In order to properly control a hot water heating system, the heat supplied to a building must equal the heat loss from the building. The heat loss is dependent on the outdoor temperature. It is therefore necessary to reset the supply water temperature based on the outdoor temperature. The ratio at which the water temperature increases as the outdoor temperature decreases is the Heating Curve. Outdoor Temperature Reset improves comfort by limiting indoor temperature swings, and provides higher energy savings by minimizing distribution and boiler stack losses.