Indirect, drainback systems require that all of the fluid in the solar loop drain back to conditioned space when the pump is shut off. This drainage prevents issues and damage as a result of extremely high or low temperatures and doesn’t require any part of the system to be constantly pressurized. In order for the fluid to drain down to a temperature-controlled space a drainback tank is needed. Drainback tanks must have sufficient volume to store the entire volume of the solar loop and have enough extra capacity to allow the pump to function properly when operating.
For small drainback systems the drainback tank is generally a small tank that sits above the main storage tank between the collector and storage tank on the solar return line. Drainback systems can be scaled up to use large, atmospheric reservoirs with hundreds or thousands of gallons of capacity. In either case the size of this tank must be calculated properly.
To size the drainback tank the total volume of the solar loop that exists in unconditioned space needs to be accounted for. For a given system this means adding up the total collector volume as well as the volume of the exposed piping. See Tables 1 & 2 for reference to the volumes of collectors and piping.