Plumbing - Anti-Siphon Spigots
Anti-siphon spigots are the homeowner's hero for those who live in cold winter climates. For an extra dollar or two you can avoid having to replace an outdoor spigot due to low temperatures. Here's why...
In areas where winter temperatures get below 20F/-7C, it's possible for trapped water in the spigot and sillcock to freeze and expand. The figure is approximate. It's certainly possible for that same water to freeze at 32F/0C, but heat from the house helps lower the 'danger point' temperature somewhat.
That ice now presents two possible problems, similar to those that cause pipes under the house to sometimes burst in winter.
The ice takes up more room than the water did. You can test that by filling a cup exactly to the rim with water then putting it into the freezer. You'll note that the ice sticks slightly above the rim. That extra volume is no problem when the top is open. But when the water/ice has nowhere to expand, it raises the pressure on the container. Raise it enough and you can break a seal.
The major effect, though, is due to the fact that the pressure builds up beyond the ice block. Air and water have nowhere to go. The air is compressible, but any trapped water will create high pressure on the internal parts of the sillcock and spigot. Even strong metals, made more brittle by the cold anyway, can be split. Plastic and cold-hardened rubber are doomed.
When the weather warms again and everything melts, you now have a spigot with a crack. If it doesn't leak spontaneously, turning it on will guarantee a drip or worse.
An anti-siphon spigot contains an additional mechanism beyond the ordinary outdoor water faucet. Hoses attach like normal, but inside and out they have features to prevent the spigot from bursting due to ice blockage and expansion.
A few things help prevent the anti-siphon spigot from cracking like an ordinary one.
The seal/valve that actually shuts off the water flow is further back in the sillcock - inside the house, which is warmer. That helps prevent cold temperatures from reaching the water. More helpful still is the ability of the sillcock to withdraw water away from areas where ice buildup can be a problem. It keeps the water from remaining near seals that can be frozen and cracked by low temperatures.
But most importantly, the design of the sillcock/spigot allows for the relief of pressure for any ice that does form. It gives a place for expanding water to go when pressure builds up in other parts of the sillcock/spigot.
Even so, in warm conditions and with normal use, it's possible for anti-siphon spigots to leak out the top pressure-relieving spout or elsewhere. Kits with replacement seals and other internal parts are available for a few dollars. For those who prefer to repair rather than replace, or if the other end of the sillcock is difficult to access, these kits can save money and time.
