Cut my hands to bits fitting a pair of 15mm double NRV's supplying bath/shower mixer. Turn water back on cold fine, no hot. Undo the verifiable nut to get water out of it. Try tapping the body with spanner to no avail. Then have to remove it, and it had to be the furthest one away. Turns out it had two inlet valves fitted in it so it locks up. I was mighty pished off.