 Solar Heating & Cooling SEIA - SEIA Solar Energy Another common type of solar water heating system design for cold climates is called drainback. This type of solar energy system typically uses water as the heat transfer fluid, and is designed to allow all of the water in the solar collector to drain back to a holding tank in Solar Heating & Cooling SEIA - SEIA Solar Energy Another common type of solar water heating system design for cold climates is called drainback. This type of solar energy system typically uses water as the heat transfer fluid, and is designed to allow all of the water in the solar collector to drain back to a holding tank in Solar Water Heater System with Two Heat Pipe Solar Collector