Solar hot water tankage while it is being installed. Alternate Energy Technologies (AET) DX-120-96. DX means drainback. 120 means 120 gallon storage tank. 96 means 96 square feet of collector area (as 3 8'x4' collectors). Drainback not only protects from freeze, it also allows system oversizing as the pumps will shut off when tank temperature gets too high.