It secures the tank and facility. The surplus part of the expanded water in the tank after heating is accumulated in this reservoir, and when the water of the facility gets cooler and the level decreases, it is taken from this reservoir. Expansion tank takes part on the top point of the system, and the decleased level of water is replenished from this expansion tank. They are open to atmosphere, and they are located on the top point of the pipe facility in hot water systems or upper point than the radiator in the highest point. Therefore, they constitute the the highest point of the facility, and opens the facility for the atmosphere. All the facility is filled with water till the level of this reservoir. The water decreased due to evaporation, various leakage, repair and maintenance etc. is replenished from this reservoir. In case of the fact that water in the open expansion tank decrease under the particular mimimum value, water is taken manually or automatically by means of a water gauge system from outer source.