What It Is
A compression valve - also called an angle stop or fixture shutoff - is a small valve installed at the wall where the water supply stub-out meets the supply line leading to a fixture. You find one under every toilet tank, under every sink, and behind washing machines. Its purpose is to let you shut off water to that single fixture without turning off water to the rest of the house. Compression valves use a rubber washer that compresses against a seat when the stem is screwed down to stop flow.
What It Does
Compression valves give you local control over each fixture. When you need to replace a toilet fill valve, swap out a faucet, or work on a supply line, you close the compression valve directly behind that fixture rather than shutting off the main water supply to the whole house. This is especially convenient in multi-bathroom homes where you want to work in one bathroom while leaving the rest of the house with full water service.
They are called "angle stops" because the most common configuration makes a 90-degree turn between the wall supply stub-out pointing horizontally and the supply line running vertically up to the fixture. This angular configuration fits neatly into the cabinet space under most sinks and behind most toilets.
Where It Is Located
You will find compression valves: on the wall or floor directly behind every toilet tank (the small oval valve with an oval handle); under every bathroom and kitchen sink in the cabinet; behind the washing machine at both hot and cold supply connections; and at the dishwasher supply connection under the sink. They are small - typically 1/2 inch on the supply side - and easy to overlook until something goes wrong.
How It Works
Turning the oval handle clockwise drives a stem downward. The stem has a rubber washer at its tip that presses against a brass seat inside the valve body, blocking water from passing through. Turning the handle counterclockwise lifts the washer off the seat and allows water to flow. Over time, the rubber washer compresses, hardens, and loses elasticity - which is why old compression valves may not fully seal when you need them to, or develop slow drips at the packing nut.
Common Problems or Signs of Failure
- Slow drip at the valve stem where the handle connects - packing nut needs tightening or valve needs replacement
- Valve does not fully stop flow when closed - rubber washer is worn
- Handle is stiff and difficult to turn - mineral deposits or corrosion on the stem
- Leak at the connection where supply line meets the valve
- Valve body shows green or white mineral deposits indicating a slow leak over time
Can a Homeowner Check It?
- Locate and identify the compression valve for each fixture in your home
- Turn each one closed and open once per year to keep the stem from seizing
- Look for any drips or mineral deposits at or around each valve
- When closing, turn gently until resistance - do not overtighten, which can crack the body
When to Call a Plumber
If a compression valve leaks at the stem or refuses to fully close, replacement is the right move. An old valve that has never been operated can seize and fail to close at the worst moment - during a fixture emergency. Replacing compression valves is a routine job that can often be done without shutting off the main supply if the main valve works. Many plumbers replace aging compression valves as a matter of course when doing fixture work nearby.
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