What type of union should I get? There's all these different types.
Would this work?
No, that is a “check valve”; also known as a one-way valve!
These aren’t recommended, primarily because they will rapidly get their sealing surface covered in organic life, subsequently rendering them useless!
These are unions:
Why does BRS recommend this? We have never heard “I put too many unions on my plumbing.” When it comes to the initial plumbing, servicing, repair, or replacement of any hardware unions will be your best friend. For a couple extra bucks right off the start, down the line, they will pay for...
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They come in white (schedule 40: cheaper, lower build quality) and (usually) grey (schedule 80, more expensive, usually last a lot longer without seizing up)
You can get them with pipe threads, sockets you cement pipe into, or a combo of both!
(You’ll find the white schedule 40 examples at your local hardware store, you can usually build your plumbing stuff right there in the aisle to wrap your head around it before making your purchase)