In many (most?) cases, the tankless heaters require both a larger gas/propane supply pipe and a larger flue to vent the exhaust. This was the main reason I decided to stick with the tank style. There have been articles in Fine Homebuilding and Journal of Light Construction over the last several years. One article by a reputable plumber was an in depth discussion about how he installs these systems with an auxillary tank. I couldn't stop laughing long enough to actually read the article to find out if it was serious.