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maybe the 10' wasn't enough length to keep things moving through that hose?
Does your spa inject air into the water? I've not tried this technique yet, but if I did, I would turn off the air injection thinking it might break the siphon.
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I used a 10' leanght of pool vac hose (1 1/2 -2" dia) and tried to get a siphon going by holding one end up to a jet on high.
The hose is rigid, right? Otherwise no vaccuum.Also, the larger the diameter of the tube, the more "flow" you'd have to create from the jet to get the syphon started. Maybe the jet is not "strong" enough for that hose. Try a bigger (higher volume, not pressure) jet? I dont know at what point the length of the hose affects the vaccuum created, but I do know that you need more volume water per second for a larger diameter hose to create a syphon.
I think you are right about the pressue. I'm not following you on the hose ridgidness. hwo would that affect the vacuum? hey....if my hose is rigid or soft is between me and Mrs Drewstar.