I have an old Crane Sunnyday 15 oil furnace with integral water heating coil for the household use. Based on my oil consumption during the Summer months, when the burner does not operate to heat the hot water cast iron baseboard radiators, I'm using ~1.25 gal/day to heat the water. I am the only one in the house, take one shower/day and wash a few dishes. I use the dishwasher for most stuff and run that every four or five days.