Hydronic heating systems use hot water to carry heat to occupied spaces. Such systems have been used for many years. In some systems, the hot water circulates by thermal convention. The circulating water is under atmospheric pressure, and an expansion tank allows changes in volume.
Most hot water systems, however, use a circulating pump to increase water flow. It carries more heat per unit of time to the room heat transfer units. Having a pump in the system permits the use of a smaller boiler.
A gas-fired domestic water boiler that uses outside air for combustion may be direct vented through the sidewall, roof, or unused chimney. A drain connection is located at the bottom side of the boiler to simplify condensate removal.