Water heating systems can be configured to provide both hot water and space heating. Combined heating sources can include boilers, tankless water heaters, solar water heaters, and other systems. Multi-function heating systems sometimes deliver space heating through a hydronic under-floor system but can also be used with other hot water and air distribution systems. One of the benefits of multi-function heating systems is that they avoid the cost and space with a separate space heating system, such as a separate furnace and water heater.