Hot-water furnaces heat, store and distribute water throughout a home or building. Various types of furnaces use different fuel sources for heating and have varying storage capacities.
Because hot-water furnaces have limited capacities, it is possible to run out of hot water if use exceeds the amount of water stored. It takes time for the tank to fill and then re-heat the water, which is why there is sometimes a lapse during which the water running out of a faucet or shower head is not hot.
According to the U.S. Department of Energy, the primary difference between boilers and furnaces is the elements that they heat. Furnaces heat air, while boilers heat water. By definition, hot-water furnaces are actually boilers.