Conventional boilers (sometimes referred to as traditional, heat only or open vent boilers) can supply central heating and hot water for sinks, baths and showers. They require a cold water supply tank and a hot water storage cylinder. Cold water supplied from the tank is heated by the boiler and then stored in the cylinder until required.
Because there is a ‘reservoir’ of stored hot water, they can supply more than one use at the same time. For example, a shower and a sink. However, once the hot water in the cylinder has been used, it can take quite a long time for it to be replaced by the boiler. They also required space for the tank and cylinder.
They may be appropriate for older heating systems where the higher pressures used by other types of boiler could not be accommodated, or where the water supply pressure is low.