These photos are actual photos taken with a thermal camera during a typical home inspection. The return air temperature (73) is taken to establish a baseline. The supply air temperature (103) was taken to show the Delta T or the difference in temperature between the return air and supply air. In this case, there is a Delta T of 28 in heating mode. Typically you want a Delta T in heating mode between 15 to 25 degrees. This example is a good delta T of 28 degrees.