The tanks are identical but the one on the right Has a ball valve and seems to be fed by the mains, the one on the left has a pipe from overhead that drips into it - this pipe goes down behind the hot water storage tank in the hot press, maybe to downstairs where the boiler is but I can’t see.
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The PRV beside the hot press that’s showing almost zero pressure? Not sure what it’s doing there but the pressure rises slightly when the heating is on (the same as the pressure gauge on the boiler)
However I can’t see anything that looks like a swing or poppet valve near the cold water tanks - is that where I would find this?