Thin film bolometers are widely used in low temperature experiments. They are always connected in an electrical measuring circuit which involves a current, so an unavoidable Joule heating effect is generated. As a result the detector temperature can be appreciably higher than that of the cell, especially in the low temperature range. It is therefore important to be able to determine the bolometer net heat loss in order to evaluate the temperature gradient which could take place. We propose a simple analysis that provides an experimental determination of the values of the heat conductance across the interfaces of a thin film bolometer deposited onto a substrate. In this way, both temperatures of the bolometer and of the part of the helium film which covers it, as well as the respective heat flows through the film and the substrate are easily obtained. This method could prove useful in the future for a better understanding of surface exchanges and surface characterization.