Wetback is something that is mostly associated with an old house with a low pressure copper cylinder connected to an old wood burner. With a Rinnai thermosiphon cylinder you can now have a new modern house with mains pressure water heated by your wetback fire, saving you money on your power bill.
The water is circulated through a coil inside the cylinder by a thermosiphon effect i.e. warmer water rises and cooler water falls, creating a natural flow. This means there is no need for a pump to circulate the water, you can still heat your water even if there is a power cut!