A boiler (1) for heating water to steam comprises a boiler housing (10) having a bottom wall (12), a top wall (13) and a circumferential wall (14) extending between these walls (12, 13). When the boiler (1) is mounted in a steam generating device for generating steam and supplying steam to an appliance such as a steam iron (20), the boiler (1) is given an inclined orientation. In a lower portion (13a) of the top wall (13) of the boiler housing (10), an inlet opening (15) for letting in water to a boiler space (11) which is enclosed by the boiler housing (10) is arranged. When water is supplied through the inlet opening (15), the water lands on a lower zone (14a) of the circumferential wall (14) and flows further in a downward direction, along the circumferential wall (14). As a result, the water is pre-heated when it reaches the water which is already present in the boiler space (11), and there is no danger of the steam production getting interrupted.