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You put the new one back the exact same way as you took the faulty one off.
If you look carefully, you will see the there are flow arrows on the mixer valve.
You know the cold water inlet is opposite the k nob.
If you look at the couplers nearly opposite each other, you will again see the arrows showing the direction of flow.
The fitting with the olive and nut is the side that threads into the heat exchanger.
If you are unsure, then start the heater and the run your wfp pump. You will quickly be able to feel the hot water inlet and the cooler outlet at the top that will eventually get hot water to your brush head.