Boiler

A boiler comprising:

• a boiler body including an upper header, a lower header, and a large number of water pipes to be heated by a burner, the water pipes connecting the upper header and the lower header to each other, wherein the water level in the water pipes when combustion by the burner is stopped is lower than the upper ends of the water pipes;
• water supply means supplying boiler feed water into the boiler body;
• control means controlling the operation of the water supply means;
• external water level detection means placed outside the boiler body, the external water level detection means communicating with an internal space of the upper header and an internal space of the lower header through communication pipes and detecting a boiler body external water level;
• and
• header water level detection means detecting a water level existing inside the upper header by boiler water inside the water pipes being pushed up by air bubbles generated by boiling of the boiler water, wherein
• the control means controls the operation of the water supply means so as to lower the boiler body water level when the water level detected by the external water level detection means reaches a low concentration set water level or a set water level lower than the low concentration set water level, and controls the operation of the water supply means so as to lower the boiler body water level when the water level detected by the header water level detection means reaches a high concentration set water level.