A tankless boiler uses a powerful heating element and heat exchanger to heat water on demand instead of maintaining a tank of hot water. These systems are more energy efficient, since no energy has to be wasted keeping a tank hot, although they can struggle when hot water demand increases.
Commonly, tankless hot water heaters or boilers are used to replace conventional hot water heaters. In some cases, a single unit may provide hot water for the entire home, but smaller, point-of-use units may offer better results for many households. Keeping the heater close to the output faucet reduces the amount of heat lost in the plumbing system, preventing heavy use in one room from affecting hot water availability elsewhere.
Tankless boilers may also serve as the centerpiece for a hydronic heating system. In a traditional hydronic system, a tank of water is kept at high temperatures, feeding water pipes that flow to heating appliances, such as baseboard heaters or radiators. The hot water may also simply flow through floors and walls, providing radiated heat for an entire home. In a tankless system, water remains inside a closed loop, but instead of a boiler tank providing the heat, a tankless heater keeps the water flowing through the system piping hot.