A block valve is a straightforward on-off valve that stops water or gas flow completely when you turn it
A block valve (also called a shutoff valve or isolation valve) is a valve designed to fully stop the flow of water, gas, or other fluids through a pipe. Unlike valves that regulate pressure or flow rate, a block valve has one job: turn it one way and flow stops; turn it the other way and flow resumes. You will find them throughout your home and building systems, and knowing where they are can save you from serious damage during an emergency.
Block valves are not complicated devices. Most operate with a straightforward handle or knob that you turn a quarter turn (90 degrees) to shut off flow. When the handle points along the pipe direction, flow is on. When it points across the pipe, flow is off. Some older block valves use a round wheel handle instead, which you turn multiple times until it stops.
Key Takeaways
- Block valves stop flow completely and are found on water supply lines, gas lines, and heating systems throughout your home.
- The main water shutoff valve is usually located where the water line enters your house, often in the basement or crawlspace.
- You should locate and test your main water shutoff before an emergency happens, so you know exactly where it is and that it works.
- Gas shutoff valves are typically located near the meter outside your home, and you should only turn one off if you smell gas or are instructed to by emergency services.
- Individual block valves under sinks and behind toilets let you shut off water to just that fixture without affecting the rest of your home.
Where the main water shutoff valve is located
Your home's main water shutoff valve controls all water entering your house. Finding it before you need it is essential. In most homes, this valve is located where the water line enters the building from the street. In cold climates, it is usually inside the house—often in the basement, crawlspace, or utility room near the foundation wall. In warmer climates, it may be outside in a box at ground level near the street or meter.
If you cannot find it inside, check outside your home for a small rectangular box (usually plastic or metal) set into the ground or mounted on the foundation. The box may have a removable lid. Inside, you will see the water meter and the shutoff valve. If you still cannot locate it, contact your water utility—they can tell you the exact location and may even mark it for you.
Once you find it, turn the handle a quarter turn to confirm it works. If it is stuck or will not turn, do not force it—call a plumber. A valve that has not been used in years may seize, and forcing it can break it. You want to know this before a pipe bursts and you are in crisis mode.
How to shut off water in an emergency
If a pipe bursts, a toilet overflows, or you have a major leak, shutting off water at the main valve stops the damage when ready. Locate your main shutoff valve and turn the handle a quarter turn so it points perpendicular to the pipe. The flow stops right away. If you have a wheel-style valve, turn it clockwise (to the right) until it stops—do not force it.
After you shut off the main valve, open a faucet on the lowest level of your home to release pressure in the pipes. This prevents water from continuing to leak from the break. Then call a plumber. If the leak is from a specific fixture like a toilet or sink, you may not need to shut off the main valve—look for the individual shutoff valve under that fixture instead.
If you cannot locate or operate the main shutoff valve during an emergency, call your water utility's emergency line. They can shut it off from the street side, though this takes longer. Having located and tested your valve beforehand means you will not waste time searching during a crisis.
Individual shutoff valves under fixtures
Most sinks, toilets, and washing machines have their own shutoff valves located on the supply line behind or under the fixture. These are smaller block valves that let you stop water to just that one item without affecting the rest of your home. If your toilet is running constantly or leaking, you can shut off its water without turning off water to your kitchen or bathroom sink.
Look under the sink or behind the toilet for a small valve on the pipe coming from the wall. It usually has a small handle or knob. Turn it a quarter turn to shut off water to that fixture. If the valve is stuck or leaks when you turn it, it may need replacement—this is a straightforward job for a plumber and costs far less than water damage from a leak.
If a fixture does not have an individual shutoff valve, consider having a plumber install one. This is inexpensive and gives you control over that fixture without affecting the rest of your home. It is especially useful for toilets, which can fail suddenly.
Gas shutoff valves and when to use them
Your gas meter has a shutoff valve near it, usually located outside your home where the gas line enters from the street. This valve looks similar to a water shutoff but controls gas flow instead. The handle typically points along the pipe when gas is flowing and perpendicular to the pipe when shut off.
You should only turn off a gas shutoff valve if you smell gas (a rotten-egg odor added to natural gas), if you suspect a gas leak, or if emergency services instruct you to do so. If you smell gas, leave your home when ready, do not use electrical switches or create sparks, and call your gas utility's emergency line from outside or from a neighbor's phone. Do not re-enter the home until the utility has inspected it and cleared it as safe.
If you must shut off gas, turn the valve handle a quarter turn so it points perpendicular to the pipe. Once gas is shut off, you will need a utility technician to turn it back on—you cannot do this yourself. Know where your gas shutoff valve is located before an emergency, but do not practice turning it unless a professional is present to help you turn it back on.
Block valves in heating and cooling systems
Furnaces, boilers, and air conditioning systems often have block valves on their water or refrigerant supply lines. These allow technicians to service the equipment without draining the entire system. You will not normally operate these valves yourself—they are there for professional maintenance.
However, if your heating or cooling system is leaking or malfunctioning, knowing that these valves exist can help you communicate with a technician. If a technician tells you they need to shut off a valve to service your system, they are likely referring to one of these isolation valves. You do not need to locate or operate them unless a professional instructs you to do so.
Frequently Asked Questions
What if my main water shutoff valve will not turn?
Do not force it. Valves that have not been used in years can seize. Call a plumber to free it or replace it. Forcing a stuck valve can break it, leaving you without a working shutoff in a real emergency. A plumber can usually free or replace it in under an hour.
Can I turn off the gas shutoff valve myself to practice?
You can locate it and look at it, but do not turn it unless a professional is present. Once you turn it off, you need a utility technician to turn it back on safely. If you want to practice, ask your gas utility if they offer a demonstration or training session.
Do I need individual shutoff valves under every sink?
It is not required, but it is a good idea, especially for toilets and washing machines. If a fixture does not have one and you want one installed, a plumber can add it for a modest cost. This gives you control over that fixture without shutting off water to your entire home.
What is the difference between a block valve and a pressure relief valve?
A block valve stops flow completely—it is either fully on or fully off. A pressure relief valve lets some flow through to control pressure in the system. Block valves are for emergencies and maintenance; pressure relief valves are part of normal system operation.
Where should I look if I cannot find my main water shutoff inside the house?
Check outside for a small box at ground level, usually near the street, meter, or foundation. The box may be plastic or metal with a removable lid. If you still cannot find it, contact your water utility with your address—they can tell you the exact location or send someone to mark it for you.