They suffer a common fault - the "pin" gets stuck in the closed (fully down) position.
Before replacing your TRV take off the top by unscrewing the collar and tap gently on the top of the pin with the side of a spanner or similar until the pin rises.
My plumber tells me that just pulling up the pin with a pair of pliers is not a solution - it must rise on its own.
This is a five minute no-cost approach.
Peccavi, October 2010
