Drainback installations use three tanks to heat the domestic water supply. The two main tanks are just like those used in pressurized solar thermal systems. However, the third is specialised – the drainback tank, which is installed on the solar loop.

The two main tanks are installed in standard fashion: the one heated by solar energy is piped in series before the backup grid-tied tank, so that the sun-heated supply is topped up with heat in the second tank if necessary.

The drainback tank is smaller than the other two tanks and is used to store solar fluid when the system is not operating. It is installed in the conditioned space, usually at the highest point to reduce head pressure (especially when the main tank is in the basement in a two-storey home). Drainback tanks are always unvented.

When a drainback solar hot water system is not operating, all the solar fluid drains out of the collectors and the pipes in every part of the system and returns to the drainback tank. Consequently, the tank must be big enough to hold all the solar fluid, plus about four gallons for safety's sake. The tank should have a sight glass on the side so you can monitor the fluid level, unless you install a low pressure drop flow meter in the feed line.

Like the other tanks, the drainback tank needs to be well insulated. It also needs a pressure relief valve : most tanks are rated to 50 pounds maximum pressure, so you can set the valve to open at that setting.

Even when the system is idle, the drainback tank can contain very hot solar fluid. It would be a shame to waste that energy, so most systems pass the accumulated heat to the main storage tank using a liquid-to-liquid heat exchanger.