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FormulaNova said.. >>I am not a plumber, but my understanding is that a normal tap (with a washer) is sufficient as the washer stops backflow.
I will wait for an actual plumber to jump in and tell me I am so wrong that I have endangered society as we know it.
That's my take. I've never seen anything like this before, and I've see a lot of taps.
I presume it may be for when retic is connected to the taps instead of directly from mains, or even perhaps when a hose with an adjustable spray nozzle can be turned off at the nozzle. The flexible hose can hold a bit of pressure. If there's a problem with the mains, some water can flow back out of the hose.
But I don't see why it needs the pressure relief holes, a simple one way flow valve should be adequate.