Pool Heat Exchanger Corrosion

I have a Raypak heater and my heat exchanger just went out a few weeks ago (green corrosion).
I replaced it. Incase this was caused by chlorine, I installed a Jandy Check valve on the output between the heater and my Rainbow In-Line chlorinator.
When I installed the check valve. I was unable to install it per the instructions. The instructions say you need 18 inches of upflow after the check valve in order for the weight of the water to keep the check valve closed. As you know, after the water leaves the heater there is not normally an upflow (it is all downflow from there).
After 2 weeks of use, I pulled the return header off and found the new one is turning green on the inside.
I had my water tested and everything was fine except hardness it was a little high (600ppm). But, the tap water in my area is pretty hard so I don't think there is much I can do about that.
I doubt the hardness would cause the green corrosion anway.
So, my conclusion is maybe even though the check valve is spring loaded, it may be allowing the water to seep backwards due to there being a downflow. The only solution I can figure out is come out of the heater and turn straight up for atleast 18 inches. then turn straight down to get to my chlorinator. I don't know if 18 inches of upflow would work followed by an imediate 30 inches of downflow (This seems it might cause it to syphon).
This does not seem like a good solution.
I would also consider changing out the chlorinator to an off-line system, but I doubt that would help my situation.
Does anyone have any suggestions.
Thanks so much,
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Jamie writes:

This is all for the quiescent state, right? So an 18-inch U (upside down) should be OK, especially if you put a reversed check valve to air at the top as a siphon breaker. You can drill and tap 1/4" NPT threads right into PVC elbows to install the siphon breaker. See my page here, about the "Bundy 3/4-inch acetal check valve":
http://www.truetex.com/poolcontrol.htm
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