Drain Back
 Solar Water Heating Systems
Single Tank Application

A Solar Storage Tank with a heating element situated in the upper half of the tank (electric backup) should be used for Drain Back Solar Systems. (See controller can prevent the tank overheating.)

1. Collector 7. Drain back Reservoir 13. IsolationValve
2. Collector Sensor 8. Hose Bibs for Filling and Flushing 14. Collector Supply
3. Manual Air Valve 9. Pressure Relief Valve 15. Heat Exchange Coil
4. Hot Water to Taps 10. Tank &  Air Vent 16. Solar Hot Water Tank
5. Tempering Valve 11. Circulating Pump with flanges or couplings 17. Immersion Heater
6. Collector Return 12. Overflow Valve 18. Differential Controller
Operation
  • A differential controller establishes that heat is available from the collector, and turns on the pump.
  • The high-head pump lifts the drain back fluid, (usually distilled water with some corrosion inhibitor) from the reservoir to the collector and then on through the heat exchanger.
  • Heat from the collector is transferred to the storage tank.
  • When no more heat is available from the collector the controller turns off the pump.
  • The drain back fluid drains from the collector back into the reservoir, providing freeze protection.
Domestic Hot Water- Single Tank Application See "Gas Backup" for Gas Backup
A Solar Storage Tank with a heating element situated just above the middle of the tank is recommended for a single tank system. This tank is designed to automatically provide an adequate hot water backup.
Regular DHW electric tanks can be used but generally do not have enough volume above the top element to provide backup. If the bottom element is turned up (they are individually controllable) the tank will be too warm in the morning to store all the energy the collector can provide. Also, most tanks have a manual reset button on the top element that will trip at about 200ºF. The high limit feature on the controller should be activated and set to a maximum of 180ºF to prevent the tank overheating.
It is possible to use two standard DHW tanks to provide adequate backup, and although this will increase overall standby heat losses, it can be a useful option when retrofitting.

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Columbia, Maryland 21045
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