Draindown solar hot water systems work as a kind of "solar add-on" to a grid-tied hot water setup. They consist of a collector array, a tank and piping which connects to the normal household plumbing, plus optional controllers, valves and gauges.

A draindown solar thermal setup is possibly the most efficient installation available when it is running. It's a direct system that heats the domestic supply directly in the collector array but it only operates when the sun is shining. Since it uses a collector (usually a flat-plate model ), it is often even more efficient than an ICS and because it only operates when the sun shines, it can use a PV-powered pump.

Typically, the system lies empty and idle until the sun comes out. Then the controller kicks in and operates a Sunspool – a specialized valve – that opens and lets the household water supply flood the system. The pump switches on and circulates the water in the normal way for direct systems.

When the sun goes away, the pump is switched off, the valve closes and the water in the drainback solar thermal system is allowed to run off through a waste pipe. That small amount of water is thrown away unused.

If you install a drainback system with a PV-powered pump, you can remove the controller and its reliance on an external power source, since it will only operate when the sun's radiation is sufficient to warm the water in the collector array.

Although they sound perfect, draindown systems do have disadvantages. They have no freeze protection, so they cannot be used in locations where temperatures fall very low (or must be completely drained and blocked off at those times). They also suffer from hard water mineral buildup problems in the same way as any direct system. On top of that, some of the specialized components are prone to failure more often than standard gear.