Combi, or combination, boilers use gas to supply both hot water and heat. They do not use storage tanks, because water comes directly from the water supply source. Combi boilers are especially useful in small homes because they take up less space than conventional household and water heating systems.
Because a combi boiler draws and heats water whenever it is needed, users do not run out of hot water, which sometimes happens with conventional water heaters. Since water is not continually warmed, this system also saves money. In addition, installation is generally faster than with conventional systems because fewer components are needed.
However, using a combi boiler does have a few drawbacks. Because there is no storage tank, tap water is not immediately hot. Also, if multiple taps are used simultaneously, the water pressure of each is often reduced. Therefore, combi boilers are not usually appropriate in large homes or houses with more than one bathroom.
One alternative to a combi boiler is a conventional boiler, which requires both a cold water tank and a hot water cylinder. Though flow does not lessen when multiple taps are used, the hot water supply is depleted when the cylinder empties. Hot water is then unavailable until the cylinder heats more water.