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Closed Loop or Drainback System? |
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Both Closed Loop and Drainback systems will provide years of reliable service. Alternative Power Distribution recommends a pre-assembled pump station for rapid installation and proven operation. The choice is dependent on many factors, and several are outlined below. Always consult with your installer to find the best fit for your situation. | | Closed Loop | Drainback | | Summary | Closed Loop Systems use a pressurized heat transfer fluid (HTF) usually consisting of water and non toxic propylene glycol *. The HTF remains in the solar loop and collectors at all times. The system is closed in that repeated exposure to oxygen is not possible, and oxygens' corrosive action is reduced. | Drainback systems use a non-pressurized Heat Transfer Fluid (HTF), usually distilled water. The HTF drains back to a small drainback tank when the system is not operating. | Freeze Protection
| Glycol provides protection to very low temperatures depending on mix concentration. | The HTF flows down to the drainback tank when the system is not operating. For example, at night or when heat demand is met. The HTF is not exposed to freezing temperatures. | Overheat Protection
| A heat dump loop is often required to prevent overheating of the HTF. Repeated overheating can cause propylene glycol to fail*. Proper collector tilt and adequate storage volume can mitigate overheating. Stagnation and fluid failure can occur due to pump failure or loss of electrical power. | The HTF drains from the collector to the drainback tank when the system is not operating. Since fluid is not in the collector, fluid overheating is not possible. Seido Models 1, 5, and SUNDA Flat Plate Collectors can tolerate continuous dry stagnation. Pump failure or loss of electrical power will not damage the system components or HTF. | | Cost | Installed costs are similar. | Installed cost are similar. | | Installation | Generally more complex to install. | Generally less complex to install with one exception. Collectors and piping must be carefully sloped at least 1/4in per foot. | System Maintenance
| Checks of glycol quality recommended every 1-2 years. Replacement of glycol necessary every 5-10 years or more frequently if stagnation occurs. | Occasionally necessary to add water. Draining or pressurizing system not required. | Power Consumption
| Low head pumps use less power. Circulation of the HTF to avoid stagnation will increase pump run time. | Drainback systems require a higher head pump that uses more power. Pump operation is not required during vacation periods or to prevent stagnation. | | Pumps | Cast iron pumps acceptable. | Bronze or stainless steel pumps required. | * Never use automotive antifreeze, also known as ethylene glycol. It is highly toxic. * Propylene Glycol inhibitors will eventually break down due to high temperatures. The resulting fluid is less effective as an antifreeze, forms deposits that can clog the pump or collector, and becomes acidic.
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