which is introduced into the first container via the manifold displaces gas via the air-tight connection from the first container into the second container thereby to expel warm water from the second container through the respective outlet port.