Anti-siphon valves prevent water from flowing backwards and contaminating the water supply. It's easy to see why they are required in toilets by virtually every building authority, and in fact, most fill valves incorporate one. This wasn't always true, and if you have an older toilet that has a fill valve without anti-siphon protection, it's time to replace it. It's also time to replace the valve if it does have anti-siphon protection, and the anti-siphon feature is broken.
What Is an Anti-Siphon Valve?
When water is flowing from your fill valve, the aperture obviously has to open, but you don't want the valve to stay open when the water stops. If it does, negative pressure in the pipes can draw water out of the tank, and although the tank sits above and separate from the bowl, the water isn't considered potable or safe. To prevent this, a spring closes the valve when water isn't flowing.
When an Anti-Siphon Valve Goes Bad
The anti-siphon feature is so well integrated into the fill valve that repairing it is impractical, if not impossible. There's little need to do that anyway, because a new fill valve sets you back less than $20, and it's so easy to install that anyone can do it.
Replacing the Fill Valve
Ballcock-style valves with float balls are still available, but cup-style valves are more common, more efficient and, because they have no metal parts, they last longer. Although there are many styles, they all have standard fittings and work in virtually any toilet, so you don't have to look far and wide to find one -- every hardware and building supply outlet stocks them.
Disconnect the top of the water supply hose at the end below the tank, using adjustable pliers on its coupling nut, then unscrew the much larger lock nut at the bottom of the fill valve. Disconnect the refill tube from the overflow tube and lift the fill valve out of the tank.
If the valve has a ball float, unscrew and remove it to get it out of the way before removing the valve.
Adjust the height of the new fill valve to fit your tank, following the directions on the container. You typically do this by unscrewing the upper section while holding the lower section steady.
Install the washer that came with the valve onto the threaded connection and insert the connection in the hole in the bottom of the tank. Screw on the lock nut and, pressing down on the fill valve's lower shank, tighten the lock nut securely by hand. Hook the refill tube connected to the outlet on the valve to the overflow tube with the clip provided.
Screw the water supply hose back onto the fill valve connection and tighten it hand-tight. Turn on the water, and if the connection leaks, tighten it more with adjustable pliers.
Wait for the tank to fill, then adjust the float to bring the water level to within 1 inch of the top of the overflow tube.