In many apartment buildings gas consumption for hot water is measured by the volume of hot water used - not volume of gas. The conversion from hot water volume to gas consumption must assume an input cold water temperature and an output hot water temperature. What are these assumed temperatures, and how are they derived (for examle is the output hotwater temperature measured at the first tap in the apartment, or at the hotwater system often 10-100's meters away)? They appear to be assumed as c ...