It is necessary to mention that the thermal energy which is measured in Giga calories, and the amount of water measured in cubic metres, are completely different physical quantities. It is known from course of physics of high school. So really we are not talking about translation gigacalories per cubic meter, and about finding the correspondence between the quantity of heat spent on heating the water, and the volume of the resulting hot water.
By definition, a calorie is the amount of heat required to heat one cubic centimeter of water by 1 degree Celsius. Gigacalorie used for measuring thermal energy in heating and utilities, it's a billion calories. 1 metre is 100 centimeters therefore, in one cubic meter is 100 x 100 x 100 = 1,000,000 centimeters. Thus, to heat a cube of water 1 degree, you will need a million calories or 0.001 Gcal.
The temperature of hot water flowing from the faucet must be at least 55oC with. If the cold water inlet to the boiler has a temperature of 5oC, it must be heated to 50oC. On heating 1 cubic meter will require to 0.05 Gcal. However, when water moves through pipes to heat losses inevitably occur, and the amount of energy expended in the provision of water supplies, in fact it is about 20% larger. The average standard of consumption of thermal energy for obtaining the cube of hot water shall be equal to 0,059 Gcal.
Consider a simple example. Let the interheating period, when all the heat is used only to provide hot water, heat consumption according to the testimony of communal counter made up of 20 Gcal per month, and tenants in the apartments which installed water meters, spent 30 cubic meters. m hot water. They account for 30 x 0,059 = 1,77 Gcal. Heat consumption of all other tenants (let them be 100): 20 – 1,77 = 18,23 Gcal. One person of 18.23/100 = 0,18 Gcal. Translating Gcal in m3, we get the hot water consumption 0,18/0,059 = 3,05 cubic meters per person.