I can believe single radiates more heat, especially with two stories involved. I had single and tried talking my installer of the new stove into single, because of the more heat. He would not back down saying single is too sloppily made and doesn't last. Now I am glad for the double, more cost, less heat out of the pipe,... But, less air for same draft, much less creosote -- creosote condenses at a higher rate in single, and the pipe is built to last. The single plus shield seemed like a good idea.