Drainback Solar Thermal System Design

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A typical solar thermal drainback system schematicAt first glance, a drainback solar hot water installation looks very much like a two-tank pressurized system. There are a few differences, though – the biggest of which is the addition of a third tank for drainback.

Drainback systems can use either flat-plate or evacuated-tube collectors to heat a solar fluid which is usually just simple water. In regions which are at risk of rare freezes, a mixture of 30% propylene glycol and 70% water is often used as insurance against catastrophic system failure.

The solar loop takes cold fluid from the heat exchanger to the collector array where it is heated by the sun's radiation. The hot fluid then heads back towards the exchanger but, en route, passes through a specialized drainback tank. The entire system is controlled by a differential temperature controller.

  • If the domestic water supply isn't hot enough, the controller switches on the pump. The pump pushes the solar fluid around the system, forcing the hot fluid in the drainback tank into the heat exchanger and completing the circuit. The domestic supply is heated by the exchanger and the cooler solar fluid heads back to the collector array.
  • If the domestic water supply has reached the required temperature, the pump switches off. All the solar fluid – in the pipes and the collector array – falls into the drainback tank, emptying the solar loop and preserving the fluid's integrity while cutting off any additional energy supply.
  • If the system is switched off for an extended vacation period or for some other reason, the solar fluid falls into the drainback tank and remains there until the controller switches on again. This ensures that the fluid does not get heated pointlessly and extends its life.

The pipes that form the solar loop are insulated and the pump is a high-head model, as it needs to overcome a great deal more resistance to get the fluid moving. A second pump may be needed if the installation has a separate heat exchanger rather than an in-tank model. Drains, gauges and valves are installed as shown in the diagram.