Buffer tanks provide additional thermal mass between a hydronic heat source and a zoned distribution system. They allow the rate of heat production by the heat source to be very different than the rate of heat dissipation by the heat emitters. When the heat source is operating, the buffer tank absorbs the difference between the rate of heat production and the rate of heat dissipation to the load. This allows the heat source to remain on for several minutes and, thus, not “short cycle.”
Buffer tanks can be piped several ways. The two most common are 4-pipe and 2-pipe configurations, as shown here in Figure 1.