Need help identifying this plumbing connection part and function


Nov 7, 2015
South Florida
I have the following setup:

- 15k pool with its own pump and filter
- spa with its own pump (no filter)
- water heater, that can in-take water from pool, spa, or both via a 3-way jandy selector valve
- water heater always outputs to spa

I see a transparent brown looking connector piece in two parts of my plumbing. I am not sure what it is, or whether its some sort of check valve. The confusing thing is this also connects between the heaters inlet and return pipes. What would be the purpose of this?

See picture, I've highlighted these two parts. Anyone recognize what these parts do?


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