Hi Dave,
I've not noticed a lot of expansion of the PEX coil, but I've not really tried to measure it.
I think that 2 inches will be plenty.
One thing I wonder about is how good the heat transfer to the PEX coil from the solar chamber will be, but I guess the easiest way to find out is to give it a try -- if it turns out to be not as good as you would like, you can think about it then.
Gary
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