A storage-type water heater includes a tank enclosing a volume of water, the tank having an upper portion and a lower portion, a chamber adjacent the lower portion of the tank and adapted to at least partially enclose a source of a heated fluid, and a helical tube heat exchanger at least partially enclosed within the tank and fluidly connected to the chamber. The helical tube directs the heated fluid from the lower portion to the upper portion of the tank and promotes heat exchange between the water in the tank and the heated fluid flowing therethrough.