Turning to the hot water circuit; the supply from the storage cistern is fed into the bottom of the hot water cylinder, and from there it travels down into a kitchen boiler or a back boiler linked with a fireplace. It enters the boiler at the bottom and is heated by the flame. Water rises as it is heated and leaves the boiler through a pipe near the top where it rises naturally back to the hot water cylinder. This hot water forces more cold from the cylinder down into the boiler and thus the cycle continues until you have a cylinder full of hot water.
If there is no way of controlling the boiler heat, as is often the case with an open fire, the water will sometimes reach boiling point and therefore, as a safety outlet, an open-ended expansion pipe is taken from the top of the cylinder and turned over the storage cistern so that steam and hot water can be harmlessly discharged. In practice, however, this state is rarely reached, because hot water is continually being drawn off, thus allowing more cold to be fed into the cylinder. But if your hot supply does approach boiling point, run some of the water off and let the fire burn low.