Could you Please guide me to a source to obtain the Schematic Diagram of Lennox G14 or G21 Pulse Gas Furnace? I could not find the company web site!

The Lennor pulse is a different animal as far as how it works and the way it burns. I would contact a qualified person to do a ON SITE inspection. Lennox probably hides the diagrams because they do not want any unqualified persons working on it. They can and do go BOOM.