Chimney expert Mark Schaub helps homeowner Terry Peterson repair the water heater flue in his chimney, preventing fireplace smoke from entering his basement. Back in the loft, plumbing and heating expert Richard Trethewey, general contractor Tom Silva, landscape contractor Roger Cook and host Kevin O'Connor try to guess "What is it?" Then Richard and homeowner Ajay Chebbi install an "anti-sweat" valve to prevent condensation on a toilet tank.