I have a partially above ground Spa and an in-ground pool. When the spa is filled to spillover with pumps off, water siphons through the spa pump and into the pool until the spa water level drops about 13 inches.

What confuses me is that the Spa drain is about 12 inches below the pool returns and the highest pipe in this circuit is about 5 inches above the highest spa water level.

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I cannot figure out why siphoning is occurring or how to stop this. Instinctively I'm guessing I should raise the highest point 13" (since the water drops 13"), but don't really want to make the change only to find out it doesn't work. Suggestions how to analyze and solve this?

  • $\begingroup$ Water always finds its level ie equilibrium... $\endgroup$
    – Solar Mike
    Sep 10 '20 at 19:37
  • $\begingroup$ so when the system sits for some time, and the water in the spa drops its ~13 inches, is the surface water level in the spa the same height/elevation as the surface water level of the pool, higher, or lower? $\endgroup$
    – Forward Ed
    Sep 14 '20 at 18:05
  • $\begingroup$ When the water levels stabilize the spa is about 12 inches above the pool water level. $\endgroup$
    – SeaSilver
    Sep 15 '20 at 20:18

This issue was resolved by adding an upward PVC loop (about 1 ft higher than the highest point of circuit) and installing a pressure relief valve, also referred to as a vacuum breaker, in this loop. A vacuum breaker is typically installed on roof pool solar heater systems to drain water off the roof when pump isn't running. This puts a small amount of air into the system which is automatically purged when a pump starts. (HELIOCOL 3/4" New Vacuum Relief Valve Pool Solar Panels).

Thanks for the feedback, it helped me think through this problem and find a solution.


You have to install a spring loaded check valve after the pump. I am attaching a link just to help you get an idea, not promoting the item.


  • $\begingroup$ A check valve won't work because the siphoning is in the same direction as the pump flow (when the pump is operating). I have check valves to prevent flow in the opposite direction. $\endgroup$
    – SeaSilver
    Sep 11 '20 at 16:07
  • $\begingroup$ 2nd thought. I currently have 2 check valves, with 10lb springs, in this circuit. So maybe the siphoning is greater than 20lb force to open these valves. I'm now wondering if I can add another check valve vertically (so gravity will help keep it closed plus this additional 10lb spring), maybe that would overcome the siphoning force but easily open when the pump is running. How can I calculate this force? Spa is 81" diameter, pipes 2" PVC, initial spa water drop is 1-2/3" per hour (I believe this is about 37.2 gallons per hour). $\endgroup$
    – SeaSilver
    Sep 11 '20 at 16:47
  • $\begingroup$ Using 8.345 lb/gallon of water, this would mean ~310.5lb of water flows per hour. However I'm struggling to calculate the spring force to keep a 2" pipe closed. Idea's? $\endgroup$
    – SeaSilver
    Sep 11 '20 at 18:03
  • $\begingroup$ Is easy its the H/10 *1kg per cm^2. H is the difference between the level of spa and pool in meters. $\endgroup$
    – kamran
    Sep 11 '20 at 18:06

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