A drum-type boiler is provided. The boiler's evaporator is divided into two sections (116A, 116B) which are connected in parallel to the drum (112). The first evaporator section (116A) relies on the feedwater pump (138) to provide forced circulation. The second evaporator section (116B) relies primarily on natural circulation but may also be assisted by the feedwater pump. This design provides a boiler which is reliable under all operating conditions, including start-up, transient conditions, and steady state conditions, without requiring the use of circulating pumps or an auxiliary energy source (steam, water, gas, etc.).