Plot: Plumbing & heating expert Richard Trethewey and host Kevin O'Connor replace an old tank-type water heater with a new, energy-efficient "tankless" unit fueled by propane. Then Richard and Kevin, along with landscaping contractor Roger Cook and general contractor Tom Silva ask, "What is it?" Then, Roger helps a homeowner level his sagging concrete walkway using a method called "mudjacking," and Tom shares a viewer tip for removing hammer dents from a piece of wood