The Seebeck effect will definitely have less than 8% efficiency with 80C water. Perhaps GP means that the hot water will already be available to use for hygiene and laundry, which for those with an electric water heater, is a large portion of the household power draw.
I'm not sure if a household sized water tank-full could provide heat over the course of a cold night, and whether a heat exchanger for air heating or water pipes would be more efficient. I suppose it depends on the insulation and placement of the ducts and pipes and how much of the heat makes it to and through a wall.
I did some very crude calculations but assuming 50 gallons at 60C and 1000W energy loss per hour from a moderately insulated house on a 50F night, the full water heater could keep the house at 70F for 5.14 hours. Someone with more recent practical physics usage is welcome to check this figure.
