Abstract: A natural circulation boiler for heating includes a water tank wherein the height of one side is formed to be lower than the height of the other side to include a water storage part wherein water is stored, and a water inlet is formed on the top of the one side, a check valve installed at the bottom of the water tank, and including a body part, a top connection part formed on the top of the body part, a bottom connection part formed at the bottom of the body part and a backflow prevention plate that opens or blocks the flow path of the top connection part, a water discharge tube wherein one end thereof is connected with the bottom connection part of the check valve and the other end thereof is connected with the top connection part of the check valve, an electric heater wherein one end thereof is connected with the bottom connection part of the check valve to heat the water supplied from the water tank, a discharge tube connected with the other end of the electric heater to feed the water heated at the electr