Also called anti-vacuum, anti-siphon, antisiphon or vacuum breaker valve. This valve allows air to escape the boiler as the water heats up, hissing and then closing off when the boiler water reaches it's operating temperature, sealed by the steam pressure created. Measure your existing valve: the outside of the male threads of a 1/4" fitting measure 3/8" because the orifice that it threads into measures 1/4" inside - is that too confusing? One may also use part W_517