The Eternal condensing hybrid water heater comes in a small size but is powerful enough for home use. Featuring a stainless steel heat exchanger and a built-in two-gallon reserve, the unit combines the flow capacity of traditional tanks with the continuity of a tankless. An infrared burner is dual-activated by thermostat similar to a tank or by flow like a tankless. It can be vented with 2- or 3-inch PVC. Cost: $1,999 to $2,699. Grand Hall. 877-934-7455.