The invention relates to a boiler, in particular to a no pressure boiler, which is characterized in that: a third layer of heated bodies is arranged on the upper part of a second layer of heated bodies and a first layer of heated bodies is arranged on the lower part of the second layer of heated bodies; a flue outlet of the second layer of heated bodies is connected with a flue connecting box to form connection with the third layer of heated bodies; the second layer of heated bodies are welded with the first layer of the heated bodies; the upper part of a round heating body is connected with aradiating lower swan neck; and the lower part of the round heating body is connected with a hearth heating pipe. The boiler has the advantages of simple structure, low cost, and safety and reliability. The hearth combustion temperature is 170-200 DEG C higher than raw coal temperature, the boiler can save energy by 30 to 35 percent in the same period. The air is induced naturally, no blower is needed; moreover, water is used to treat smoke and dust, the operation is dust-free and noiseless. The water in the boiler can keep a temperature between 60 and 70 DEG C in working state without stopping, and the backwater is between 45 and 55 DEG C. The heated area of the boiler is 3 to 4 times of that of a normal boiler, the capacity of hot water is 5 times larger than that of the boiler with thesame type; and the boiler can be maintained by replacing pipes.