A passive differential calorimeter, comprising an inflow control valve (2″), differential measurement means (4″) including the driving part of an input water meter (15) and the driving part of an output water meter (16), a back flow control valve (9″) and connecting pipes between them, wherein both the inflow control valve and the output flow control valve being temperature controlled valve, i.e. the flow through the valve is controlled according to the medium temperature, and the inlet of the inflow meter being connected with the high temperature medium end and the inlet of the output water meter being connected with the low temperature medium end, characterized in that: the medium inlet (1″) is connected with tee (17) which separates the medium flow that into the calorimeter in two lines, one of which (bypass line) flows into the inlet of the inflow control valve (2″), of which the outlet is connected with another tee (7″), and the other of the two lines (measurement line) flows into the inlet (18) of the flow meter (15), of which the outlet (19) is connected with the tee (7″), the tee (7″) being connected with the inlet of a radiator (8″).