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What's a Drain Back System?

DIY drainback heater pool solar solar water heater water

You've purchased your solar water heater, and winter is coming. How do you make sure leftover/standing water doesn't freeze inside the system? This is where a drain back system comes in.

Drainback systems are closed-loop, indirect, active systems. A heat-transfer fluid (HTF, usually water) contained in an unpressurized, closed loop is pumped through the collectors and is separate from the end-use water being heated through a heat exchanger. When the pump is off, the HTF drains out of the properly sloped collectors and pipe, leaving them empty and protected from freezing. 

While this article will primarily focus on residential domestic hot water systems, many options exist for system configurations, ranging from pool heating to space heating and combined domestic hot water and space heating. Regardless of what the heat is being used for, the basic components of a drainback system are:

  • A storage tank, to hold the end-use water being heated. This could be a domestic hot water tank, a several-thousand-gallon tank for a space-heating system, or, in the case of a pool system, the pool itself.
  • One or more solar hot water collectors.
  • A differential controller, which monitors the water temperature in the tank and the collector temperature. When the collector temperature exceeds the storage temperature by a set differential (number of degrees), the controller activates the HTF pump. In some cases, a DC pump, powered by a PV module, acts as the controller—when appropriately sized, the PV module receives enough sunlight to operate the pump when there is enough heat in the thermal collector for it to be worthwhile to circulate the HTF.
  • A reservoir—a tank in the HTF loop plumbing—which holds the HTF in the drainback/collector loop. The draining of the HTF into the reservoir creates the characteristic gurgling sound of these systems, due to the air space required in the unpressurized reservoir.
  • A heat exchanger to transfer heat between the drainback loop and the end-use water (unless it is a pool system). The heat exchanger is often inside the drainback reservoir, and a second pump circulates domestic water through the exchanger, which is immersed in the HTF. Most pool-heating systems circulate the pool water through collectors, eliminating the need for a heat exchanger.

But... do you really need a drain back system if you have a solar water heater kit? Not necessarily. The way our solar water heater is designed, it can be mounted 90 degrees so the water will drain naturally out of the copper tubes. It's ultimately up to you and your project needs, but our system does not need to have a drainback system installed in it in order to work. So pick up your Solar Water Heater kit today!

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