A ducting system has a first upstream location is to be connected to a first source of air. A downstream location in the duct is to be connected to a second source of air. The upstream location has a first valve, and the downstream location has a second valve. An end location of the duct is to be connected to a sink. A control for the valves achieves a desired pressure and temperature of air at the end location. The first temperature sensor is located at a position intermediate the upstream location and the downstream location. A second temperature sensor is located at a position intermediate the downstream location and the end location. A control is programmed to determine the health of the first and second valves based upon a difference between temperatures sensed by the first and second sensors. An air use system and a method are also disclosed.