Hot water reset, also called outdoor reset (ODR), is an energy-saving automatic control algorithm for heating boilers that are typically fired with fuel oil or natural gas. A hot water reset control loop measures the outside air temperature; this information is used to estimate demand or heating load as the outdoor temperature varies. The supply hot water temperature is modulated up and down range in an inverse linear ratio to outside air temperature. The typical range for conventional boilers is to vary the supply water temperature from 60 to 82 °C (140 to 180 °F) as the outside temperature varies from 18 to −18 °C (64 to 0 °F).