The project: the Carlisle, Mass., farmstead. The motorized back-porch screen system is installed, as are mahogany countertops for the kitchen and office and shower walls in the guest bath. Also: the wine cellar is finished and wallpaper is hung in the master bedroom. Meanwhile, plumbing-heating expert Richard Trethewey visits a New Bern, N.C., dishwasher maker. On “Ask This Old House”: replacing a rotting threshold; shopping for a rider lawn mower; a primer on garbage disposals.