Drain Back Solar Water Heating Systems
Pools and Spas Heating


An excellent application for Solar Thermal Energy is heating pools and spas. In mild and moderate climates the pool/spa pump can be utilized to circulate the water through the collector. In extreme climates a closed loop system should be used, with the bypass described here.


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 4. Collector Supply 7. Pool/Spa Circulating Pump
2. Vacuum Breaker 5. Globe Valve (Bypass) 8. Motorized Valve
3. Collector Return 6. Supply from Pool/Spa 9. Return to Pool/Spa
  • A Differential Temperature Controller establishes that heat is available from the collector (1) and rotates the three way valve (8) directing the flow towards the collector.
  • Pool/Spa water is heated as it circulates through the collector.
  • There would be too much back pressure on the pump if all of the flow went through the collector, so a proportion is allowed to bypass the collector through a globe valve (5).
  • When no more heat is available from the collector, the controller rotates the three way valve to direct the flow away from the collector and to the pool/spa.
  • Air enters the system through the vacuum breaker (2) as the water in the collector drains back into the pool/spa.
Advantages Disadvantages
  • Excellent freeze protection, even in a power outage.
  • Simple installation with few components. The existing pump drives the system.
  • Care must be taken to plumb the supply and return so that they will drain back.


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