There is a really good section in the service manual for pressurization that covers test procedures to test the heat exchanger and heat valves as well as fresh air and others. This video gives you a quick walk though on how to easily see those valves operate to insure you have everything working properly as well as a quick did you know about the valves and butterflies that your shop probably doesn't even realize.
http://youtu.be/efd1GpGsyVUGlenn
