Drain Down Solar Water Heating System
Single Tank Application


Drain Down Systems are suitable for single application systems, particularly domestic hot water, in moderate climates. One or two tanks can be used in various configurations and they retrofit easily into existing systems.


NOTE: The information presented in this page is for guidance only - no part of this may be used for any agreement, whether express or implied, or to form any contract. THERMO TECHNOLOGIES reserves the right to change specifications and prices without prior notice.


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1. Solar Collector 6. Collector Return  Breaker 11. Tank Drain                    
2. Vent/Vacuum 7. Collector Supply 12. Tank Drain Fitting
3. Hot Water to Taps 8. Drain Down Valve 13. DHW Electric Tank
4. Cold Feed 9. Circulating Pump 14. Immersion Heater
The drain down valve provides system freeze protection. Without power, the valve closes the supply and return lines from the tank and the collector water drains away, preventing freeze-ups.
  • City water enters the storage tank at line pressure.
  • A controller establishes that heat is available from the collector, and turns on the drain down valve and the pump.
  • The drain down valve opens allowing city water to fill the collector at line pressure.
  • The small pump circulates water from the tank to be heated by the collector.
  • When no more heat is available from the collector, the controller turns off the pump and valve.
  • The drain down valve seals off the supply and return lines from the tank, and allows the water to drain from the supply and return lines to the collector.
  • A tempering valve governs the tap water temperature by adding cold water.

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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