A hot water boiler heats water which moves through a system of piping and delivers heat using radiators, baseboard, or radiant convection systems. Gravity simply means that the energy from the heat forces the hot water to move out of the boiler, like in a steam system, forcing the cooler water back into the boiler for reheating. Most hot water systems today are forced systems using circulator pumps to move the water through one or more zones. Most modern homes have multiple zones for maximum efficiency and effectiveness. A popular newer form of distribution using hot water is hydronic forced air. In this system the water is delivered to a coil in a duct. A fan blows on the hot coil to move air across it and deliver the warm air to the rooms through registers. This is often seen when the duct system is also used for air conditioning and saves on the installation of a separate distribution system for the heat.