An isolation valve is any valve that lets you shut water off to one part of the system, such as a whole house, a water heater, a fixture or a softener, without shutting off the rest. Plumbing code calls them full-open or shutoff valves. Yours sit at the house entry, the water heater, under sinks and behind toilets.
What counts as an isolation valve?
Any valve that isolates a branch, a fixture or a piece of equipment for service. The common types are the ball valve (a quarter turn closes it), the gate valve (a round handle you spin several times) and the small angle stop under a sink or behind a toilet. Our page on the difference between a ball valve and a gate valve covers how each works and fails.
Moen describes the basic idea in its repair guidance: find "where the faucet shutoffs are located," rotate the shutoffs "clockwise until they are tightly closed," then turn the faucet handle to the open position "to insure the water is completely off." That open-the-faucet check works for any isolation valve. Close it, open a tap downstream, and confirm the water stops.
Where do you find the isolation valves in your home?
Walk the house once and tag each of these.
- 1The house valve (main shutoff). The City of Phoenix says that "for most houses, this valve is usually located at the front of the home, near a hose bib," and tells you to "look for a pipe coming out of the ground and going into the house." It also says that some older houses may have it at the rear of the property. If you cannot find it, the City says to call Customer Services at 602-262-6251 and a technician will turn the water off at the meter. Our guide to testing your main shut-off valve shows how to confirm it still closes.
- 2The water heater. A.O. Smith's manual says to "install a shut-off valve in the cold water inlet line" that "should be located close to the water heater and be easily accessible," and to know "the location of this valve and how to shut off the water to the heater." Look on the cold line above the tank.
- 3Fixture stops. Moen says these are generally "located under the sink, in a closet access behind a shower or in an access panel around the tub." Toilets have one behind or beside the bowl. Our page on shutting off water to a single fixture walks through it. Many tubs and showers have no stop at the fixture.
- 4Hose bibs and appliances. The code in the next section calls for a shutoff on the supply to each outdoor hose bib and to each appliance, so check the washing machine, the dishwasher and the outdoor spigots.
- 5The water softener. Morton's manual says that "bypass valves allow you to turn off water to the softener for maintenance if needed, but still have water in house pipes." See when to use the softener bypass.
Tankless water heaters use isolation valves too. Our page on tankless water heater maintenance covers how they come into service work.
What does plumbing code require?
Phoenix allows plumbing to be designed to either the Uniform Plumbing Code or the International Plumbing Code, both as amended, and the UPC has its own valve section, 606. This page uses the 2024 IPC text. Phoenix's IPC amendment list changes sections such as 604.4, 605.25 and 608 but does not list section 606, so the model wording applies. Section 606.1 says full-open valves "shall be installed" on the water distribution supply pipe at the entrance into the structure, and "on the water supply pipe to every water heater." Section 606.2 requires shutoff valves "on the fixture supply to each plumbing fixture other than bathtubs and showers in one- and two-family residential occupancies," on the supply "to each sill cock" (the code's word for an outdoor hose bib), and on the supply "to each appliance or mechanical equipment."
Two more rules matter when you go hunting. Section 606.3 says "access shall be provided to all full-open valves and shutoff valves." Section 606.4 says valves "installed in locations that are not adjacent to the fixture or appliance shall be identified, indicating the fixture or appliance served." So a valve you cannot reach, or a remote valve with no tag, is a defect worth fixing.
Which valve is not yours to turn?
The meter and the valve at the curb. The City of Phoenix says: "It's important for you to NOT shut your water off at the meter. This requires a special tool and you may be held responsible for any repairs to a damaged water meter." Our page on who can shut off water at the curb stop in Phoenix covers the curb valve.
What if a valve is stuck, leaking or missing?
Valves that are never turned tend to seize. If a handle will not move, do not force it with a long wrench. If a stop drips at the stem, see why a shut-off valve leaks at the handle. If a fixture has no stop at all, or the house valve will not close fully, that is a job for a licensed plumber, since changing a valve means cutting into a pressurized line.
HQ Plumbing and Air is licensed and insured, and we give free estimates and upfront pricing before work begins. Our residential plumbing team can add fixture stops, replace a failed house valve and tag every valve in the house so you know what each one does.