I recently purchased and installed a tap (pics below). I found that if I keep the main tap handle open but keep the outlets closed, I hear hiss sound of the water as if the water is still running. I ignored this and then water started overflowing from an overhead tank in the attic.
A handy-man told me that the tap hasn't got a non-return valve. I should put a NRV. Is he correct? Can I add an NRV to the existing pipework? Have I bought a wrong type of tap?