- Abstract A simple method for the estimation of energy flows in building mechanical systems is suggested for energy auditing purposes. The method involves a non-intrusive pipe flow measurement technique which entails the recording of naturally occurring variations of temperatures of the fluid with two sensors separated a known distance along the pipe's surface. When the two resulting temperature-time series are cross-correlated, a time lag between the two series can be determined. The bulk fluid velocity or the flow rate is estimated from the distance between the two sensors and the time lag. Experiments indicate that the accuracy of the method is about 10% which should suffice for identification of major heat losses in the building's heating systems. With the aid of a computer simulation of the interactions between pipe and fluid flow, the observed systematic errors are explored.