a pipe which turns down and runs along the side of the tank. When it's almost at the bottom of the tank, the pipe turns back upwards for a little way, then runs through a circulating pump which pushes water away from the tank and into the space heating equipment.

The pump must always be below the water level in the tank. Remember that you should not over-tighten the fittings on a fiberglass tank.

Direct drainback systems

This setup is very rare these days, but it is another alternative which bears mentioning. In this case, the tank acts as a heat storage medium (like in a closed loop) but is also the drainback tank and provides the content of the solar loop!

The same bulkhead fitting is installed as for a direct system but, instead of pumping water to the space heating equipment, a high-head pump circulates antifreeze mix around the solar loop. The solar fluid leaves the tank, drops down the pipe and is pumped up through the collector array. It gathers heat and drains back to the storage tank, where the energy is transferred to the space heating (and hot water) equipment. The entire content of the storage tank is solar fluid, which must be an antifreeze mix in locations with cold climates.

Such a drainback system needs a high-head AC pump capable of overcoming the static head measured from the water level in the storage tank to the top of the collector array, plus 4 feet (1.2 meters).